+ * Get the GL base format of a specified GL compressed texture format
+ *
+ * From page 232 of the OpenGL 3.3 (Compatiblity Profile) spec:
+ *
+ * "Compressed Internal Format Base Internal Format Type
+ * --------------------------- -------------------- ---------
+ * COMPRESSED_ALPHA ALPHA Generic
+ * COMPRESSED_LUMINANCE LUMINANCE Generic
+ * COMPRESSED_LUMINANCE_ALPHA LUMINANCE_ALPHA Generic
+ * COMPRESSED_INTENSITY INTENSITY Generic
+ * COMPRESSED_RED RED Generic
+ * COMPRESSED_RG RG Generic
+ * COMPRESSED_RGB RGB Generic
+ * COMPRESSED_RGBA RGBA Generic
+ * COMPRESSED_SRGB RGB Generic
+ * COMPRESSED_SRGB_ALPHA RGBA Generic
+ * COMPRESSED_SLUMINANCE LUMINANCE Generic
+ * COMPRESSED_SLUMINANCE_ALPHA LUMINANCE_ALPHA Generic
+ * COMPRESSED_RED_RGTC1 RED Specific
+ * COMPRESSED_SIGNED_RED_RGTC1 RED Specific
+ * COMPRESSED_RG_RGTC2 RG Specific
+ * COMPRESSED_SIGNED_RG_RGTC2 RG Specific"
+ *
+ * \return
+ * The base format of \c format if \c format is a compressed format (either
+ * generic or specific. Otherwise 0 is returned.
+ */
+GLenum
+_mesa_gl_compressed_format_base_format(GLenum format)
+{
+ switch (format) {
+ case GL_COMPRESSED_RED:
+ case GL_COMPRESSED_RED_RGTC1:
+ case GL_COMPRESSED_SIGNED_RED_RGTC1:
+ return GL_RED;
+
+ case GL_COMPRESSED_RG:
+ case GL_COMPRESSED_RG_RGTC2:
+ case GL_COMPRESSED_SIGNED_RG_RGTC2:
+ return GL_RG;
+
+ case GL_COMPRESSED_RGB:
+ case GL_COMPRESSED_SRGB:
+ case GL_COMPRESSED_RGB_S3TC_DXT1_EXT:
+ case GL_COMPRESSED_RGB_FXT1_3DFX:
+ case GL_COMPRESSED_SRGB_S3TC_DXT1_EXT:
+ case GL_ETC1_RGB8_OES:
+ return GL_RGB;
+
+ case GL_COMPRESSED_RGBA:
+ case GL_COMPRESSED_SRGB_ALPHA:
+ case GL_COMPRESSED_RGBA_BPTC_UNORM_ARB:
+ case GL_COMPRESSED_SRGB_ALPHA_BPTC_UNORM_ARB:
+ case GL_COMPRESSED_RGB_BPTC_SIGNED_FLOAT_ARB:
+ case GL_COMPRESSED_RGB_BPTC_UNSIGNED_FLOAT_ARB:
+ case GL_COMPRESSED_RGBA_S3TC_DXT1_EXT:
+ case GL_COMPRESSED_RGBA_S3TC_DXT3_EXT:
+ case GL_COMPRESSED_RGBA_S3TC_DXT5_EXT:
+ case GL_COMPRESSED_RGBA_FXT1_3DFX:
+ case GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT:
+ case GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT:
+ case GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT:
+ return GL_RGBA;
+
+ case GL_COMPRESSED_ALPHA:
+ return GL_ALPHA;
+
+ case GL_COMPRESSED_LUMINANCE:
+ case GL_COMPRESSED_SLUMINANCE:
+ case GL_COMPRESSED_LUMINANCE_LATC1_EXT:
+ case GL_COMPRESSED_SIGNED_LUMINANCE_LATC1_EXT:
+ return GL_LUMINANCE;
+
+ case GL_COMPRESSED_LUMINANCE_ALPHA:
+ case GL_COMPRESSED_SLUMINANCE_ALPHA:
+ case GL_COMPRESSED_LUMINANCE_ALPHA_LATC2_EXT:
+ case GL_COMPRESSED_SIGNED_LUMINANCE_ALPHA_LATC2_EXT:
+ case GL_COMPRESSED_LUMINANCE_ALPHA_3DC_ATI:
+ return GL_LUMINANCE_ALPHA;
+
+ case GL_COMPRESSED_INTENSITY:
+ return GL_INTENSITY;
+
+ default:
+ return 0;
+ }
+}
+
+/**
+ * Return list of (and count of) all specific texture compression
+ * formats that are supported.
+ *
+ * Some formats are \b not returned by this function. The
+ * \c GL_COMPRESSED_TEXTURE_FORMATS query only returns formats that are
+ * "suitable for general-purpose usage." All texture compression extensions
+ * have taken this to mean either linear RGB or linear RGBA.
+ *
+ * The GL_ARB_texture_compress_rgtc spec says:
+ *
+ * "19) Should the GL_NUM_COMPRESSED_TEXTURE_FORMATS and
+ * GL_COMPRESSED_TEXTURE_FORMATS queries return the RGTC formats?
+ *
+ * RESOLVED: No.
+ *
+ * The OpenGL 2.1 specification says "The only values returned
+ * by this query [GL_COMPRESSED_TEXTURE_FORMATS"] are those
+ * corresponding to formats suitable for general-purpose usage.
+ * The renderer will not enumerate formats with restrictions that
+ * need to be specifically understood prior to use."
+ *
+ * Compressed textures with just red or red-green components are
+ * not general-purpose so should not be returned by these queries
+ * because they have restrictions.
+ *
+ * Applications that seek to use the RGTC formats should do so
+ * by looking for this extension's name in the string returned by
+ * glGetString(GL_EXTENSIONS) rather than
+ * what GL_NUM_COMPRESSED_TEXTURE_FORMATS and
+ * GL_COMPRESSED_TEXTURE_FORMATS return."
+ *
+ * There is nearly identical wording in the GL_EXT_texture_compression_rgtc
+ * spec.