alpha: Add documentation

docs/plugins/gst-plugins-good-plugins-docs.sgml

@@ -20,6 +20,7 @@
     <title>gst-plugins-good Elements</title>
     <xi:include href="xml/element-aasink.xml" />
     <xi:include href="xml/element-agingtv.xml" />
+    <xi:include href="xml/element-alpha.xml" />
     <xi:include href="xml/element-alphacolor.xml" />
     <xi:include href="xml/element-alawdec.xml" />
     <xi:include href="xml/element-alawenc.xml" />

docs/plugins/gst-plugins-good-plugins-sections.txt

@@ -39,6 +39,21 @@ GST_IS_ALAW_ENC_CLASS
 gst_alaw_enc_get_type
 </SECTION>
 
+<SECTION>
+<FILE>element-alpha</FILE>
+<TITLE>alpha</TITLE>
+GstAlpha
+GstAlphaMethod
+<SUBSECTION Standard>
+GstAlphaClass
+GST_ALPHA
+GST_IS_ALPHA
+GST_TYPE_ALPHA
+gst_alpha_get_type
+GST_ALPHA_CLASS
+GST_IS_ALPHA_CLASS
+</SECTION>
+
 <SECTION>
 <FILE>element-alphacolor</FILE>
 <TITLE>alphacolor</TITLE>

gst/alpha/gstalpha.c

@@ -20,6 +20,25 @@
  * Boston, MA 02111-1307, USA.
  */
 
+/**
+ * SECTION:element-alpha
+ * 
+ * The alpha element adds an alpha channel to a video stream. The values
+ * of the alpha channel can be either be set to a constant or can be
+ * dynamically calculated via chroma keying, e.g. blue can be set as
+ * the transparent color.
+ *
+ * Sample pipeline:
+ * |[
+ * gst-launch videotestsrc pattern=smpte75 ! alpha method=green ! \
+ *   videomixer name=mixer ! ffmpegcolorspace ! autovideosink     \
+ *   videotestsrc pattern=snow ! mixer.
+ * ]| This pipeline adds a alpha channel to the SMPTE color bars
+ * with green as the transparent color and mixes the output with
+ * a snow video stream.
+ */
+
+
 #ifdef HAVE_CONFIG_H
 #include "config.h"
 #endif

gst/alpha/gstalpha.h

@@ -44,6 +44,12 @@ G_BEGIN_DECLS
 typedef struct _GstAlpha GstAlpha;
 typedef struct _GstAlphaClass GstAlphaClass;
 
+/** GstAlphaMethod:
+ * @ALPHA_METHOD_SET: Set/adjust alpha channel
+ * @ALPHA_METHOD_GREEN: Chroma Key green
+ * @ALPHA_METHOD_BLUE: Chroma Key blue
+ * @ALPHA_METHOD_CUSTOM: Chroma Key on target_r/g/b
+ */
 typedef enum
 {
   ALPHA_METHOD_SET,