[vlc-devel] [PATCH] Added fallback support for set_alpha_mode for libpng (fallback required for libpng < 1.5.4)

[Diogo Silva]

It was required to add fallback support, png_set_alpha_mode was only added
in version 1.5.4.
The fallback was implemented by converting the pixels with any alpha by a
background color, black. The color is the same used for representing 
alpha when it exists in VLC.
---
 modules/codec/png.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/modules/codec/png.c b/modules/codec/png.c
index 75f3ad440b..c4b6898cfa 100644
--- a/modules/codec/png.c
+++ b/modules/codec/png.c
@@ -261,8 +261,6 @@ static int DecodeBlock( decoder_t *p_dec, block_t *p_block )
     if( i_color_type == PNG_COLOR_TYPE_GRAY ||
         i_color_type == PNG_COLOR_TYPE_GRAY_ALPHA )
           png_set_gray_to_rgb( p_png );
-    if( i_color_type & PNG_COLOR_MASK_ALPHA )
-        png_set_alpha_mode( p_png, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR );
 
     /* Strip to 8 bits per channel */
     if( i_bit_depth == 16 ) png_set_strip_16( p_png );
@@ -276,6 +274,18 @@ static int DecodeBlock( decoder_t *p_dec, block_t *p_block )
         p_dec->fmt_out.i_codec = VLC_CODEC_RGB24;
     }
 
+    if ( i_color_type & PNG_COLOR_MASK_ALPHA ) {
+#if PNG_LIBPNG_VER >= 10504
+        png_set_alpha_mode( p_png, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR );
+#else
+        /* Fallback - Older version does not allow to specify alpha encoding */
+        /* png_color_16 - index (unused), red, green, blue, gray (unused) */
+        png_color_16 black_bg = { 0xf, 0x00, 0x00, 0x00, 0xff };
+        png_set_background( p_png, &black_bg,
+                PNG_BACKGROUND_GAMMA_SCREEN, 0.0, 1.0 );
+        p_dec->fmt_out.i_codec = VLC_CODEC_RGB24;
+#endif
+    }
     /* Get a new picture */
     if( decoder_UpdateVideoFormat( p_dec ) )
         goto error;
-- 
2.12.2