Commit 16f25d2e for libheif
commit 16f25d2e13fb7445509a18a081903208272ceb15
Author: Dirk Farin <dirk.farin@gmail.com>
Date: Mon Oct 26 08:55:36 2026 +0100
PNG loader: convert tRNS transparency to alpha (#1947)
RGB and grayscale PNGs can mark one color as transparent with a tRNS
chunk instead of storing an alpha channel. The loader did not expand
this, so heif-enc dropped the transparency. Palette images were not
affected because libpng's palette expansion already applies tRNS.
Call png_set_tRNS_to_alpha() whenever a tRNS chunk is present.
diff --git a/heifio/decoder_png.cc b/heifio/decoder_png.cc
index 7285c178..bf0e5b37 100644
--- a/heifio/decoder_png.cc
+++ b/heifio/decoder_png.cc
@@ -196,6 +196,13 @@ heif_error loadPNG(const char* filename, int output_bit_depth, InputImage *input
bit_depth = 8;
}
+ /* Expand a tRNS chunk (single-color transparency key for RGB and grayscale images,
+ * or per-entry alpha for palette images) into a full alpha channel.
+ */
+ if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
+ png_set_tRNS_to_alpha(png_ptr);
+ }
+
/* Set the background color to draw transparent and alpha images over.
* It is possible to set the red, green, and blue components directly
* for paletted images instead of supplying a palette index. Note that
diff --git a/tests/data/heifio/gray16_trns.png b/tests/data/heifio/gray16_trns.png
new file mode 100644
index 00000000..006f667c
Binary files /dev/null and b/tests/data/heifio/gray16_trns.png differ
diff --git a/tests/data/heifio/gray8_trns.png b/tests/data/heifio/gray8_trns.png
new file mode 100644
index 00000000..58eaf898
Binary files /dev/null and b/tests/data/heifio/gray8_trns.png differ
diff --git a/tests/data/heifio/palette_trns.png b/tests/data/heifio/palette_trns.png
new file mode 100644
index 00000000..742bb7df
Binary files /dev/null and b/tests/data/heifio/palette_trns.png differ
diff --git a/tests/data/heifio/rgb16_trns.png b/tests/data/heifio/rgb16_trns.png
new file mode 100644
index 00000000..308ccfca
Binary files /dev/null and b/tests/data/heifio/rgb16_trns.png differ
diff --git a/tests/data/heifio/rgb8_trns.png b/tests/data/heifio/rgb8_trns.png
new file mode 100644
index 00000000..471f0d83
Binary files /dev/null and b/tests/data/heifio/rgb8_trns.png differ
diff --git a/tests/pngdecode.cc b/tests/pngdecode.cc
index 269b35cb..8d95c6a3 100644
--- a/tests/pngdecode.cc
+++ b/tests/pngdecode.cc
@@ -70,3 +70,90 @@ TEST_CASE("Grayscale+alpha 16 bit") {
REQUIRE(*(const uint16_t*) (pa + y * a_stride + x * 2) == 0x2000 * x + 0x100 * y);
}
}
+
+
+// PNG files without an alpha channel can still carry transparency in a tRNS chunk:
+// a single transparent color for RGB and grayscale images, or per-entry alpha for
+// palette images. The PNG loader has to turn this into an alpha channel (#1947).
+// In the *_trns.png images, pixel (x,y) is transparent when (x+y)%3==0.
+
+static bool is_transparent(int x, int y)
+{
+ return (x + y) % 3 == 0;
+}
+
+TEST_CASE("RGB 8 bit with tRNS color key") {
+ InputImage input_image = load("rgb8_trns.png", 8);
+ const heif_image* image = input_image.image.get();
+ REQUIRE(heif_image_get_chroma_format(image) == heif_chroma_interleaved_RGBA);
+
+ size_t stride;
+ const uint8_t* p = heif_image_get_plane_readonly2(image, heif_channel_interleaved, &stride);
+ for (int y = 0; y < height; y++)
+ for (int x = 0; x < width; x++) {
+ INFO("x=" << x << " y=" << y);
+ REQUIRE(p[y * stride + x * 4 + 3] == (is_transparent(x, y) ? 0 : 255));
+ }
+}
+
+TEST_CASE("RGB 16 bit with tRNS color key") {
+ InputImage input_image = load("rgb16_trns.png", 16);
+ const heif_image* image = input_image.image.get();
+ REQUIRE(heif_image_get_chroma_format(image) == heif_chroma_interleaved_RRGGBBAA_LE);
+
+ size_t stride;
+ const uint8_t* p = heif_image_get_plane_readonly2(image, heif_channel_interleaved, &stride);
+ for (int y = 0; y < height; y++)
+ for (int x = 0; x < width; x++) {
+ INFO("x=" << x << " y=" << y);
+ const uint8_t* a = p + y * stride + x * 8 + 6;
+ REQUIRE((a[0] | (a[1] << 8)) == (is_transparent(x, y) ? 0 : 0xFFFF));
+ }
+}
+
+TEST_CASE("Grayscale 8 bit with tRNS color key") {
+ InputImage input_image = load("gray8_trns.png", 8);
+ const heif_image* image = input_image.image.get();
+ REQUIRE(heif_image_get_chroma_format(image) == heif_chroma_monochrome);
+ REQUIRE(heif_image_has_channel(image, heif_channel_Alpha));
+
+ size_t stride;
+ const uint8_t* p = heif_image_get_plane_readonly2(image, heif_channel_Alpha, &stride);
+ for (int y = 0; y < height; y++)
+ for (int x = 0; x < width; x++) {
+ INFO("x=" << x << " y=" << y);
+ REQUIRE(p[y * stride + x] == (is_transparent(x, y) ? 0 : 255));
+ }
+}
+
+TEST_CASE("Grayscale 16 bit with tRNS color key") {
+ InputImage input_image = load("gray16_trns.png", 16);
+ const heif_image* image = input_image.image.get();
+ REQUIRE(heif_image_get_chroma_format(image) == heif_chroma_monochrome);
+ REQUIRE(heif_image_has_channel(image, heif_channel_Alpha));
+
+ size_t stride;
+ const uint8_t* p = heif_image_get_plane_readonly2(image, heif_channel_Alpha, &stride);
+ for (int y = 0; y < height; y++)
+ for (int x = 0; x < width; x++) {
+ INFO("x=" << x << " y=" << y);
+ const uint16_t a = *(const uint16_t*) (p + y * stride + x * 2);
+ REQUIRE(a == (is_transparent(x, y) ? 0 : 0xFFFF));
+ }
+}
+
+TEST_CASE("Palette with tRNS alpha entries") {
+ InputImage input_image = load("palette_trns.png", 8);
+ const heif_image* image = input_image.image.get();
+ REQUIRE(heif_image_get_chroma_format(image) == heif_chroma_interleaved_RGBA);
+
+ // palette index 0: alpha 0, index 1: alpha 128, index 2: no tRNS entry (opaque)
+ size_t stride;
+ const uint8_t* p = heif_image_get_plane_readonly2(image, heif_channel_interleaved, &stride);
+ for (int y = 0; y < height; y++)
+ for (int x = 0; x < width; x++) {
+ INFO("x=" << x << " y=" << y);
+ int expected = is_transparent(x, y) ? 0 : (x % 2 == 0 ? 128 : 255);
+ REQUIRE(p[y * stride + x * 4 + 3] == expected);
+ }
+}