Document use cases, image type support, plans

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Nils Dagsson Moskopp 2022-05-16 20:34:33 +02:00
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@ -6,3 +6,82 @@ Created by fleckenstein for MineClone2, then improved by erlehmann.
May be used as a Minetest mod.
See `examples.lua` for example code and usage hints.
## Use Cases for `tga_encoder`
### Encoding Textures for Editing
TGA images of types 1/2/3 consist of header data followed by a pixel array.
This makes it trivial to parse TGA files and even edit pixels in-place.
No checksums need to be updated on any kind of in-place texture editing.
**Tip**: When storing an editable image in item meta, use zlib compression.
### Legacy Minetest Texture Encoding
Minetest 5.4 did not include `minetest.encode_png()` (or any equvivalent).
Since `tga_encoder` is written in pure Lua, it does not need engine support.
**Tip:** Look at `examples.lua` and the Minetest mod `mcl_maps` for guidance.
### Advanced Texture Format Control
The function `minetest.encode_png()` always encodes images as 32bpp RGBA.
`tga_encoder` allows saving images as grayscale, 16bpp RGBA and 24bpp RGB.
For generating maps from terrain, color-mapped formats can be more useful.
### Encoding Very Small Textures
Images of size 8×8 or below are often smaller than an equivalent PNG file.
Note that on many filesystems files use at least 4096 bytes (i.e. 64×64).
Therefore, saving bytes on files up to a few 100 bytes is often useless.
### Encoding Reference Textures
TGA is a simple format, which makes it easy to create reference textures.
Using a hex editor, one can trivially see how all the pixels are stored.
## Supported Image Types
For all types, images are encoded in a fast single pass (i.e. append-only).
### Color-Mapped Images (Type 1)
These images contain a palette, followed by pixel data.
* `A1R5G5B5` (8bpp RGB)
* `B8G8R8` (8bpp RGB)
### True-Color Images (Type 2)
These images contain uncompressed RGB(A) pixel data.
* `A1R5G5B5` (16bpp RGBA)
* `B8G8R8` (24bpp RGB)
* `B8G8R8A8` (32bpp RGBA)
### Grayscale Images (Type 3)
* `Y8` (8bpp grayscale)
### Run-Length Encoded (RLE), True-Color Images (Type 10)
These images contain compressed RGB(A) pixel data.
* `A1R5G5B5` (16bpp RGBA)
* `B8G8R8` (24bpp RGB)
* `B8G8R8A8` (32bpp RGBA)
## TODO
* Support Type 1 `B8G8R8A8` output (color-mapped 8bpp RGBA)
* Actually support `R8G8B8A8` input for `A1R5G5B5` output
* Add both zoomable and explorable maps to `mcl_maps`.