Emit tRNS PNG chunk for images with alpha channel in palette. (#47)

* Emit tRNS PNG chunk for images with alpha channel in palette.

* Add test case for PNG tRNS round-tripping

Author: filipnavara

src/pngcodec.c

@@ -708,6 +708,7 @@ gdip_save_png_image_to_file_or_stream (FILE *fp, PutBytesDelegate putBytesFunc,
 
 	if (gdip_is_an_indexed_pixelformat (image->active_bitmap->pixel_format)) {
 		png_color palette[256];
+		png_byte trans_alpha[256];
 
 		int palette_entries = image->active_bitmap->palette->Count;
 		if (image->active_bitmap->pixel_format == PixelFormat4bppIndexed) {
@@ -716,13 +717,13 @@ gdip_save_png_image_to_file_or_stream (FILE *fp, PutBytesDelegate putBytesFunc,
 
 		for (i=0; i < palette_entries; i++) {
 			ARGB entry = image->active_bitmap->palette->Entries[i];
-
-			int dummy;
-
-			get_pixel_bgra(entry, palette[i].blue, palette[i].green, palette[i].red, dummy);
+			get_pixel_bgra(entry, palette[i].blue, palette[i].green, palette[i].red, trans_alpha[i]);
 		}
 
 		png_set_PLTE (png_ptr, info_ptr, palette, palette_entries);
+		if ((image->active_bitmap->palette->Flags & PaletteFlagsHasAlpha) == PaletteFlagsHasAlpha) {
+			png_set_tRNS (png_ptr, info_ptr, trans_alpha, palette_entries, NULL);
+		}
 	}
 
 	png_set_filter (png_ptr, 0, PNG_NO_FILTERS);

tests/.gitignore

@@ -7,4 +7,4 @@ testbits
 testclip
 testgdi
 testreversepath
-
+testpng

tests/Makefile.am

@@ -17,7 +17,7 @@ LDADDS =					\
 	-lm
 
 noinst_PROGRAMS =			\
-	testbits testclip testreversepath
+	testbits testclip testreversepath testpng
 
 if HAS_X11
 noinst_PROGRAMS =
@@ -48,14 +48,22 @@ testreversepath_SOURCES =	\
 testreversepath_DEPENDENCIES = $(TEST_DEPS)
 testreversepath_LDADD = $(LDADDS)
 
+testpng_DEPENDENCIES = $(TEST_DEPS)
+testpng_LDADD = $(LDADDS)
+
+testpng_SOURCES =		\
+	testpng.c
+
 EXTRA_DIST =			\
 	$(testgdi_SOURCES)	\
 	$(testbits_SOURCES)	\
 	$(testclip_SOURCES)	\
+	$(testpng_SOURCES)	\
 	$(testreversepath_SOURCES)
 
 TESTS = \
 	testbits \
 	testclip \
 	testreversepath \
+	testpng \
 	$(NULL)

tests/test-trns.png

@@ -0,0 +1,71 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+
+#include "GdiPlusFlat.h"
+
+static int status_counter = 0;
+
+#define CHECK_STATUS(x) do { if (status != Ok) { printf ("status[%d] == %d!\n", status_counter++, status); if(x) { exit(-1); } } else { printf ("status[%d] == Ok\n", status_counter++); } } while (0)
+#define CHECK_ASSERT(x) do { if (!(x)) { printf ("check %s at %s:%d failed\n", #x, __FILE__, __LINE__); exit(-1); } else { printf("check %s at %s:%d passed\n", #x, __FILE__, __LINE__); }  } while (0)
+ 
+CLSID png_clsid = { 0x557cf406, 0x1a04, 0x11d3, { 0x9a, 0x73, 0x0, 0x0, 0xf8, 0x1e, 0xf3, 0x2e } };
+
+int
+main (int argc, char **argv)
+{
+    GpImage *img;
+    gunichar2 *unis;
+    GpBitmap *bitmap;
+    GpStatus status;
+    int original_palette_size;
+    int reloaded_palette_size;
+    ColorPalette *original_palette;
+    ColorPalette *reloaded_palette;
+    GdiplusStartupInput gdiplusStartupInput;
+    ULONG_PTR gdiplusToken;
+
+    GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);
+
+    // PNG resave should preserve the palette transparency. Let's test it
+    // by loading a PNG file and its palette, then resaving it and loading
+    // it again for comparison.
+    unis = g_utf8_to_utf16 ("test-trns.png", -1, NULL, NULL, NULL);
+    status = GdipLoadImageFromFile (unis, &img);
+    CHECK_STATUS(1);
+    g_free (unis);
+
+    status = GdipGetImagePaletteSize (img, &original_palette_size);
+    CHECK_STATUS(1);
+    CHECK_ASSERT(original_palette_size > 0);
+    original_palette = malloc (original_palette_size);
+    GdipGetImagePalette (img, original_palette, original_palette_size);
+    CHECK_STATUS(1);
+
+    unis = g_utf8_to_utf16 ("test-trns-resave.png", -1, NULL, NULL, NULL);
+    status = GdipSaveImageToFile (img, unis, &png_clsid, NULL);
+    CHECK_STATUS(1);
+    GdipDisposeImage (img);
+    status = GdipLoadImageFromFile (unis, &img);
+    CHECK_STATUS(1);
+    g_free (unis);
+
+    status = GdipGetImagePaletteSize (img, &reloaded_palette_size);
+    CHECK_STATUS(1);
+    CHECK_ASSERT(reloaded_palette_size > 0);
+    CHECK_ASSERT(reloaded_palette_size == original_palette_size);
+    reloaded_palette = malloc (reloaded_palette_size);
+    GdipGetImagePalette (img, reloaded_palette, reloaded_palette_size);
+    CHECK_STATUS(1);
+
+    CHECK_ASSERT(memcmp (original_palette, reloaded_palette, original_palette_size) == 0);
+
+    GdipDisposeImage (img);
+    img = NULL;
+    unlink ("test-trns-resave.png");
+    free (original_palette);
+    free (reloaded_palette);
+
+    return 0;
+}