gles: Support the GL_EXT_texture_format_BGRA8888 extension

This extension allows an application to upload data in BGRA format. We
can use this to avoid a conversion in Cogl whenever it is given BGRA
data. This is quite useful when uploading data generated by Cairo
because at least on little-endian architectures that ends up as BGRA.

The patch just makes the pixel_format_to_gl implementation return
GL_BGRA_EXT for the data format and internal format whenever
COGL_PIXEL_FORMAT_BGRA_8888{,_PRE} is used.

A small caveat with this patch is that once a texture is created as
GL_BGRA, when later using glTexSubImage2D to update the texture it
must always be given data as GL_BGRA. Currently this just works out
because we store the internal format of a texture as a CoglPixelFormat
and we already swizzle the data if it does not match exactly on GLES.
However if we later switch to using a different enum for internal
formats then we might lose the ability to store the component ordering
so we'll have to think of another way to do this.

cogl/cogl-internal.h

@@ -100,7 +100,8 @@ typedef enum
   COGL_PRIVATE_FEATURE_PBOS = 1L<<5,
   COGL_PRIVATE_FEATURE_VBOS = 1L<<6,
   COGL_PRIVATE_FEATURE_EXT_PACKED_DEPTH_STENCIL = 1L<<7,
-  COGL_PRIVATE_FEATURE_OES_PACKED_DEPTH_STENCIL = 1L<<8
+  COGL_PRIVATE_FEATURE_OES_PACKED_DEPTH_STENCIL = 1L<<8,
+  COGL_PRIVATE_FEATURE_TEXTURE_FORMAT_BGRA8888 = 1L<<9
 } CoglPrivateFeatureFlags;
 
 /* Sometimes when evaluating pipelines, either during comparisons or

cogl/cogl-texture.c

@@ -166,6 +166,15 @@ _cogl_texture_determine_internal_format (CoglPixelFormat src_format,
         return src_format;
     }
   else
+    /* XXX: It might be nice to make this match the component ordering
+       of the source format when the formats are otherwise the same
+       because on GL there is no way to specify the ordering of the
+       internal format. However when using GLES with the
+       GL_EXT_texture_format_BGRA8888 the order of the internal format
+       becomes important because it must exactly match the format of
+       the uploaded data. That means that if someone creates a texture
+       with some RGBA data and then later tries to upload BGRA data we
+       do actually have to swizzle the components */
     return dst_format;
 }
 

cogl/driver/gles/cogl-gles.c

@@ -69,6 +69,22 @@ _cogl_driver_pixel_format_to_gl (CoglContext *context,
       gltype = GL_UNSIGNED_BYTE;
       break;
 
+    case COGL_PIXEL_FORMAT_BGRA_8888:
+    case COGL_PIXEL_FORMAT_BGRA_8888_PRE:
+      /* There is an extension to support this format */
+      if ((context->private_feature_flags &
+           COGL_PRIVATE_FEATURE_TEXTURE_FORMAT_BGRA8888))
+        {
+          /* For some reason the extension says you have to specify
+             BGRA for the internal format too */
+          glintformat = GL_BGRA_EXT;
+          glformat = GL_BGRA_EXT;
+          gltype = GL_UNSIGNED_BYTE;
+          required_format = format;
+          break;
+        }
+      /* flow through */
+
       /* Just one 24-bit ordering supported */
     case COGL_PIXEL_FORMAT_RGB_888:
     case COGL_PIXEL_FORMAT_BGR_888:
@@ -81,8 +97,6 @@ _cogl_driver_pixel_format_to_gl (CoglContext *context,
       /* Just one 32-bit ordering supported */
     case COGL_PIXEL_FORMAT_RGBA_8888:
     case COGL_PIXEL_FORMAT_RGBA_8888_PRE:
-    case COGL_PIXEL_FORMAT_BGRA_8888:
-    case COGL_PIXEL_FORMAT_BGRA_8888_PRE:
     case COGL_PIXEL_FORMAT_ARGB_8888:
     case COGL_PIXEL_FORMAT_ARGB_8888_PRE:
     case COGL_PIXEL_FORMAT_ABGR_8888:
@@ -276,6 +290,9 @@ _cogl_driver_update_features (CoglContext *context,
   if (_cogl_check_extension ("GL_OES_packed_depth_stencil", gl_extensions))
     private_flags |= COGL_PRIVATE_FEATURE_OES_PACKED_DEPTH_STENCIL;
 
+  if (_cogl_check_extension ("GL_EXT_texture_format_BGRA8888", gl_extensions))
+    private_flags |= COGL_PRIVATE_FEATURE_TEXTURE_FORMAT_BGRA8888;
+
   /* Cache features */
   context->private_feature_flags |= private_flags;
   context->feature_flags |= flags;