oot/docs/assets/images.md

Images

Images in the rom are in N64 image formats. On extraction, they are converted to png and written to extracted/VERSION/. On build, they are converted back to N64 formats and written as C arrays to .inc.c files in build/VERSION/.

The build system will also pick up images in assets/, allowing modders to add or even replace images.

PNG files have suffixes indicating how they are to be converted. For example, a gDekuStickTex.i8.png file will be converted to i8.

The valid formats are rgba32, rgba16, i4, i8, ia4, ia8, ia16, ci4, ci4_rgba16, ci4_ia16, ci8, ci8_rgba16, ci8_ia16.

An optional suffix can be used to indicate the element type of the C array written out, for example .u32 in gHylianMan1ShirtTex.i8.u32.png. The valid array element types are u64 and u32. If omitted, the element type defaults to u64. u32 is only used for unaligned textures.

The tool implementing png->n64 conversion is build_from_png, using n64texconv as its backbone.

Replacing images

The contents of extracted/ are meant to hold the baserom assets and should not be modified. Instead, replacing an image can be done by creating a png file at the same relative path under assets/ as the image to replace.

For example, replacing gLinkHairTex (extracted/VERSION/assets/objects/gameplay_keep/gLinkHairTex.rgba16.png) with the texture for gHylianMan1BeardedEyeOpenTex (extracted/VERSION/assets/objects/object_ahg/gHylianMan1BeardedEyeOpenTex.ci8.tlut_gHylianMan1TLUT_u32.u32.png):

# for VERSION=gc-eu-mq-dbg

mkdir -p assets/objects/gameplay_keep/
cp \
    extracted/gc-eu-mq-dbg/assets/objects/object_ahg/gHylianMan1BeardedEyeOpenTex.ci8.tlut_gHylianMan1TLUT_u32.u32.png \
    assets/objects/gameplay_keep/gLinkHairTex.rgba16.png

# Cause make to rebuild gameplay_keep, where gLinkHairTex is
touch extracted/gc-eu-mq-dbg/assets/objects/gameplay_keep/gameplay_keep.c

make VERSION=gc-eu-mq-dbg

CI images

CI (Color Indexed) images also have a palette or TLUT (Texture Look-Up Table).

The CI formats are ci4, ci4_rgba16, ci4_ia16, ci8, ci8_rgba16, ci8_ia16. The first three indicate a CI4 image. The last three indicate a CI8 image. The _rgba16 and _ia16 suffixes indicate that the palette is to be in RGBA16 or IA16 format respectively. By default (if using plain ci4 or ci8) the palette is RGBA16.

PNG images to be converted to CI formats may have a .tlut_gNameTLUT[_<u32|u64>] suffix indicating the name and element type (optional, defaults to u64) of the TLUT gNameTLUT.tlut.rgba16[.<u32|u64>].inc.c file to write the palette to.

If this suffix is omitted, the TLUT will be written to a gNameTex.tlut.rgba16.inc.c file named after the CI image.

For example without the .tlut_ suffix, gGanonHairFringeTex.ci8.png:

• extracted to extracted/VERSION/assets/objects/object_ganon2/gGanonHairFringeTex.ci8.png
• texture written to build/VERSION/assets/objects/object_ganon2/gGanonHairFringeTex.ci8.inc.c
• palette written to build/VERSION/assets/objects/object_ganon2/gGanonHairFringeTex.tlut.rgba16.inc.c

For example with the .tlut_ suffix, gCowNoseTex.ci8.tlut_gCowTLUT.png:

• extracted to extracted/gc-eu-mq-dbg/assets/objects/object_cow/gCowNoseTex.ci8.tlut_gCowTLUT.png
• texture written to build/gc-eu-mq-dbg/assets/objects/object_cow/gCowNoseTex.ci8.tlut_gCowTLUT.inc.c
• palette written to build/gc-eu-mq-dbg/assets/objects/object_cow/gCowTLUT.tlut.rgba16.bin

CI images with a .tlut_ suffix have a shared palette: there are several CI images using the same palette. The build system (build_from_png) will find images sharing the same palette by looking at the .tlut_ suffixes of png files in the same folder and in the corresponding assets/ folder.

In the matching case of shared palettes, all png files have the same palette, which is written out. Otherwise the images are automatically co-quantized and the resulting images and palette are written out.

Automatic Palette Generation

n64texconv supports automatically generating palettes when converting a png without a stored palette to a CI formatted image. However note that this process makes a number of assumptions about alpha channel handling for RGBA16 palettes. If using a CI image with an alpha channel it is strongly recommended to pre-generate a palette externally to ensure the results are optimal for the particular use. n64texconv will convert the alpha channel to single-bit with a fixed threshold of 128 and set the rgb content of all transparent pixels to transparent black, prioritizing filling the generated palette with visible pixels. This comes at the cost of black texture samples bleeding into visible pixels when filtered. Especially for CI4, since there are only 16 channels it is inadvisable to rely on correct bilinear sampling adjacent to alpha pixels, as storing multiple transparent pixels with different rgb contents will starve the palette of visible colors.

Skybox textures

Skybox textures, located in (extracted/VERSION/)assets/textures/skyboxes, are ci8 images with the additional specificity of being limited to a palette of 128 colors that can be loaded as either the first or last 128 colors of a 256-colors palette, depending on the skybox.

The build system identifies such textures with the additional "sub-format" suffix .split_lo or .split_hi right after the .ci8 suffix. split_lo corresponds to textures where the half palette is mapped to the lower half, and split_hi to the higher half.

Like for regular ci images, images are automatically converted, quantized and written within those restrictions by the build system.