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Honor PNG tRNS color-key transparency. #154

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Merged
bkaradzic merged 1 commit into from
Sep 29, 2026
Merged

Honor PNG tRNS color-key transparency. #154

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180 changes: 163 additions & 17 deletions src/image_decode.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -161,6 +161,20 @@ BX_PRAGMA_DIAGNOSTIC_POP();

namespace bimg
{
#if BIMG_CONFIG_PARSE_PNG
static uint32_t pngReadPackedSample(const uint8_t* _data, uint32_t _index, uint32_t _bitDepth)
{
const uint64_t bitOffset = uint64_t(_index)*_bitDepth;
const uint32_t shift = 8 - _bitDepth - uint32_t(bitOffset&7);
return (_data[bitOffset>>3]>>shift) & ( (1u<<_bitDepth)-1);
}

static uint8_t pngExpandGraySample(uint32_t _sample, uint32_t _bitDepth)
{
return uint8_t(_sample*255 / ( (1u<<_bitDepth)-1) );
}
#endif // BIMG_CONFIG_PARSE_PNG

static ImageContainer* imageParseLodePng(bx::AllocatorI* _allocator, const void* _data, uint32_t _size, bx::Error* _err)
{
BX_ERROR_SCOPE(_err);
Expand Down Expand Up @@ -191,24 +205,37 @@ namespace bimg
}
else
{
bool palette = false;
bool supported = false;
const bool colorKey = 0 != state.info_raw.key_defined;
bool palette = false;
bool supported = false;

switch (state.info_raw.bitdepth)
{
case 1:
case 2:
case 4:
palette = LCT_PALETTE == state.info_raw.colortype;
format = palette ? bimg::TextureFormat::RGBA8 : bimg::TextureFormat::R8;
supported = true;

if (palette)
{
format = bimg::TextureFormat::RGBA8;
}
else if (colorKey)
{
format = bimg::TextureFormat::RG8;
}
else
{
format = bimg::TextureFormat::R8;
}
break;

case 8:
switch (state.info_raw.colortype)
{
case LCT_GREY:
format = bimg::TextureFormat::R8;
format = colorKey ? bimg::TextureFormat::RG8 : bimg::TextureFormat::R8;
supported = true;
break;

Expand All @@ -218,7 +245,7 @@ namespace bimg
break;

case LCT_RGB:
format = bimg::TextureFormat::RGB8;
format = colorKey ? bimg::TextureFormat::RGBA8 : bimg::TextureFormat::RGB8;
supported = true;
break;

Expand Down Expand Up @@ -247,7 +274,7 @@ namespace bimg
uint16_t* rgba = (uint16_t*)data + ii;
rgba[0] = bx::toHostEndian(rgba[0], false);
}
format = bimg::TextureFormat::R16;
format = colorKey ? bimg::TextureFormat::RG16 : bimg::TextureFormat::R16;
supported = true;
break;

Expand Down Expand Up @@ -304,7 +331,9 @@ namespace bimg
const uint8_t* copyData = data;

TextureFormat::Enum dstFormat = format;
if (palette) {
if (palette
|| colorKey)
{
copyData = NULL;
}
else if (1 == state.info_raw.bitdepth
Expand Down Expand Up @@ -382,6 +411,104 @@ namespace bimg
}
}
}
else if (colorKey)
{
const uint32_t bitDepth = state.info_raw.bitdepth;
const uint32_t keyR = state.info_raw.key_r;
const uint32_t keyG = state.info_raw.key_g;
const uint32_t keyB = state.info_raw.key_b;

bool hasAlpha = false;

if (LCT_GREY == state.info_raw.colortype)
{
if (8 > bitDepth)
{
for (uint32_t ii = 0, num = width*height; ii < num; ++ii)
{
const uint32_t sample = pngReadPackedSample(data, ii, bitDepth);
const bool transparent = keyR == sample;

uint8_t* dst = (uint8_t*)output->m_data + ii*2;
dst[0] = pngExpandGraySample(sample, bitDepth);
dst[1] = transparent ? 0 : UINT8_MAX;

hasAlpha |= transparent;
}
}
else if (8 == bitDepth)
{
for (uint32_t ii = 0, num = width*height; ii < num; ++ii)
{
const uint8_t* src = (uint8_t*)data + ii;
uint8_t* dst = (uint8_t*)output->m_data + ii*2;
const bool transparent = keyR == src[0];

dst[0] = src[0];
dst[1] = transparent ? 0 : UINT8_MAX;

hasAlpha |= transparent;
}
}
else
{
for (uint32_t ii = 0, num = width*height; ii < num; ++ii)
{
const uint16_t* src = (uint16_t*)data + ii;
uint16_t* dst = (uint16_t*)output->m_data + ii*2;
const bool transparent = keyR == src[0];

dst[0] = src[0];
dst[1] = transparent ? 0 : UINT16_MAX;

hasAlpha |= transparent;
}
}
}
else
{
if (8 == bitDepth)
{
for (uint32_t ii = 0, num = width*height; ii < num; ++ii)
{
const uint8_t* src = (uint8_t*)data + ii*3;
uint8_t* dst = (uint8_t*)output->m_data + ii*4;
const bool transparent = keyR == src[0]
&& keyG == src[1]
&& keyB == src[2]
;

dst[0] = src[0];
dst[1] = src[1];
dst[2] = src[2];
dst[3] = transparent ? 0 : UINT8_MAX;

hasAlpha |= transparent;
}
}
else
{
for (uint32_t ii = 0, num = width*height; ii < num; ++ii)
{
const uint16_t* src = (uint16_t*)data + ii*3;
uint16_t* dst = (uint16_t*)output->m_data + ii*4;
const bool transparent = keyR == src[0]
&& keyG == src[1]
&& keyB == src[2]
;

dst[0] = src[0];
dst[1] = src[1];
dst[2] = src[2];
dst[3] = transparent ? 0 : UINT16_MAX;

hasAlpha |= transparent;
}
}
}

output->m_hasAlpha = hasAlpha;
}
else if (1 == state.info_raw.bitdepth)
{
for (uint32_t ii = 0, num = width*height/8; ii < num; ++ii)
Expand Down Expand Up @@ -1453,6 +1580,13 @@ namespace bimg
return false;
}

const uint8_t* begin = (const uint8_t*)_data + 8;
const uint8_t* end = (const uint8_t*)_data + _size;
const bool colorKey = true
&& (LCT_GREY == colortype || LCT_RGB == colortype)
&& NULL != lodepng_chunk_find_const(begin, end, "tRNS")
;

// Mirror the final container format selection in imageParseLodePng.
bimg::TextureFormat::Enum format = bimg::TextureFormat::RGBA8;
bool supported = false;
Expand All @@ -1462,29 +1596,41 @@ namespace bimg
case 1:
case 2:
case 4:
format = (LCT_PALETTE == colortype) ? bimg::TextureFormat::RGBA8 : bimg::TextureFormat::R8;
supported = true;

if (LCT_PALETTE == colortype)
{
format = bimg::TextureFormat::RGBA8;
}
else if (colorKey)
{
format = bimg::TextureFormat::RG8;
}
else
{
format = bimg::TextureFormat::R8;
}
break;

case 8:
switch (colortype)
{
case LCT_GREY: format = bimg::TextureFormat::R8; supported = true; break;
case LCT_GREY_ALPHA: format = bimg::TextureFormat::RG8; supported = true; break;
case LCT_RGB: format = bimg::TextureFormat::RGB8; supported = true; break;
case LCT_RGBA: format = bimg::TextureFormat::RGBA8; supported = true; break;
case LCT_PALETTE: format = bimg::TextureFormat::RGBA8; supported = true; break;
case LCT_GREY: format = colorKey ? bimg::TextureFormat::RG8 : bimg::TextureFormat::R8; supported = true; break;
case LCT_GREY_ALPHA: format = bimg::TextureFormat::RG8; supported = true; break;
case LCT_RGB: format = colorKey ? bimg::TextureFormat::RGBA8 : bimg::TextureFormat::RGB8; supported = true; break;
case LCT_RGBA: format = bimg::TextureFormat::RGBA8; supported = true; break;
case LCT_PALETTE: format = bimg::TextureFormat::RGBA8; supported = true; break;
default: break;
}
break;

case 16:
switch (colortype)
{
case LCT_GREY: format = bimg::TextureFormat::R16; supported = true; break;
case LCT_GREY_ALPHA: format = bimg::TextureFormat::RG16; supported = true; break;
case LCT_RGB: format = bimg::TextureFormat::RGBA16; supported = true; break;
case LCT_RGBA: format = bimg::TextureFormat::RGBA16; supported = true; break;
case LCT_GREY: format = colorKey ? bimg::TextureFormat::RG16 : bimg::TextureFormat::R16; supported = true; break;
case LCT_GREY_ALPHA: format = bimg::TextureFormat::RG16; supported = true; break;
case LCT_RGB: format = bimg::TextureFormat::RGBA16; supported = true; break;
case LCT_RGBA: format = bimg::TextureFormat::RGBA16; supported = true; break;
default: break;
}
break;
Expand Down
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