Panda3DS/src/core/renderer_gl/etc1.cpp

#include <algorithm>
#include "colour.hpp"
#include "renderer_gl/renderer_gl.hpp"
#include "renderer_gl/textures.hpp"

static constexpr u32 signExtend3To32(u32 val) {
    return (u32)(s32(val) << 29 >> 29);
}

u32 Texture::getTexelETC(bool hasAlpha, u32 u, u32 v, u32 width, const void* data) {
    // Pixel offset of the 8x8 tile based on u, v and the width of the texture
    u32 offs = ((u & ~7) * 8) + ((v & ~7) * width);
    if (!hasAlpha)
        offs >>= 1;

    // In-tile offsets for u/v
    u &= 7;
    v &= 7;

    // ETC1(A4) also subdivide the 8x8 tile to 4 4x4 tiles
    // Each tile is 8 bytes for ETC1, but since ETC1A4 has 4 alpha bits per pixel, that becomes 16 bytes
    const u32 subTileSize = hasAlpha ? 16 : 8;
    const u32 subTileIndex = (u / 4) + 2 * (v / 4); // Which of the 4 subtiles is this texel in?

    // In-subtile offsets for u/v
    u &= 3;
    v &= 3;
    offs += subTileSize * subTileIndex;

    u32 alpha;
    const u8* tmp = static_cast<const u8*>(data) + offs; // Pointer to colour and alpha data as u8*
    const u64* ptr = reinterpret_cast<const u64*>(tmp); // Cast to u64*

    if (hasAlpha) {
        // First 64 bits of the 4x4 subtile are alpha data
        const u64 alphaData = *ptr++;
        alpha = Colour::convert4To8Bit((alphaData >> (4 * (u * 4 + v))) & 0xf);
    }
    else {
        alpha = 0xff; // ETC1 without alpha uses ff for every pixel
    }

    // Next 64 bits of the subtile are colour data
    u64 colourData = *ptr;
    return decodeETC(alpha, u, v, colourData);
}

u32 Texture::decodeETC(u32 alpha, u32 u, u32 v, u64 colourData) {
    static constexpr u32 modifiers[8][2] = {
        { 2, 8 },
        { 5, 17 },
        { 9, 29 },
        { 13, 42 },
        { 18, 60 },
        { 24, 80 },
        { 33, 106 },
        { 47, 183 },
    };

    // Parse colour data for 4x4 block
    const u32 subindices = colourData & 0xffff;
    const u32 negationFlags = (colourData >> 16) & 0xffff;
    const bool flip = (colourData >> 32) & 1;
    const bool diffMode = (colourData >> 33) & 1;

    // Note: index1 is indeed stored on the higher bits, with index2 in the lower bits
    const u32 tableIndex1 = (colourData >> 37) & 7;
    const u32 tableIndex2 = (colourData >> 34) & 7;
    const u32 texelIndex = u * 4 + v; // Index of the texel in the block

    if (flip)
        std::swap(u, v);

    s32 r, g, b;
    if (diffMode) {
        r = (colourData >> 59) & 0x1f;
        g = (colourData >> 51) & 0x1f;
        b = (colourData >> 43) & 0x1f;

        if (u >= 2) {
            r += signExtend3To32((colourData >> 56) & 0x7);
            g += signExtend3To32((colourData >> 48) & 0x7);
            b += signExtend3To32((colourData >> 40) & 0x7);
        }

        // Expand from 5 to 8 bits per channel
        r = Colour::convert5To8Bit(r);
        g = Colour::convert5To8Bit(g);
        b = Colour::convert5To8Bit(b);
    } else {
        if (u < 2) {
            r = (colourData >> 60) & 0xf;
            g = (colourData >> 52) & 0xf;
            b = (colourData >> 44) & 0xf;
        } else {
            r = (colourData >> 56) & 0xf;
            g = (colourData >> 48) & 0xf;
            b = (colourData >> 40) & 0xf;
        }

        // Expand from 4 to 8 bits per channel
        r = Colour::convert4To8Bit(r);
        g = Colour::convert4To8Bit(g);
        b = Colour::convert4To8Bit(b);
    }

    const u32 index = (u < 2) ? tableIndex1 : tableIndex2;
    s32 modifier = modifiers[index][(subindices >> texelIndex) & 1];

    if (((negationFlags >> texelIndex) & 1) != 0) {
        modifier = -modifier;
    }

    r = std::clamp(r + modifier, 0, 255);
    g = std::clamp(g + modifier, 0, 255);
    b = std::clamp(b + modifier, 0, 255);

    return (alpha << 24) | (u32(b) << 16) | (u32(g) << 8) | u32(r);
}