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Establish png lazy transparency detection

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This commit is contained in:
jpirnay
2026-03-02 11:43:03 +01:00
parent b8109a2c5e
commit 34233b6ebb
1 changed files with 15 additions and 8 deletions
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src/activities/boot_sleep/SleepActivity.cpp
+15 -8
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@@ -34,8 +34,11 @@ struct PngOverlayCtx {
float yScale;
int lastDstY;
// Color-key transparency (tRNS chunk) for TRUECOLOR and GRAYSCALE images.
// -1 means no color key. For TRUECOLOR: 0x00RRGGBB; for GRAYSCALE: low byte only.
// Initialized lazily on the first draw callback because tRNS is processed during decode(),
// not during open() — so hasAlpha()/getTransparentColor() are only valid once decode() starts.
// -2 = not yet read; -1 = no color key; >=0 = 0x00RRGGBB (TRUECOLOR) or low-byte gray.
int32_t transparentColor;
PNG* pngObj; // for lazy-init of transparentColor on first callback
};
// PNGdec file I/O callbacks — mirror the pattern in PngToFramebufferConverter.cpp.
@@ -71,6 +74,15 @@ int32_t pngSleepSeek(PNGFILE* pFile, int32_t pos) {
int pngOverlayDraw(PNGDRAW* pDraw) {
PngOverlayCtx* ctx = reinterpret_cast<PngOverlayCtx*>(pDraw->pUser);
// Lazy-init: tRNS chunk is processed during decode() before any IDAT data, so by the time
// the first draw callback fires, hasAlpha() / getTransparentColor() are already valid.
if (ctx->transparentColor == -2) {
const int pt = pDraw->iPixelType;
ctx->transparentColor = (pDraw->iHasAlpha && (pt == PNG_PIXEL_TRUECOLOR || pt == PNG_PIXEL_GRAYSCALE))
? (int32_t)ctx->pngObj->getTransparentColor()
: -1;
}
const int destY = ctx->dstY + (int)(pDraw->y * ctx->yScale);
if (destY == ctx->lastDstY) return 1; // skip duplicate rows from Y scaling
ctx->lastDstY = destY;
@@ -520,13 +532,8 @@ void SleepActivity::renderOverlaySleepScreen() const {
ctx.dstY = (pageHeight - dstH) / 2;
ctx.yScale = yScale;
ctx.lastDstY = -1;
// Populate color-key transparency for TRUECOLOR/GRAYSCALE PNGs with a tRNS chunk.
// TRUECOLOR_ALPHA and GRAY_ALPHA carry per-pixel alpha, handled directly in the callback.
// INDEXED+tRNS stores per-palette alpha at pPalette[768+idx], also handled there.
const int pixType = png->getPixelType();
ctx.transparentColor = (png->hasAlpha() && (pixType == PNG_PIXEL_TRUECOLOR || pixType == PNG_PIXEL_GRAYSCALE))
? (int32_t)png->getTransparentColor()
: -1;
ctx.transparentColor = -2; // will be resolved on first draw callback (after tRNS is parsed)
ctx.pngObj = png;
rc = png->decode(&ctx, 0);
png->close();