BPTC Texture Compression
|Core in version||4.6|
|Core since version||4.2|
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BPTC Texture Compression is the collective name for a pair of compression formats. One of them is for unsigned normalized images, while the other is for floating-point values. They both use 4x4 blocks, and each block in both is 128-bits in size. Unlike S3 Texture Compression, the blocks are taken as byte streams, and thus they are endian-independent.
The unsigned normalized formats come in two flavors that are mathematically equivalent: one for linear RGB values (GL_COMPRESSED_RGBA_BPTC_UNORM) and one for sRGB values(GL_COMPRESSED_SRGB_ALPHA_BPTC_UNORM). They both have an alpha channel as well.
Both this format and S3TC work based on gradients: defining a block based on a pair of colors and then linearly interpolated values between them. The general difference between this format and the nearest S3TC size equivalent (GL_COMPRESSED_RGBA_S3TC_DXT5_EXT) is that this BPTC format allows for blocks to contain multiple gradients for different color channels. This is similar to how DXT5 has a block for RGB and a separate block for alpha. But BPTC uses a much more flexible mechanism that allows different groups of channels to be used.
Each block has a selector that picks between one of 7 different modes. The different modes represent how many channel gradients there are and which channels they map to.