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refactor: name ADT vertex constants, add BLP decompression comments

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- adt_loader: replace magic 145 with kMCVTVertexCount and 17 with
  kMCVTRowStride — MCVT height grid is 9 outer + 8 inner vertices
  per row across 9 rows
- adt_loader: replace 999999.0f sentinels with numeric_limits
- blp_loader: add why-comments on RGB565→RGB888 bit layout
  (R=bits[15:11], G=[10:5], B=[4:0])
- blp_loader: explain DXT3 4-bit alpha scaling (n * 255 / 15)
- blp_loader: explain palette 4-bit alpha multiply-by-17 trick
  (equivalent to n * 255 / 15, exact for all 16 values)
This commit is contained in:
Kelsi 2026-03-30 14:28:22 -07:00
parent 683e171fd1
commit 1151785381
2 changed files with 21 additions and 18 deletions
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8
src/pipeline/blp_loader.cpp
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@ -209,7 +209,8 @@ void BLPLoader::decompressDXT1(const uint8_t* src, uint8_t* dst, int width, int
uint16_t c0 = block[0] | (block[1] << 8);
uint16_t c1 = block[2] | (block[3] << 8);
// Convert RGB565 to RGB888
// Convert RGB565 to RGB888: extract 5/6/5-bit channels and scale to [0..255].
// R = bits[15:11] (5-bit, /31), G = bits[10:5] (6-bit, /63), B = bits[4:0] (5-bit, /31)
uint8_t r0 = ((c0 >> 11) & 0x1F) * 255 / 31;
uint8_t g0 = ((c0 >> 5) & 0x3F) * 255 / 63;
uint8_t b0 = (c0 & 0x1F) * 255 / 31;
@ -303,7 +304,7 @@ void BLPLoader::decompressDXT3(const uint8_t* src, uint8_t* dst, int width, int
case 3: pixel[0] = (r0 + 2*r1) / 3; pixel[1] = (g0 + 2*g1) / 3; pixel[2] = (b0 + 2*b1) / 3; break;
}
// Apply 4-bit alpha
// Apply 4-bit alpha: scale [0..15] → [0..255] via (n * 255 / 15)
int alphaIndex = py * 4 + px;
uint8_t alpha4 = (alphaBlock >> (alphaIndex * 4)) & 0xF;
pixel[3] = alpha4 * 255 / 15;
@ -416,7 +417,8 @@ void BLPLoader::decompressPalette(const uint8_t* src, uint8_t* dst, const uint32
if (alphaDepth == 8) {
dst[i * 4 + 3] = alphaData[i];
} else if (alphaDepth == 4) {
// 4-bit alpha: 2 pixels per byte
// 4-bit alpha: 2 pixels packed per byte (low nibble first).
// Multiply by 17 to scale [0..15] → [0..255] (equivalent to n * 255 / 15).
uint8_t alphaByte = alphaData[i / 2];
dst[i * 4 + 3] = (i % 2 == 0) ? ((alphaByte & 0x0F) * 17) : ((alphaByte >> 4) * 17);
} else if (alphaDepth == 1) {