diff --git a/.gitignore b/.gitignore
index 921dc23..bf8a301 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,7 +17,7 @@ videos/
 .manim-slides.toml
 
 slides/
-!tests/slides/
+!tests/data/slides/
 
 slides_assets/
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..1fd967e
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,26 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [v5 (Unreleased)](https://github.com/jeertmans/languagetool-rust/compare/v4.16.0...HEAD)
+
+Prior to v5, there was no real CHANGELOG other than the GitHub releases,
+with most of the content automatically generated by GitHub from merged
+pull requests.
+
+In an effort to better document changes, this CHANGELOG document is now created.
+
+### Chore
+
+- Automatically concatenate all animations from a slide into one.
+  This is a **breaking change** because the config file format is
+  different from the previous one. For migration help, see associated PR.
+  [#242](https://github.com/jeertmans/manim-slides/pull/242)
+
+### Removed
+
+- Removed `--start-at-animation-number` option from `manim-slides present`.
+  [#242](https://github.com/jeertmans/manim-slides/pull/242)
diff --git a/example.py b/example.py
index 1062c8b..d6e2283 100644
--- a/example.py
+++ b/example.py
@@ -2,16 +2,14 @@
 # type: ignore
 import sys
 
-if "manim" in sys.modules:
-    from manim import *
-
-    MANIMGL = False
-elif "manimlib" in sys.modules:
+if "manimlib" in sys.modules:
     from manimlib import *
 
     MANIMGL = True
 else:
-    raise ImportError("This script must be run with either `manim` or `manimgl`")
+    from manim import *
+
+    MANIMGL = False
 
 from manim_slides import Slide, ThreeDSlide
 
diff --git a/manim_slides/config.py b/manim_slides/config.py
index f62378a..f69c757 100644
--- a/manim_slides/config.py
+++ b/manim_slides/config.py
@@ -1,8 +1,5 @@
-import hashlib
 import json
 import shutil
-import subprocess
-import tempfile
 from enum import Enum
 from pathlib import Path
 from typing import Any, Dict, List, Optional, Set, Tuple, Union
@@ -19,30 +16,9 @@ from pydantic import (
 from pydantic_extra_types.color import Color
 from PySide6.QtCore import Qt
 
-from .defaults import FFMPEG_BIN
 from .logger import logger
 
 
-def merge_basenames(files: List[FilePath]) -> Path:
-    """
-    Merge multiple filenames by concatenating basenames.
-    """
-    logger.info(f"Generating a new filename for animations: {files}")
-
-    dirname: Path = files[0].parent
-    ext = files[0].suffix
-
-    basenames = (file.stem for file in files)
-
-    basenames_str = ",".join(f"{len(b)}:{b}" for b in basenames)
-
-    # We use hashes to prevent too-long filenames, see issue #123:
-    # https://github.com/jeertmans/manim-slides/issues/123
-    basename = hashlib.sha256(basenames_str.encode()).hexdigest()
-
-    return dirname.joinpath(basename + ext)
-
-
 class Key(BaseModel):  # type: ignore
     """Represents a list of key codes, with optionally a name."""
 
@@ -124,12 +100,10 @@ class SlideType(str, Enum):
     last = "last"
 
 
-class SlideConfig(BaseModel):  # type: ignore
+class PreSlideConfig(BaseModel):  # type: ignore
     type: SlideType
     start_animation: int
     end_animation: int
-    number: int
-    terminated: bool = Field(False, exclude=True)
 
     @field_validator("start_animation", "end_animation")
     @classmethod
@@ -138,13 +112,6 @@ class SlideConfig(BaseModel):  # type: ignore
             raise ValueError("Animation index (start or end) cannot be negative")
         return v
 
-    @field_validator("number")
-    @classmethod
-    def number_is_strictly_posint(cls, v: int) -> int:
-        if v <= 0:
-            raise ValueError("Slide number cannot be negative or zero")
-        return v
-
     @model_validator(mode="before")
     def start_animation_is_before_end(
         cls, values: Dict[str, Union[SlideType, int, bool]]
@@ -161,6 +128,23 @@ class SlideConfig(BaseModel):  # type: ignore
 
         return values
 
+    @property
+    def slides_slice(self) -> slice:
+        return slice(self.start_animation, self.end_animation)
+
+
+class SlideConfig(BaseModel):  # type: ignore
+    type: SlideType
+    file: FilePath
+    rev_file: FilePath
+    terminated: bool = Field(False, exclude=True)
+
+    @classmethod
+    def from_pre_slide_config_and_files(
+        cls, pre_slide_config: PreSlideConfig, file: Path, rev_file: Path
+    ) -> "SlideConfig":
+        return cls(type=pre_slide_config.type, file=file, rev_file=rev_file)
+
     def is_slide(self) -> bool:
         return self.type == SlideType.slide
 
@@ -170,14 +154,9 @@ class SlideConfig(BaseModel):  # type: ignore
     def is_last(self) -> bool:
         return self.type == SlideType.last
 
-    @property
-    def slides_slice(self) -> slice:
-        return slice(self.start_animation, self.end_animation)
-
 
 class PresentationConfig(BaseModel):  # type: ignore
     slides: List[SlideConfig] = Field(min_length=1)
-    files: List[FilePath]
     resolution: Tuple[PositiveInt, PositiveInt] = (1920, 1080)
     background_color: Color = "black"
 
@@ -187,12 +166,15 @@ class PresentationConfig(BaseModel):  # type: ignore
         with open(path, "r") as f:
             obj = json.load(f)
 
-            if files := obj.get("files", None):
-                # First parent is ../slides
-                # so we take the parent of this parent
-                parent = Path(path).parents[1]
-                for i in range(len(files)):
-                    files[i] = parent / files[i]
+            slides = obj.setdefault("slides", [])
+            parent = path.parent.parent  # Never fails, but parents[1] can fail
+
+            for slide in slides:
+                if file := slide.get("file", None):
+                    slide["file"] = parent / file
+
+                if rev_file := slide.get("rev_file", None):
+                    slide["rev_file"] = parent / rev_file
 
             return cls.model_validate(obj)  # type: ignore
 
@@ -201,104 +183,25 @@ class PresentationConfig(BaseModel):  # type: ignore
         with open(path, "w") as f:
             f.write(self.model_dump_json(indent=2))
 
-    @model_validator(mode="after")
-    def animation_indices_match_files(
-        cls, config: "PresentationConfig"
-    ) -> "PresentationConfig":
-        files = config.files
-        slides = config.slides
-
-        n_files = len(files)
-
-        for slide in slides:
-            if slide.end_animation > n_files:
-                raise ValueError(
-                    f"The following slide's contains animations not listed in files {files}: {slide}"
-                )
-
-        return config
-
-    def copy_to(self, dest: Path, use_cached: bool = True) -> "PresentationConfig":
+    def copy_to(self, folder: Path, use_cached: bool = True) -> "PresentationConfig":
         """
         Copy the files to a given directory.
         """
-        n = len(self.files)
-        for i in range(n):
-            file = self.files[i]
-            dest_path = dest / self.files[i].name
-            self.files[i] = dest_path
-            if use_cached and dest_path.exists():
-                logger.debug(f"Skipping copy of {file}, using cached copy")
-                continue
-            logger.debug(f"Copying {file} to {dest_path}")
-            shutil.copy(file, dest_path)
+        for slide_config in self.slides:
+            file = slide_config.file
+            rev_file = slide_config.rev_file
 
-        return self
+            dest = folder / file.name
+            rev_dest = folder / rev_file.name
 
-    def concat_animations(
-        self, dest: Optional[Path] = None, use_cached: bool = True
-    ) -> "PresentationConfig":
-        """
-        Concatenate animations such that each slide contains one animation.
-        """
+            slide_config.file = dest
+            slide_config.rev_file = rev_dest
 
-        dest_paths = []
+            if not use_cached or not dest.exists():
+                shutil.copy(file, dest)
 
-        for i, slide_config in enumerate(self.slides):
-            files = self.files[slide_config.slides_slice]
-
-            slide_config.start_animation = i
-            slide_config.end_animation = i + 1
-
-            if len(files) > 1:
-                dest_path = merge_basenames(files)
-                dest_paths.append(dest_path)
-
-                if use_cached and dest_path.exists():
-                    logger.debug(f"Concatenated animations already exist for slide {i}")
-                    continue
-
-                f = tempfile.NamedTemporaryFile(mode="w", delete=False)
-                f.writelines(f"file '{path.absolute()}'\n" for path in files)
-                f.close()
-
-                command: List[str] = [
-                    str(FFMPEG_BIN),
-                    "-f",
-                    "concat",
-                    "-safe",
-                    "0",
-                    "-i",
-                    f.name,
-                    "-c",
-                    "copy",
-                    str(dest_path),
-                    "-y",
-                ]
-                logger.debug(" ".join(command))
-                process = subprocess.Popen(
-                    command, stdout=subprocess.PIPE, stderr=subprocess.PIPE
-                )
-                output, error = process.communicate()
-
-                if output:
-                    logger.debug(output.decode())
-
-                if error:
-                    logger.debug(error.decode())
-
-                if not dest_path.exists():
-                    raise ValueError(
-                        "could not properly concatenate animations, use `-v INFO` for more details"
-                    )
-
-            else:
-                dest_paths.append(files[0])
-
-        self.files = dest_paths
-
-        if dest:
-            return self.copy_to(dest)
+            if not use_cached or not rev_dest.exists():
+                shutil.copy(rev_file, rev_dest)
 
         return self
 
diff --git a/manim_slides/convert.py b/manim_slides/convert.py
index 27d8633..de4731b 100644
--- a/manim_slides/convert.py
+++ b/manim_slides/convert.py
@@ -362,7 +362,7 @@ class RevealJS(Converter):
         """Generates a sequence of sections, one per slide, that will be included into the html template."""
         for presentation_config in self.presentation_configs:
             for slide_config in presentation_config.slides:
-                file = presentation_config.files[slide_config.start_animation]
+                file = slide_config.file
 
                 logger.debug(f"Writing video section with file {file}")
 
@@ -399,9 +399,6 @@ class RevealJS(Converter):
         """Converts this configuration into a RevealJS HTML presentation, saved to DEST."""
         if self.data_uri:
             assets_dir = Path("")  # Actually we won't care.
-
-            for presentation_config in self.presentation_configs:
-                presentation_config.concat_animations()
         else:
             dirname = dest.parent
             basename = dest.stem
@@ -417,7 +414,7 @@ class RevealJS(Converter):
             full_assets_dir.mkdir(parents=True, exist_ok=True)
 
             for presentation_config in self.presentation_configs:
-                presentation_config.concat_animations().copy_to(full_assets_dir)
+                presentation_config.copy_to(full_assets_dir)
 
         with open(dest, "w") as f:
             sections = "".join(self.get_sections_iter(assets_dir))
@@ -470,15 +467,14 @@ class PDF(Converter):
         images = []
 
         for i, presentation_config in enumerate(self.presentation_configs):
-            presentation_config.concat_animations()
             for slide_config in tqdm(
                 presentation_config.slides,
                 desc=f"Generating video slides for config {i + 1}",
                 leave=False,
             ):
-                file = presentation_config.files[slide_config.start_animation]
-
-                images.append(read_image_from_video_file(file, self.frame_index))
+                images.append(
+                    read_image_from_video_file(slide_config.file, self.frame_index)
+                )
 
         images[0].save(
             dest,
@@ -531,7 +527,7 @@ class PowerPoint(Converter):
             return etree.ElementBase.xpath(el, query, namespaces=nsmap)
 
         def save_first_image_from_video_file(file: Path) -> Optional[str]:
-            cap = cv2.VideoCapture(str(file))
+            cap = cv2.VideoCapture(file.as_posix())
             ret, frame = cap.read()
 
             if ret:
@@ -543,13 +539,12 @@ class PowerPoint(Converter):
                 return None
 
         for i, presentation_config in enumerate(self.presentation_configs):
-            presentation_config.concat_animations()
             for slide_config in tqdm(
                 presentation_config.slides,
                 desc=f"Generating video slides for config {i + 1}",
                 leave=False,
             ):
-                file = presentation_config.files[slide_config.start_animation]
+                file = slide_config.file
 
                 mime_type = mimetypes.guess_type(file)[0]
 
diff --git a/manim_slides/present.py b/manim_slides/present.py
index f73a69d..af0dbe7 100644
--- a/manim_slides/present.py
+++ b/manim_slides/present.py
@@ -88,14 +88,13 @@ class Presentation:
         self.config = config
 
         self.__current_slide_index: int = 0
-        self.current_animation: int = self.current_slide.start_animation
-        self.current_file: Path = Path("")
+        self.current_file: Path = self.current_slide.file
 
-        self.loaded_animation_cap: int = -1
+        self.loaded_slide_cap: int = -1
         self.cap = None  # cap = cv2.VideoCapture
 
         self.reverse: bool = False
-        self.reversed_animation: int = -1
+        self.reversed_slide: int = -1
 
         self.lastframe: Optional[np.ndarray] = None
 
@@ -109,11 +108,6 @@ class Presentation:
         """Returns the list of slides."""
         return self.config.slides
 
-    @property
-    def files(self) -> List[Path]:
-        """Returns the list of animation files."""
-        return self.config.files
-
     @property
     def resolution(self) -> Tuple[int, int]:
         """Returns the resolution."""
@@ -133,39 +127,12 @@ class Presentation:
         if value is not None:
             if -len(self) <= value < len(self):
                 self.__current_slide_index = value
-                self.current_animation = self.current_slide.start_animation
                 logger.debug(f"Set current slide index to {value}")
             else:
                 logger.error(
                     f"Could not load slide number {value}, playing first slide instead."
                 )
 
-    def set_current_animation_and_update_slide_number(
-        self, value: Optional[int]
-    ) -> None:
-        if value is not None:
-            n_files = len(self.files)
-            if -n_files <= value < n_files:
-                if value < 0:
-                    value += n_files
-
-                for i, slide in enumerate(self.slides):
-                    if value < slide.end_animation:
-                        self.current_slide_index = i
-                        self.current_animation = value
-
-                        logger.debug(f"Playing animation {value}, at slide index {i}")
-                        return
-
-                assert (
-                    False
-                ), f"An error occurred when setting the current animation to {value}, please create an issue on GitHub!"
-
-            else:
-                logger.error(
-                    f"Could not load animation number {value}, playing first animation instead."
-                )
-
     @property
     def current_slide(self) -> SlideConfig:
         """Returns currently playing slide."""
@@ -186,54 +153,47 @@ class Presentation:
         if self.cap is not None:
             self.cap.release()
 
-        self.loaded_animation_cap = -1
+        self.loaded_slide_cap = -1
 
-    def load_animation_cap(self, animation: int) -> None:
-        """Loads video file of given animation."""
+    def load_slide_cap(self, slide: int) -> None:
+        """Loads video file of given slide."""
         # We must load a new VideoCapture file if:
-        if (self.loaded_animation_cap != animation) or (
-            self.reverse and self.reversed_animation != animation
+        if (self.loaded_slide_cap != slide) or (
+            self.reverse and self.reversed_slide != slide
         ):  # cap already loaded
-            logger.debug(f"Loading new cap for animation #{animation}")
+            logger.debug(f"Loading new cap for slide #{slide}")
 
             self.release_cap()
 
-            file: Path = self.files[animation]
-
             if self.reverse:
-                file = file.parent / f"{file.stem}_reversed{file.suffix}"
-                self.reversed_animation = animation
+                file = self.current_slide.rev_file
+                self.reversed_slide = slide
+            else:
+                file = self.current_slide.file
 
             self.current_file = file
 
-            self.cap = cv2.VideoCapture(str(file))
-            self.loaded_animation_cap = animation
+            self.cap = cv2.VideoCapture(file.as_posix())
+            self.loaded_slide_cap = slide
 
     @property
     def current_cap(self) -> cv2.VideoCapture:
         """Returns current VideoCapture object."""
-        self.load_animation_cap(self.current_animation)
+        self.load_slide_cap(self.current_slide_index)
         return self.cap
 
     def rewind_current_slide(self) -> None:
         """Rewinds current slide to first frame."""
         logger.debug("Rewinding current slide")
         self.current_slide.terminated = False
-
-        if self.reverse:
-            self.current_animation = self.current_slide.end_animation - 1
-        else:
-            self.current_animation = self.current_slide.start_animation
-
-        cap = self.current_cap
-        cap.set(cv2.CAP_PROP_POS_FRAMES, 0)
+        self.current_cap.set(cv2.CAP_PROP_POS_FRAMES, 0)
 
     def cancel_reverse(self) -> None:
         """Cancels any effet produced by a reversed slide."""
         if self.reverse:
             logger.debug("Cancelling effects from previous 'reverse' action'")
             self.reverse = False
-            self.reversed_animation = -1
+            self.reversed_slide = -1
             self.release_cap()
 
     def reverse_current_slide(self) -> None:
@@ -276,36 +236,20 @@ class Presentation:
 
     def reset(self) -> None:
         """Rests current presentation."""
-        self.current_animation = 0
-        self.load_animation_cap(0)
+        self.load_slide_cap(0)
         self.current_slide_index = 0
         self.slides[-1].terminated = False
 
     def load_last_slide(self) -> None:
         """Loads last slide."""
         self.current_slide_index = len(self.slides) - 1
-        assert (
-            self.current_slide_index >= 0
-        ), "Slides should be at list of a least one element"
-        self.current_animation = self.current_slide.start_animation
-        self.load_animation_cap(self.current_animation)
+        self.load_slide_cap(self.current_slide_index)
         self.slides[-1].terminated = False
 
     @property
-    def next_animation(self) -> int:
-        """Returns the next animation."""
-        if self.reverse:
-            return self.current_animation - 1
-        else:
-            return self.current_animation + 1
-
-    @property
-    def is_last_animation(self) -> bool:
-        """Returns True if current animation is the last one of current slide."""
-        if self.reverse:
-            return self.current_animation == self.current_slide.start_animation
-        else:
-            return self.next_animation == self.current_slide.end_animation
+    def is_last_slide(self) -> bool:
+        """Returns True if current slide is the last one."""
+        return self.current_slide_index == len(self.slides) - 1
 
     @property
     def current_frame_number(self) -> int:
@@ -331,25 +275,17 @@ class Presentation:
             return self.lastframe, State.PLAYING
 
         # Video was terminated
-        if self.is_last_animation:
-            if self.current_slide.is_loop():
-                if self.reverse:
-                    state = State.WAIT
-
-                else:
-                    self.current_animation = self.current_slide.start_animation
-                    state = State.PLAYING
-                    self.rewind_current_slide()
-            elif self.current_slide.is_last():
-                state = State.END
-            else:
+        if self.current_slide.is_loop():
+            if self.reverse:
                 state = State.WAIT
+
+            else:
+                state = State.PLAYING
+                self.rewind_current_slide()
+        elif self.current_slide.is_last():
+            state = State.END
         else:
-            # Play next video!
-            self.current_animation = self.next_animation
-            self.load_animation_cap(self.current_animation)
-            # Reset video to position zero if it has been played before
-            self.current_cap.set(cv2.CAP_PROP_POS_FRAMES, 0)
+            state = State.WAIT
 
         return self.lastframe, state
 
@@ -372,7 +308,6 @@ class Display(QThread):  # type: ignore
         exit_after_last_slide: bool = False,
         start_at_scene_number: Optional[int] = None,
         start_at_slide_number: Optional[int] = None,
-        start_at_animation_number: Optional[int] = None,
     ) -> None:
         super().__init__()
         self.presentations = presentations
@@ -388,9 +323,6 @@ class Display(QThread):  # type: ignore
         self.__current_presentation_index = 0
         self.current_presentation_index = start_at_scene_number  # type: ignore
         self.current_presentation.current_slide_index = start_at_slide_number  # type: ignore
-        self.current_presentation.set_current_animation_and_update_slide_number(
-            start_at_animation_number
-        )
 
         self.run_flag = True
 
@@ -535,9 +467,8 @@ class Display(QThread):  # type: ignore
         """Shows updated information about presentations."""
         self.change_info_signal.emit(
             {
-                "animation": self.current_presentation.current_animation,
                 "state": self.state,
-                "slide_index": self.current_presentation.current_slide.number,
+                "slide_index": self.current_presentation.current_slide_index + 1,
                 "n_slides": len(self.current_presentation.slides),
                 "type": self.current_presentation.current_slide.type,
                 "scene_index": self.current_presentation_index + 1,
@@ -612,13 +543,11 @@ class Info(QWidget):  # type: ignore
 
         self.setLayout(self.layout)
 
-        self.animationLabel = QLabel()
         self.stateLabel = QLabel()
         self.slideLabel = QLabel()
         self.typeLabel = QLabel()
         self.sceneLabel = QLabel()
 
-        self.layout.addWidget(self.animationLabel, 0, 0, 1, 2)
         self.layout.addWidget(self.stateLabel, 1, 0)
         self.layout.addWidget(self.slideLabel, 1, 1)
         self.layout.addWidget(self.typeLabel, 2, 0)
@@ -628,7 +557,6 @@ class Info(QWidget):  # type: ignore
 
     @Slot(dict)
     def update_info(self, info: Dict[str, Union[str, int]]) -> None:
-        self.animationLabel.setText("Animation: {}".format(info.get("animation", "na")))
         self.stateLabel.setText("State: {}".format(info.get("state", "unknown")))
         self.slideLabel.setText(
             "Slide: {}/{}".format(
@@ -889,8 +817,8 @@ def get_scenes_presentation_config(
 def start_at_callback(
     ctx: Context, param: Parameter, values: str
 ) -> Tuple[Optional[int], ...]:
-    if values == "(None, None, None)":
-        return (None, None, None)
+    if values == "(None, None)":
+        return (None, None)
 
     def str_to_int_or_none(value: str) -> Optional[int]:
         if value.lower().strip() == "":
@@ -907,11 +835,11 @@ def start_at_callback(
 
     values_tuple = values.split(",")
     n_values = len(values_tuple)
-    if n_values == 3:
+    if n_values == 2:
         return tuple(map(str_to_int_or_none, values_tuple))
 
     raise click.BadParameter(
-        f"exactly 3 arguments are expected but you gave {n_values}, please use commas to separate them",
+        f"exactly 2 arguments are expected but you gave {n_values}, please use commas to separate them",
         ctx=ctx,
         param=param,
     )
@@ -991,11 +919,11 @@ def start_at_callback(
     "--sa",
     "--start-at",
     "start_at",
-    metavar="<SCENE,SLIDE,ANIMATION>",
+    metavar="<SCENE,SLIDE>",
     type=str,
     callback=start_at_callback,
-    default=(None, None, None),
-    help="Start presenting at (x, y, z), equivalent to --sacn x --sasn y --saan z, and overrides values if not None.",
+    default=(None, None),
+    help="Start presenting at (x, y), equivalent to --sacn x --sasn y, and overrides values if not None.",
 )
 @click.option(
     "--sacn",
@@ -1015,15 +943,6 @@ def start_at_callback(
     default=None,
     help="Start presenting at a given slide number (0 is first, -1 is last).",
 )
-@click.option(
-    "--saan",
-    "--start-at-animation-number",
-    "start_at_animation_number",
-    metavar="INDEX",
-    type=int,
-    default=0,
-    help="Start presenting at a given animation number (0 is first, -1 is last). This conflicts with slide number since animation number is absolute to the presentation.",
-)
 @click.option(
     "--screen",
     "screen_number",
@@ -1051,7 +970,6 @@ def present(
     start_at: Tuple[Optional[int], Optional[int], Optional[int]],
     start_at_scene_number: Optional[int],
     start_at_slide_number: Optional[int],
-    start_at_animation_number: Optional[int],
     screen_number: Optional[int] = None,
 ) -> None:
     """
@@ -1115,9 +1033,6 @@ def present(
     if start_at[1]:
         start_at_slide_number = start_at[1]
 
-    if start_at[2]:
-        start_at_animation_number = start_at[2]
-
     if not QApplication.instance():
         app = QApplication(sys.argv)
     else:
@@ -1150,7 +1065,6 @@ def present(
         resize_mode=RESIZE_MODES[resize_mode],
         start_at_scene_number=start_at_scene_number,
         start_at_slide_number=start_at_slide_number,
-        start_at_animation_number=start_at_animation_number,
         screen=screen,
     )
 
diff --git a/manim_slides/slide.py b/manim_slides/slide.py
index c23508a..3b6f940 100644
--- a/manim_slides/slide.py
+++ b/manim_slides/slide.py
@@ -1,6 +1,4 @@
 import platform
-import shutil
-import subprocess
 from pathlib import Path
 from typing import (
     Any,
@@ -17,10 +15,9 @@ from warnings import warn
 import numpy as np
 from tqdm import tqdm
 
-from .config import PresentationConfig, SlideConfig, SlideType
+from .config import PresentationConfig, PreSlideConfig, SlideConfig, SlideType
 from .defaults import FOLDER_PATH
 from .manim import (
-    FFMPEG_BIN,
     LEFT,
     MANIMGL,
     AnimationGroup,
@@ -32,20 +29,7 @@ from .manim import (
     config,
     logger,
 )
-
-
-def reverse_video_file(src: Path, dst: Path) -> None:
-    """Reverses a video file, writting the result to `dst`."""
-    command = [str(FFMPEG_BIN), "-y", "-i", str(src), "-vf", "reverse", str(dst)]
-    logger.debug(" ".join(command))
-    process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-    output, error = process.communicate()
-
-    if output:
-        logger.debug(output.decode())
-
-    if error:
-        logger.debug(error.decode())
+from .utils import concatenate_video_files, merge_basenames, reverse_video_file
 
 
 class Slide(Scene):  # type:ignore
@@ -69,7 +53,7 @@ class Slide(Scene):  # type:ignore
         super().__init__(*args, **kwargs)
 
         self.__output_folder: Path = output_folder
-        self.__slides: List[SlideConfig] = []
+        self.__slides: List[PreSlideConfig] = []
         self.__current_slide = 1
         self.__current_animation = 0
         self.__loop_start_animation: Optional[int] = None
@@ -369,7 +353,7 @@ class Slide(Scene):  # type:ignore
             self.wait(self.wait_time_between_slides)
 
         self.__slides.append(
-            SlideConfig(
+            PreSlideConfig(
                 type=SlideType.slide,
                 start_animation=self.__pause_start_animation,
                 end_animation=self.__current_animation,
@@ -404,7 +388,7 @@ class Slide(Scene):  # type:ignore
             return
 
         self.__slides.append(
-            SlideConfig(
+            PreSlideConfig(
                 type=SlideType.last,
                 start_animation=self.__pause_start_animation,
                 end_animation=self.__current_animation,
@@ -458,7 +442,7 @@ class Slide(Scene):  # type:ignore
             self.__loop_start_animation is not None
         ), "You have to start a loop before ending it"
         self.__slides.append(
-            SlideConfig(
+            PreSlideConfig(
                 type=SlideType.loop,
                 start_animation=self.__loop_start_animation,
                 end_animation=self.__current_animation,
@@ -484,51 +468,57 @@ class Slide(Scene):  # type:ignore
 
         scene_files_folder.mkdir(parents=True, exist_ok=True)
 
-        files = []
-        for src_file in tqdm(
-            self.__partial_movie_files,
-            desc=f"Copying animation files to '{scene_files_folder}' and generating reversed animations",
-            leave=self.__leave_progress_bar,
-            ascii=True if platform.system() == "Windows" else None,
-            disable=not self.__show_progress_bar,
-        ):
-            if src_file is None and not MANIMGL:
-                # This happens if rendering with -na,b (manim only)
-                # where animations not in [a,b] will be skipped
-                # but animations before a will have a None src_file
-                continue
-
-            dst_file = scene_files_folder / src_file.name
-            rev_file = scene_files_folder / f"{src_file.stem}_reversed{src_file.suffix}"
-
-            # We only copy animation if it was not present
-            if not use_cache or not dst_file.exists():
-                shutil.copyfile(src_file, dst_file)
-
-            # We only reverse video if it was not present
-            if not use_cache or not rev_file.exists():
-                reverse_video_file(src_file, rev_file)
-
-            files.append(dst_file)
+        # When rendering with -na,b (manim only)
+        # the animations not in [a,b] will be skipped,
+        # but animation before a will have a None source file.
+        files: List[Path] = list(filter(None, self.__partial_movie_files))
 
+        # We must filter slides that end before the animation offset
         if offset := self.__start_at_animation_number:
             self.__slides = [
                 slide for slide in self.__slides if slide.end_animation > offset
             ]
-
             for slide in self.__slides:
-                slide.start_animation -= offset
+                slide.start_animation = max(0, slide.start_animation - offset)
                 slide.end_animation -= offset
 
+        slides: List[SlideConfig] = []
+
+        for pre_slide_config in tqdm(
+            self.__slides,
+            desc=f"Concatenating animation files to '{scene_files_folder}' and generating reversed animations",
+            leave=self.__leave_progress_bar,
+            ascii=True if platform.system() == "Windows" else None,
+            disable=not self.__show_progress_bar,
+        ):
+            slide_files = files[pre_slide_config.slides_slice]
+
+            file = merge_basenames(slide_files)
+            dst_file = scene_files_folder / file.name
+            rev_file = scene_files_folder / f"{file.stem}_reversed{file.suffix}"
+
+            # We only concat animations if it was not present
+            if not use_cache or not dst_file.exists():
+                concatenate_video_files(slide_files, dst_file)
+
+            # We only reverse video if it was not present
+            if not use_cache or not rev_file.exists():
+                reverse_video_file(dst_file, rev_file)
+
+            slides.append(
+                SlideConfig.from_pre_slide_config_and_files(
+                    pre_slide_config, dst_file, rev_file
+                )
+            )
+
         logger.info(
-            f"Copied {len(files)} animations to '{scene_files_folder.absolute()}' and generated reversed animations"
+            f"Generated {len(slides)} slides to '{scene_files_folder.absolute()}'"
         )
 
         slide_path = self.__output_folder / f"{scene_name}.json"
 
         PresentationConfig(
-            slides=self.__slides,
-            files=files,
+            slides=slides,
             resolution=self.__resolution,
             background_color=self.__background_color,
         ).to_file(slide_path)
diff --git a/manim_slides/utils.py b/manim_slides/utils.py
new file mode 100644
index 0000000..457e6cb
--- /dev/null
+++ b/manim_slides/utils.py
@@ -0,0 +1,80 @@
+import hashlib
+import subprocess
+import tempfile
+from pathlib import Path
+from typing import List
+
+from .manim import FFMPEG_BIN, logger
+
+
+def concatenate_video_files(files: List[Path], dest: Path) -> None:
+    """
+    Concatenate multiple video files into one.
+    """
+
+    f = tempfile.NamedTemporaryFile(mode="w", delete=False)
+    f.writelines(f"file '{path.absolute()}'\n" for path in files)
+    f.close()
+
+    command: List[str] = [
+        str(FFMPEG_BIN),
+        "-f",
+        "concat",
+        "-safe",
+        "0",
+        "-i",
+        f.name,
+        "-c",
+        "copy",
+        str(dest),
+        "-y",
+    ]
+    logger.debug(" ".join(command))
+    process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    output, error = process.communicate()
+
+    if output:
+        logger.debug(output.decode())
+
+    if error:
+        logger.debug(error.decode())
+
+    if not dest.exists():
+        raise ValueError(
+            "could not properly concatenate files, use `-v DEBUG` for more details"
+        )
+
+
+def merge_basenames(files: List[Path]) -> Path:
+    """
+    Merge multiple filenames by concatenating basenames.
+    """
+
+    dirname: Path = files[0].parent
+    ext = files[0].suffix
+
+    basenames = list(file.stem for file in files)
+
+    basenames_str = ",".join(f"{len(b)}:{b}" for b in basenames)
+
+    # We use hashes to prevent too-long filenames, see issue #123:
+    # https://github.com/jeertmans/manim-slides/issues/123
+    basename = hashlib.sha256(basenames_str.encode()).hexdigest()
+
+    logger.info(f"Generated a new basename for basenames: {basenames} -> '{basename}'")
+
+    return dirname.joinpath(basename + ext)
+
+
+def reverse_video_file(src: Path, dst: Path) -> None:
+    """Reverses a video file, writting the result to `dst`."""
+    command = [str(FFMPEG_BIN), "-y", "-i", str(src), "-vf", "reverse", str(dst)]
+    logger.debug(" ".join(command))
+    process = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    output, error = process.communicate()
+
+    if output:
+        logger.debug(output.decode())
+
+    if error:
+        logger.debug(error.decode())
diff --git a/tests/conftest.py b/tests/conftest.py
index 4772df9..e6bdb40 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,13 +1,62 @@
+import random
+import string
 from pathlib import Path
-from typing import Iterator
+from typing import Generator, Iterator, List
 
 import pytest
 
+from manim_slides.config import PresentationConfig
 from manim_slides.logger import make_logger
 
 _ = make_logger()  # This is run so that "PERF" level is created
 
 
 @pytest.fixture
-def folder_path() -> Iterator[Path]:
-    yield (Path(__file__).parent / "slides").resolve()
+def data_folder() -> Iterator[Path]:
+    path = (Path(__file__).parent / "data").resolve()
+    assert path.exists()
+    yield path
+
+
+@pytest.fixture
+def slides_folder(data_folder: Path) -> Iterator[Path]:
+    path = (data_folder / "slides").resolve()
+    assert path.exists()
+    yield path
+
+
+@pytest.fixture
+def slides_file(data_folder: Path) -> Iterator[Path]:
+    path = (data_folder / "slides.py").resolve()
+    assert path.exists()
+    yield path
+
+
+def random_path(
+    length: int = 20,
+    dirname: Path = Path("./media/videos/example"),
+    suffix: str = ".mp4",
+    touch: bool = False,
+) -> Path:
+    basename = "".join(random.choices(string.ascii_letters, k=length))
+
+    filepath = dirname.joinpath(basename + suffix)
+
+    if touch:
+        filepath.touch()
+
+    return filepath
+
+
+@pytest.fixture
+def paths() -> Generator[List[Path], None, None]:
+    random.seed(1234)
+
+    yield [random_path() for _ in range(20)]
+
+
+@pytest.fixture
+def presentation_config(
+    slides_folder: Path,
+) -> Generator[PresentationConfig, None, None]:
+    yield PresentationConfig.from_file(slides_folder / "BasicSlide.json")
diff --git a/tests/data/slides.py b/tests/data/slides.py
new file mode 100644
index 0000000..5503ae4
--- /dev/null
+++ b/tests/data/slides.py
@@ -0,0 +1,24 @@
+# flake8: noqa: F403, F405
+# type: ignore
+from manim import *
+
+from manim_slides import Slide
+
+
+class BasicSlide(Slide):
+    def construct(self):
+        circle = Circle(radius=3, color=BLUE)
+        dot = Dot()
+
+        self.play(GrowFromCenter(circle))
+        self.next_slide()
+
+        self.start_loop()
+        self.play(MoveAlongPath(dot, circle), run_time=2, rate_func=linear)
+        self.wait(2.0)
+        self.end_loop()
+
+        self.play(dot.animate.move_to(ORIGIN))
+        self.next_slide()
+
+        self.play(self.wipe(Group(dot, circle), []))
diff --git a/tests/data/slides/BasicSlide.json b/tests/data/slides/BasicSlide.json
new file mode 100644
index 0000000..cf8d7a2
--- /dev/null
+++ b/tests/data/slides/BasicSlide.json
@@ -0,0 +1,29 @@
+{
+  "slides": [
+    {
+      "type": "slide",
+      "file": "slides/files/BasicSlide/275756d906c706ca0125660866bb925b8927e2b2589d31a7a578079b70076ef4.mp4",
+      "rev_file": "slides/files/BasicSlide/275756d906c706ca0125660866bb925b8927e2b2589d31a7a578079b70076ef4_reversed.mp4"
+    },
+    {
+      "type": "loop",
+      "file": "slides/files/BasicSlide/05b94f634a049cd83daa9b47e483183d1ccdbc485687cee79c6ffbd4f02698da.mp4",
+      "rev_file": "slides/files/BasicSlide/05b94f634a049cd83daa9b47e483183d1ccdbc485687cee79c6ffbd4f02698da_reversed.mp4"
+    },
+    {
+      "type": "slide",
+      "file": "slides/files/BasicSlide/d09707faa0d68c55e98c628c5da51d66c92d0f79ac48647526817c377f843810.mp4",
+      "rev_file": "slides/files/BasicSlide/d09707faa0d68c55e98c628c5da51d66c92d0f79ac48647526817c377f843810_reversed.mp4"
+    },
+    {
+      "type": "last",
+      "file": "slides/files/BasicSlide/c10cc5deb3630a8259712288913b2bd6e79d093356d379d518cc929812194bd9.mp4",
+      "rev_file": "slides/files/BasicSlide/c10cc5deb3630a8259712288913b2bd6e79d093356d379d518cc929812194bd9_reversed.mp4"
+    }
+  ],
+  "resolution": [
+    854,
+    480
+  ],
+  "background_color": "black"
+}
diff --git a/tests/data/slides/files/BasicSlide/05b94f634a049cd83daa9b47e483183d1ccdbc485687cee79c6ffbd4f02698da.mp4 b/tests/data/slides/files/BasicSlide/05b94f634a049cd83daa9b47e483183d1ccdbc485687cee79c6ffbd4f02698da.mp4
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diff --git a/tests/slides/BasicExample.json b/tests/slides/BasicExample.json
deleted file mode 100644
index cdfbecb..0000000
--- a/tests/slides/BasicExample.json
+++ /dev/null
@@ -1,32 +0,0 @@
-{
-  "slides": [
-    {
-      "type": "slide",
-      "start_animation": 0,
-      "end_animation": 1,
-      "number": 1
-    },
-    {
-      "type": "loop",
-      "start_animation": 1,
-      "end_animation": 2,
-      "number": 2
-    },
-    {
-      "type": "last",
-      "start_animation": 2,
-      "end_animation": 3,
-      "number": 3
-    }
-  ],
-  "files": [
-    "slides/files/BasicExample/1413466013_3346521118_223132457.mp4",
-    "slides/files/BasicExample/1672018281_3136302242_2191168284.mp4",
-    "slides/files/BasicExample/1672018281_1369283980_3942561600.mp4"
-  ],
-  "resolution": [
-    1920,
-    1080
-  ],
-  "background_color": "black"
-}
diff --git a/tests/slides/files/BasicExample/1413466013_3346521118_223132457.mp4 b/tests/slides/files/BasicExample/1413466013_3346521118_223132457.mp4
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deleted file mode 100644
index a457ea5..0000000
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diff --git a/tests/test_config.py b/tests/test_config.py
index d2b8d09..f1e5846 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -1,80 +1,9 @@
-import random
-import string
-import tempfile
-from pathlib import Path
-from typing import Any, Generator, List
+from typing import Any
 
 import pytest
 from pydantic import ValidationError
 
-from manim_slides.config import (
-    Key,
-    PresentationConfig,
-    SlideConfig,
-    SlideType,
-    merge_basenames,
-)
-
-
-def random_path(
-    length: int = 20,
-    dirname: Path = Path("./media/videos/example"),
-    suffix: str = ".mp4",
-    touch: bool = False,
-) -> Path:
-    basename = "".join(random.choices(string.ascii_letters, k=length))
-
-    filepath = dirname.joinpath(basename + suffix)
-
-    if touch:
-        filepath.touch()
-
-    return filepath
-
-
-@pytest.fixture
-def paths() -> Generator[List[Path], None, None]:
-    random.seed(1234)
-
-    yield [random_path() for _ in range(20)]
-
-
-@pytest.fixture
-def presentation_config(paths: List[Path]) -> Generator[PresentationConfig, None, None]:
-    dirname = Path(tempfile.mkdtemp())
-    files = [random_path(dirname=dirname, touch=True) for _ in range(10)]
-
-    slides = [
-        SlideConfig(
-            type=SlideType.slide,
-            start_animation=0,
-            end_animation=5,
-            number=1,
-        ),
-        SlideConfig(
-            type=SlideType.loop,
-            start_animation=5,
-            end_animation=6,
-            number=2,
-        ),
-        SlideConfig(
-            type=SlideType.last,
-            start_animation=6,
-            end_animation=10,
-            number=3,
-        ),
-    ]
-
-    yield PresentationConfig(
-        slides=slides,
-        files=files,
-    )
-
-
-def test_merge_basenames(paths: List[Path]) -> None:
-    path = merge_basenames(paths)
-    assert path.suffix == paths[0].suffix
-    assert path.parent == paths[0].parent
+from manim_slides.config import Key, PresentationConfig
 
 
 class TestKey:
diff --git a/tests/test_main.py b/tests/test_main.py
index f52f009..196c1b0 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -16,29 +16,29 @@ def test_help() -> None:
     assert results.exit_code == 0
 
 
-def test_defaults_to_present(folder_path: Path) -> None:
+def test_defaults_to_present(slides_folder: Path) -> None:
     runner = CliRunner()
 
     with runner.isolated_filesystem():
         results = runner.invoke(
-            cli, ["BasicExample", "--folder", str(folder_path), "-s"]
+            cli, ["BasicSlide", "--folder", str(slides_folder), "-s"]
         )
 
         assert results.exit_code == 0
 
 
-def test_present(folder_path: Path) -> None:
+def test_present(slides_folder: Path) -> None:
     runner = CliRunner()
 
     with runner.isolated_filesystem():
         results = runner.invoke(
-            cli, ["present", "BasicExample", "--folder", str(folder_path), "-s"]
+            cli, ["present", "BasicSlide", "--folder", str(slides_folder), "-s"]
         )
 
         assert results.exit_code == 0
 
 
-def test_convert(folder_path: Path) -> None:
+def test_convert(slides_folder: Path) -> None:
     runner = CliRunner()
 
     with runner.isolated_filesystem():
@@ -46,10 +46,10 @@ def test_convert(folder_path: Path) -> None:
             cli,
             [
                 "convert",
-                "BasicExample",
+                "BasicSlide",
                 "basic_example.html",
                 "--folder",
-                str(folder_path),
+                str(slides_folder),
             ],
         )
 
@@ -71,7 +71,7 @@ def test_init() -> None:
         assert results.exit_code == 0
 
 
-def test_list_scenes(folder_path: Path) -> None:
+def test_list_scenes(slides_folder: Path) -> None:
     runner = CliRunner()
 
     with runner.isolated_filesystem():
@@ -80,12 +80,12 @@ def test_list_scenes(folder_path: Path) -> None:
             [
                 "list-scenes",
                 "--folder",
-                str(folder_path),
+                str(slides_folder),
             ],
         )
 
         assert results.exit_code == 0
-        assert "BasicExample" in results.output
+        assert "BasicSlide" in results.output
 
 
 def test_wizard() -> None:
diff --git a/tests/test_slide.py b/tests/test_slide.py
index 70c6cec..146ce25 100644
--- a/tests/test_slide.py
+++ b/tests/test_slide.py
@@ -1,7 +1,12 @@
+from pathlib import Path
+
 import pytest
+from click.testing import CliRunner
 from manim import Text
+from manim.__main__ import main as cli
 from pydantic import ValidationError
 
+from manim_slides.config import PresentationConfig
 from manim_slides.slide import Slide
 
 
@@ -14,6 +19,41 @@ def assert_construct(cls: type) -> type:
     return Wrapper
 
 
+def test_render_basic_examples(
+    slides_file: Path, presentation_config: PresentationConfig
+) -> None:
+    runner = CliRunner()
+
+    with runner.isolated_filesystem():
+        results = runner.invoke(cli, [str(slides_file), "BasicSlide", "-ql"])
+
+        assert results.exit_code == 0
+
+        local_slides_folder = Path("slides")
+
+        assert local_slides_folder.exists()
+
+        local_config_file = local_slides_folder / "BasicSlide.json"
+
+        assert local_config_file.exists()
+
+        local_presentation_config = PresentationConfig.from_file(local_config_file)
+
+        assert len(local_presentation_config.slides) == len(presentation_config.slides)
+
+        assert (
+            local_presentation_config.background_color
+            == presentation_config.background_color
+        )
+
+        assert (
+            local_presentation_config.background_color
+            == presentation_config.background_color
+        )
+
+        assert local_presentation_config.resolution == presentation_config.resolution
+
+
 class TestSlide:
     @assert_construct
     class TestLoop(Slide):
diff --git a/tests/test_utils.py b/tests/test_utils.py
new file mode 100644
index 0000000..c0bccff
--- /dev/null
+++ b/tests/test_utils.py
@@ -0,0 +1,23 @@
+from pathlib import Path
+from typing import List
+
+from manim_slides.utils import merge_basenames
+
+
+def test_merge_basenames(paths: List[Path]) -> None:
+    path = merge_basenames(paths)
+    assert path.suffix == paths[0].suffix
+    assert path.parent == paths[0].parent
+
+
+def test_merge_basenames_same_with_different_parent_directories(
+    paths: List[Path],
+) -> None:
+    d1 = Path("a/b/c")
+    d2 = Path("d/e/f")
+    p1 = d1 / "one.txt"
+    p2 = d1 / "a/b/c/two.txt"
+    p3 = d2 / "d/e/f/one.txt"
+    p4 = d2 / "d/e/f/two.txt"
+
+    assert merge_basenames([p1, p2]).name == merge_basenames([p3, p4]).name