png: do alpha pre-multiplication manually

Something's up with libpng's alpha-mode... it's required to get proper
alpha (transparency) on some images, but at the same time, that breaks
other images (they are rendered way too dark).

Doing the pre-multiplication manually seems to handle all images
correctly.

2 files changed, 31 insertions, 3 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d487564..607d8d6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,10 @@
 ### Deprecated
 ### Removed
 ### Fixed
+
+* Alpha not being applied correctly to PNG images.
+
+
 ### Security
 ### Contributors
 
diff --git a/png.c b/png.c
index 71a4f3f..8f3896f 100644
--- a/png.c
+++ b/png.c
@@ -68,12 +68,10 @@ png_load(FILE *fp, const char *path)
     png_set_strip_16(png_ptr);  /* "pack" 16-bit colors to 8-bit */
     png_set_bgr(png_ptr);
 
-    /* pixman expects pre-multiplied alpha */
-    //png_set_alpha_mode(png_ptr, PNG_ALPHA_PREMULTIPLIED, 2.2);
-
     /* Tell libpng to expand to RGB(A) when necessary, and tell pixman
      * whether we have alpha or not */
     pixman_format_code_t format;
+
     switch (color_type) {
     case PNG_COLOR_TYPE_GRAY:
     case PNG_COLOR_TYPE_GRAY_ALPHA:
@@ -125,6 +123,32 @@ png_load(FILE *fp, const char *path)
 
     png_read_image(png_ptr, row_pointers);
 
+    /* pixman expects pre-multiplied alpha */
+    if (format == PIXMAN_x8r8g8b8) {
+        for (int i = 0; i < height; i++) {
+            uint32_t *p = (uint32_t *)row_pointers[i];
+            for (int j = 0; j < width; j++, p++) {
+                uint8_t a = (*p >> 24) & 0xff;
+                uint8_t r = (*p >> 16) & 0xff;
+                uint8_t g = (*p >> 8) & 0xff;
+                uint8_t b = (*p >> 0) & 0xff;
+
+                if (a == 0xff)
+                    continue;
+
+                if (a == 0) {
+                    r = g = b = 0;
+                } else {
+                    r = r * a / 0xff;
+                    g = g * a / 0xff;
+                    b = b * a / 0xff;
+                }
+
+                *p = (uint32_t)a << 24 | r << 16 | g << 8 | b;
+            }
+        }
+    }
+
     pix = pixman_image_create_bits_no_clear(
         format, width, height, (uint32_t *)image_data, stride);