Merge commit '46ad9ac9641d1fe8292ec1f46bbd2e4456621ca7'

[ffmpeg] / libavutil / pixdesc.h

diff --git a/libavutil/pixdesc.h b/libavutil/pixdesc.h
index 705377df8b7932f902110c46000516b61126707b..c4d0548ad68729408b1db186bc1e51c44edeb3d9 100644 (file)
--- a/libavutil/pixdesc.h
+++ b/libavutil/pixdesc.h
@@ -139,8 +139,19 @@ typedef struct AVPixFmtDescriptor {
  * An example of a pseudo-paletted format is AV_PIX_FMT_GRAY8
  */
 #define AV_PIX_FMT_FLAG_PSEUDOPAL    (1 << 6)
+
 /**
- * The pixel format has an alpha channel.
+ * The pixel format has an alpha channel. This is set on all formats that
+ * support alpha in some way. The exception is AV_PIX_FMT_PAL8, which can
+ * carry alpha as part of the palette. Details are explained in the
+ * AVPixelFormat enum, and are also encoded in the corresponding
+ * AVPixFmtDescriptor.
+ *
+ * The alpha is always straight, never pre-multiplied.
+ *
+ * If a codec or a filter does not support alpha, it should set all alpha to
+ * opaque, or use the equivalent pixel formats without alpha component, e.g.
+ * AV_PIX_FMT_RGB0 (or AV_PIX_FMT_RGB24 etc.) instead of AV_PIX_FMT_RGBA.
  */
 #define AV_PIX_FMT_FLAG_ALPHA        (1 << 7)
 
@@ -299,8 +310,6 @@ int av_pix_fmt_get_chroma_sub_sample(enum AVPixelFormat pix_fmt,
  */
 int av_pix_fmt_count_planes(enum AVPixelFormat pix_fmt);
 
-void ff_check_pixfmt_descriptors(void);
-
 /**
  * Utility function to swap the endianness of a pixel format.
  *