Support TGA color-mapped files with expansion

[walbourn]

The current TGA loader does not support TGA_COLOR_MAPPED or TGA_COLOR_MAPPED_RLE images. Legacy D3DX9 did support them, but loaded them as D3DFMT_P8 (i.e. 256-index palette).

While there are no palette DXGI formats, such TGA files could be expanded to B5G5R5A1_UNORM (bColorMapSize of 15 or of 16) or R8G8B8A8_UNORM (bColorMapSize of 24 or 32).

[walbourn]

There's generally not much call for color-mapped TGA files as their primary value is being able to encode 24bpp or 32bpp pixel data. 8-bit palette textures are usually created as png or bmp which can already be expanded by DirectXTex.

[dingsheng999]

Thanks a lot Chuck for this great library. I tested DirectXTex for .tga sample files on http://www.fileformat.info/format/tga/sample/index.htm, and only one file, 'CCM8.TGA', cannot be correctly decoded because it's a color-mapped TGA. It's still nice to have DirectXTex support this kind of file so that people can use it as a general TGA reader, and along with other awesome functions such as pixel format conversion.

[walbourn]

@dingsheng999 Thanks for the feedback. The vast majority of use-cases for TGA is 24bpp/32bpp bitmaps because at the time it was one of the few formats that supported it. I've also not found any examples of color-mapped TGA files except those sample files from the original test suite. Have you run across any others?

[dingsheng999]

@walbourn I tried to search on Bing, Baidu, Google and a even a few image websites for professional artists but cannot find a single real case color-mapped TGA file. That sample file is the only one I can find.

[walbourn]

It seems like color-mapped TGAs just never got any real traction in the real world since other formats cover that scenario well. As such, I'm closing this issue.