Use GL_ARB_texture_swizzle to emulate GL_ALPHA textures

The core profile of GL3 has removed support for component-alpha
textures. Previously the GL3 driver would just ignore this and try to
create them anyway. This would generate a GL error on Mesa.

To fix this the GL texture driver will now create a GL_RED texture
when GL_ALPHA textures are not supported natively. It will then set a
texture swizzle using the GL_ARB_texture_swizzle extension so that the
alpha component will be taken from the red component of the texture.
The swizzle is part of the texture object state so it only needs to be
set once when the texture is created.

The ‘gen’ virtual function of the texture driver has been changed to
also take the internal format as a parameter. The GL driver will now
set the swizzle as appropriate here.

The GL3 driver now reports an error if the texture swizzle extension
is not available because Cogl can't really work properly without out
it. The extension is part of GL 3.3 so it is quite likely that it has
wide support from drivers. Eventually we could get rid of this
requirement if we have our own GLSL front-end and we could generate
the swizzle ourselves.

When uploading or downloading texture data to or from a
component-alpha texture, we can no longer rely on GL to do the
conversion. The swizzle doesn't have any effect on the texture data
functions. In these cases Cogl will now force an intermediate buffer
to be used and it will manually do the conversion as it does for the
GLES drivers.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

(cherry picked from commit 32bacf81ebaa3be21a8f26af07d8f6eed6607652)

cogl/cogl-internal.h

@@ -114,7 +114,9 @@ typedef enum
   COGL_PRIVATE_FEATURE_BLEND_CONSTANT = 1L<<18,
   COGL_PRIVATE_FEATURE_QUERY_FRAMEBUFFER_BITS = 1L<<19,
   COGL_PRIVATE_FEATURE_BUILTIN_POINT_SIZE_UNIFORM = 1L<<20,
-  COGL_PRIVATE_FEATURE_QUERY_TEXTURE_PARAMETERS = 1L<<21
+  COGL_PRIVATE_FEATURE_QUERY_TEXTURE_PARAMETERS = 1L<<21,
+  COGL_PRIVATE_FEATURE_ALPHA_TEXTURES = 1L<<22,
+  COGL_PRIVATE_FEATURE_TEXTURE_SWIZZLE = 1L<<23
 } CoglPrivateFeatureFlags;
 
 /* Sometimes when evaluating pipelines, either during comparisons or

cogl/cogl-texture-3d.c

@@ -234,7 +234,8 @@ cogl_texture_3d_new_with_size (CoglContext *ctx,
                                          width, height, depth,
                                          internal_format);
 
-  ctx->texture_driver->gen (ctx, GL_TEXTURE_3D, 1, &tex_3d->gl_texture);
+  tex_3d->gl_texture =
+    ctx->texture_driver->gen (ctx, GL_TEXTURE_3D, internal_format);
   _cogl_bind_gl_texture_transient (GL_TEXTURE_3D,
                                    tex_3d->gl_texture,
                                    FALSE);
@@ -308,7 +309,8 @@ cogl_texture_3d_new_from_bitmap (CoglBitmap *bmp,
       _cogl_bitmap_unmap (dst_bmp);
     }
 
-  ctx->texture_driver->gen (ctx, GL_TEXTURE_3D, 1, &tex_3d->gl_texture);
+  tex_3d->gl_texture =
+    ctx->texture_driver->gen (ctx, GL_TEXTURE_3D, internal_format);
 
   ctx->texture_driver->upload_to_gl_3d (ctx,
                                         GL_TEXTURE_3D,

cogl/cogl-texture-driver.h

@@ -33,11 +33,10 @@ struct _CoglTextureDriver
    * non-mipmap filters when creating textures. This is to save some memory as
    * the driver will not allocate room for the mipmap tree.
    */
-  void
+  GLuint
   (* gen) (CoglContext *ctx,
            GLenum gl_target,
-           GLsizei n,
+           CoglPixelFormat internal_format);
-           GLuint *textures);
 
   /*
    * This sets up the glPixelStore state for an upload to a destination with

cogl/cogl-texture-rectangle.c

@@ -219,10 +219,10 @@ cogl_texture_rectangle_new_with_size (CoglContext *ctx,
                                                   width, height,
                                                   internal_format);
 
+  tex_rect->gl_texture =
     ctx->texture_driver->gen (ctx,
                               GL_TEXTURE_RECTANGLE_ARB,
-                            1, /* num textures */
+                              internal_format);
-                            &tex_rect->gl_texture);
   _cogl_bind_gl_texture_transient (GL_TEXTURE_RECTANGLE_ARB,
                                    tex_rect->gl_texture,
                                    tex_rect->is_foreign);
@@ -274,10 +274,10 @@ cogl_texture_rectangle_new_from_bitmap (CoglBitmap *bmp,
                                                   cogl_bitmap_get_height (bmp),
                                                   internal_format);
 
+  tex_rect->gl_texture =
     ctx->texture_driver->gen (ctx,
                               GL_TEXTURE_RECTANGLE_ARB,
-                            1, /* num textures */
+                              internal_format);
-                            &tex_rect->gl_texture);
   ctx->texture_driver->upload_to_gl (ctx,
                                      GL_TEXTURE_RECTANGLE_ARB,
                                      tex_rect->gl_texture,

cogl/cogl-texture.c

@@ -206,7 +206,15 @@ _cogl_texture_prepare_for_upload (CoglBitmap      *src_bmp,
      limited number of formats so we must convert using the Cogl
      bitmap code instead */
 
-  if ((ctx->private_feature_flags & COGL_PRIVATE_FEATURE_FORMAT_CONVERSION))
+  /* If the driver doesn't natively support alpha textures then it
+   * won't work correctly to convert to/from component-alpha
+   * textures */
+
+  if ((ctx->private_feature_flags & COGL_PRIVATE_FEATURE_FORMAT_CONVERSION) &&
+      ((ctx->private_feature_flags & COGL_PRIVATE_FEATURE_ALPHA_TEXTURES) ||
+       (src_format != COGL_PIXEL_FORMAT_A_8 &&
+        dst_format != COGL_PIXEL_FORMAT_A_8) ||
+       src_format == dst_format))
     {
       /* If the source format does not have the same premult flag as the
          dst format then we need to copy and convert it */
@@ -1202,6 +1210,34 @@ cogl_texture_get_data (CoglTexture *texture,
     closest_format = ((closest_format & ~COGL_PREMULT_BIT) |
                       (texture_format & COGL_PREMULT_BIT));
 
+  /* If the application is requesting a conversion from a
+   * component-alpha texture and the driver doesn't support them
+   * natively then we can only read into an alpha-format buffer. In
+   * this case the driver will be faking the alpha textures with a
+   * red-component texture and it won't swizzle to the correct format
+   * while reading */
+  if ((ctx->private_feature_flags & COGL_PRIVATE_FEATURE_ALPHA_TEXTURES) == 0)
+    {
+      if (texture_format == COGL_PIXEL_FORMAT_A_8)
+        {
+          closest_format = COGL_PIXEL_FORMAT_A_8;
+          closest_gl_format = GL_RED;
+          closest_gl_type = GL_UNSIGNED_BYTE;
+        }
+      else if (format == COGL_PIXEL_FORMAT_A_8)
+        {
+          /* If we are converting to a component-alpha texture then we
+           * need to read all of the components to a temporary buffer
+           * because there is no way to get just the 4th component.
+           * Note: it doesn't matter whether the texture is
+           * pre-multiplied here because we're only going to look at
+           * the alpha component */
+          closest_format = COGL_PIXEL_FORMAT_RGBA_8888;
+          closest_gl_format = GL_RGBA;
+          closest_gl_type = GL_UNSIGNED_BYTE;
+        }
+    }
+
   /* Is the requested format supported? */
   if (closest_format == format)
     /* Target user data directly */

cogl/driver/gl/cogl-texture-2d-gl.c

@@ -108,7 +108,8 @@ _cogl_texture_2d_gl_new_with_size (CoglContext *ctx,
                                          width, height,
                                          internal_format);
 
-  ctx->texture_driver->gen (ctx, GL_TEXTURE_2D, 1, &tex_2d->gl_texture);
+  tex_2d->gl_texture =
+    ctx->texture_driver->gen (ctx, GL_TEXTURE_2D, internal_format);
   _cogl_bind_gl_texture_transient (GL_TEXTURE_2D,
                                    tex_2d->gl_texture,
                                    tex_2d->is_foreign);
@@ -164,7 +165,8 @@ _cogl_texture_2d_gl_new_from_bitmap (CoglBitmap *bmp,
       _cogl_bitmap_unmap (dst_bmp);
     }
 
-  ctx->texture_driver->gen (ctx, GL_TEXTURE_2D, 1, &tex_2d->gl_texture);
+  tex_2d->gl_texture =
+    ctx->texture_driver->gen (ctx, GL_TEXTURE_2D, internal_format);
   ctx->texture_driver->upload_to_gl (ctx,
                                      GL_TEXTURE_2D,
                                      tex_2d->gl_texture,
@@ -198,7 +200,8 @@ _cogl_egl_texture_2d_gl_new_from_image (CoglContext *ctx,
                                          width, height,
                                          format);
 
-  ctx->texture_driver->gen (ctx, GL_TEXTURE_2D, 1, &tex_2d->gl_texture);
+  tex_2d->gl_texture =
+    ctx->texture_driver->gen (ctx, GL_TEXTURE_2D, format);
   _cogl_bind_gl_texture_transient (GL_TEXTURE_2D,
                                    tex_2d->gl_texture,
                                    FALSE);

cogl/driver/gl/gl/cogl-driver-gl.c

@@ -54,6 +54,10 @@ _cogl_driver_pixel_format_from_gl_internal (CoglContext *context,
     {
     case GL_ALPHA: case GL_ALPHA4: case GL_ALPHA8:
     case GL_ALPHA12: case GL_ALPHA16:
+      /* Cogl only supports one single-component texture so if we have
+       * ended up with a red texture then it is probably being used as
+       * a component-alpha texture */
+    case GL_RED:
 
       *out_format = COGL_PIXEL_FORMAT_A_8;
       return TRUE;
@@ -98,8 +102,20 @@ _cogl_driver_pixel_format_to_gl (CoglContext *context,
   switch (format)
     {
     case COGL_PIXEL_FORMAT_A_8:
+      /* If the driver doesn't natively support alpha textures then we
+       * will use a red component texture with a swizzle to implement
+       * the texture */
+      if ((context->private_feature_flags &
+           COGL_PRIVATE_FEATURE_ALPHA_TEXTURES) == 0)
+        {
+          glintformat = GL_RED;
+          glformat = GL_RED;
+        }
+      else
+        {
           glintformat = GL_ALPHA;
           glformat = GL_ALPHA;
+        }
       gltype = GL_UNSIGNED_BYTE;
       break;
     case COGL_PIXEL_FORMAT_G_8:
@@ -564,11 +580,17 @@ _cogl_driver_update_features (CoglContext *ctx,
   if (ctx->glGenSamplers)
     private_flags |= COGL_PRIVATE_FEATURE_SAMPLER_OBJECTS;
 
+  if (COGL_CHECK_GL_VERSION (gl_major, gl_minor, 3, 3) ||
+      _cogl_check_extension ("GL_ARB_texture_swizzle", gl_extensions) ||
+      _cogl_check_extension ("GL_EXT_texture_swizzle", gl_extensions))
+    private_flags |= COGL_PRIVATE_FEATURE_TEXTURE_SWIZZLE;
+
   if (ctx->driver == COGL_DRIVER_GL)
     /* Features which are not available in GL 3 */
     private_flags |= (COGL_PRIVATE_FEATURE_FIXED_FUNCTION |
                       COGL_PRIVATE_FEATURE_ALPHA_TEST |
-                      COGL_PRIVATE_FEATURE_QUADS);
+                      COGL_PRIVATE_FEATURE_QUADS |
+                      COGL_PRIVATE_FEATURE_ALPHA_TEXTURES);
 
   private_flags |= (COGL_PRIVATE_FEATURE_READ_PIXELS_ANY_FORMAT |
                     COGL_PRIVATE_FEATURE_ANY_GL |
@@ -583,6 +605,17 @@ _cogl_driver_update_features (CoglContext *ctx,
 
   g_strfreev (gl_extensions);
 
+  if ((private_flags & (COGL_PRIVATE_FEATURE_ALPHA_TEXTURES |
+                        COGL_PRIVATE_FEATURE_TEXTURE_SWIZZLE)) == 0)
+    {
+      _cogl_set_error (error,
+                       COGL_DRIVER_ERROR,
+                       COGL_DRIVER_ERROR_NO_SUITABLE_DRIVER_FOUND,
+                       "The GL_ARB_texture_swizzle extension is required "
+                       "to use the GL3 driver");
+      return FALSE;
+    }
+
   return TRUE;
 }
 

cogl/driver/gl/gl/cogl-texture-driver-gl.c

@@ -45,21 +45,20 @@
 #include <stdlib.h>
 #include <math.h>
 
-static void
+#ifndef GL_TEXTURE_SWIZZLE_RGBA
+#define GL_TEXTURE_SWIZZLE_RGBA 0x8E46
+#endif
+
+static GLuint
 _cogl_texture_driver_gen (CoglContext *ctx,
                           GLenum gl_target,
-                          GLsizei n,
+                          CoglPixelFormat internal_format)
-                          GLuint *textures)
 {
-  unsigned int i;
+  GLuint tex;
 
-  GE (ctx, glGenTextures (n, textures));
+  GE (ctx, glGenTextures (1, &tex));
 
-  for (i = 0; i < n; i++)
+  _cogl_bind_gl_texture_transient (gl_target, tex, FALSE);
-    {
-      _cogl_bind_gl_texture_transient (gl_target,
-                                       textures[i],
-                                       FALSE);
 
   switch (gl_target)
     {
@@ -79,7 +78,22 @@ _cogl_texture_driver_gen (CoglContext *ctx,
     default:
       g_assert_not_reached();
     }
+
+  /* If the driver doesn't support alpha textures directly then we'll
+   * fake them by setting the swizzle parameters */
+  if (internal_format == COGL_PIXEL_FORMAT_A_8 &&
+      (ctx->private_feature_flags & (COGL_PRIVATE_FEATURE_ALPHA_TEXTURES |
+                                     COGL_PRIVATE_FEATURE_TEXTURE_SWIZZLE)) ==
+      COGL_PRIVATE_FEATURE_TEXTURE_SWIZZLE)
+    {
+      static const GLint red_swizzle[] = { GL_ZERO, GL_ZERO, GL_ZERO, GL_RED };
+
+      GE( ctx, glTexParameteriv (gl_target,
+                                 GL_TEXTURE_SWIZZLE_RGBA,
+                                 red_swizzle) );
     }
+
+  return tex;
 }
 
 /* OpenGL - unlike GLES - can upload a sub region of pixel data from a larger

cogl/driver/gl/gles/cogl-driver-gles.c

@@ -271,7 +271,8 @@ _cogl_driver_update_features (CoglContext *context,
                       COGL_PRIVATE_FEATURE_BUILTIN_POINT_SIZE_UNIFORM);
 
   private_flags |= (COGL_PRIVATE_FEATURE_VBOS |
-                    COGL_PRIVATE_FEATURE_ANY_GL);
+                    COGL_PRIVATE_FEATURE_ANY_GL |
+                    COGL_PRIVATE_FEATURE_ALPHA_TEXTURES);
 
   /* Both GLES 1.1 and GLES 2.0 support point sprites in core */
   flags |= COGL_FEATURE_POINT_SPRITE;

cogl/driver/gl/gles/cogl-texture-driver-gles.c

@@ -64,19 +64,16 @@
 #define GL_UNPACK_SKIP_PIXELS 0x0CF4
 #endif
 
-static void
+static GLuint
 _cogl_texture_driver_gen (CoglContext *ctx,
                           GLenum gl_target,
-                          GLsizei n,
+                          CoglPixelFormat internal_format)
-                          GLuint *textures)
 {
-  unsigned int i;
+  GLuint tex;
 
-  GE (ctx, glGenTextures (n, textures));
+  GE (ctx, glGenTextures (1, &tex));
 
-  for (i = 0; i < n; i++)
+  _cogl_bind_gl_texture_transient (gl_target, tex, FALSE);
-    {
-      _cogl_bind_gl_texture_transient (gl_target, textures[i], FALSE);
 
   switch (gl_target)
     {
@@ -91,7 +88,8 @@ _cogl_texture_driver_gen (CoglContext *ctx,
     default:
       g_assert_not_reached();
     }
-    }
+
+  return tex;
 }
 
 static void