OpenGL texture format enums table

Note: This content was written before the ASTC LDR 4x4-12x12 and XUASTC LDR 4x4-12x12 codecs were added to the system. This table needs to be updated, as we now support transcoding all 14 ASTC block sizes from these two codec formats.

Here's a table of OpenGL texture formats/internal formats enums corresponding to each compressed texture format in the transcoder_texture_format enum in transcoder/basisu_transcoder.h. The command line tool uses these OpenGL enums when it writes .KTX format texture files.

The transcoder supports all of these GPU texture formats, and the encoder library has code to decompress all of these formats.

transcoder_texture_format	GL internal format	GL base internal format
cTFBC1_RGB	GL_COMPRESSED_RGB_S3TC_DXT1_EXT	GL_RGB
cTFBC3_RGBA	GL_COMPRESSED_RGBA_S3TC_DXT5_EXT	GL_RGBA
cTFBC4_R	GL_COMPRESSED_RED_RGTC1_EXT	GL_RED
cTFBC5_RG	GL_COMPRESSED_RED_GREEN_RGTC2_EXT	GL_RG
cTFETC1_RGB	GL_ETC1_RGB8_OES	GL_RGB
cTFETC2_RGBA	GL_COMPRESSED_RGBA8_ETC2_EAC	GL_RGBA
cTFBC7_RGBA	GL_COMPRESSED_RGBA_BPTC_UNORM	GL_RGBA
cTFPVRTC1_4_RGB	GL_COMPRESSED_RGB_PVRTC_4BPPV1_IMG	GL_RGB
cTFPVRTC1_4_RGBA	GL_COMPRESSED_RGBA_PVRTC_4BPPV1_IMG	GL_RGBA
cTFASTC_4x4_RGBA	GL_COMPRESSED_RGBA_ASTC_4x4_KHR	GL_RGBA
cTFATC_RGB	GL_ATC_RGB_AMD	GL_RGB
cTFATC_RGBA	GL_ATC_RGBA_INTERPOLATED_ALPHA_AMD	GL_RGBA
cTFETC2_EAC_R11	GL_COMPRESSED_R11_EAC	GL_RED
cTFETC2_EAC_RG11	GL_COMPRESSED_RG11_EAC	GL_RG
cTFFXT1_RGB	GL_COMPRESSED_RGB_FXT1_3DFX	GL_RGB
cTFPVRTC2_4_RGB, cTFPVRTC2_4_RGBA	GL_COMPRESSED_RGBA_PVRTC_4BPPV2_IMG	GL_RGBA