Add a specification for the IMF format

docs/file-formats/IMF.md

@@ -1,10 +1,68 @@
 ---
 slug: /file-formats/imf
-title: IMF (Placeholder)
+title: IMF
 ---
 
-:::info
+This document describes the IMF file format used in the Ragnarok Online client.
 
-This section is a placeholder. If you know anything about the topic, please help fill it with content!
+## Contents
 
-:::
+IMF files store layers that determine the drawing order for the game's sprites, compiled as a single binary file:
+
+- These can be combined with [SPR](/file-formats/spr) and [ACT](/file-formats/act) to render 2D objects in the world or as part of the user interface
+- Sprites drawn using the IMF data are known to glitch, which can (for example) be observed in the character screen
+- They seem to be somewhat redundant and of limited usefulness, but more research is needed on the format
+
+Information on these files is relatively scarce, but interestingly enough they don't appear to be used in [Arcturus](/arcturus) at all.
+
+## Layout
+
+### Versions
+
+import Tabs from '@theme/Tabs';
+import TabItem from '@theme/TabItem';
+
+<Tabs>
+<TabItem value="1.01" label="Version 1.01">
+
+```cpp title="IMF File Format (v1.01)"
+struct AnimationFrameLayer {
+    int32_t Index;
+    int32_t OriginU;
+    int32_t OriginV;
+};
+
+struct SpriteDrawLayer {
+    int32_t AnimationFrameCount;
+    struct AnimationFrameLayer FrameLayers[AnimationFrameCount];
+};
+
+struct RagnarokIMF {
+    float Version;
+    int32_t Checksum;
+    int32_t DrawLayerCount;
+    struct SpriteDrawLayer DrawLayers[DrawLayerCount];
+};
+```
+
+</TabItem>
+</Tabs>
+
+### Fields
+
+#### Version
+
+Only one version is known to appear in modern builds of the RO client. It is approximated by the value `1.0099999904632569`.
+
+#### DrawLayers
+
+Presumably, layers are used similarly to the [z-index CSS property](https://developer.mozilla.org/en-US/docs/Web/CSS/z-index), but it's unclear how sprite drawing works exactly.
+
+## References
+
+Multiple open-source IMF decoders exist:
+
+- [GPL-licensed implementation in C++ (Dolori)](https://gitlab.com/Dolori/Dolori/-/blob/master/src/Files/ImfRes.cpp)
+- [MPL-licensed implementation in LuaJIT (RagLite SDK)](https://github.com/RagnarokResearchLab/RagLite/blob/main/Core/FileFormats/RagnarokIMF.lua)
+
+There don't appear to be any other implementations, which is likely due to the limited usefulness of the format.