Images with high contrast regions such as print or cartoon-like colors and borders will likely generate visible artifacts, particularly with smaller resolutions. What is stored as compressed data is not the same as the original image, and on a fine level it may not even be close. This can result in huge video memory savings.ĭXT codecs are lossy. Compression ratio is 6:1 if no alpha channel is used, or 4:1 if an alpha channel is used. (And uncompressing them takes time and resources.) DDS/DXT files remain in their compressed state in video RAM, using special algorithms on the video card to retrieve data on demand. All image formats except DDS/DXT are loaded into graphics memory in flat, uncompressed state. More on mipmaps in a moment.ĭata remains compressed in video memory. This is another savings in load times and gives the graphic designer control over mipmap construction. Mipmaps can be pre-generated and included in the DDS files. In situations where many files are being constantly swapped in/out of the graphics unit, this can be a substantial savings and can reduce "lag", especially with big texture files. DDS files are ready to be used by the graphics system and can be read straight into graphics memory with little overhead. What are the advantages of DDS files and DXT compression?įast load times. Note that the actual image size will likely be larger, as it will frequently include mipmap data. For an image using DXT3 or DXT5 incorporating an alpha channel, the requirements will be 8 bpp. So, for an image with no alpha using DXT1, compression results in an image using 4 bpp (bits per pixel). This allows the alpha channel to be represented more accurately than DXT5, but with less subtle transitions. Each pixel gets 4 bits to represent its alpha, for a total of 16 unique values of transparency. The alpha values are represented by 8 bits each, and the range indices by 3 bits each, allowing for gradients of up to 8 shades.ĭXT3 handles the alpha channel a little differently. Two alpha values are selected and used as the extremes for a range of transparency values. DXT5 stores alpha information in a way that is almost the same as color information. In DXT5, the alpha channel is encoded using a second set of 64 bits for each texel. What if the image has an alpha channel? How DXT handles this depends on the codec used. So for any set of images having the same dimensions, compressed size will always be the same. So each texel requires 2x16 bits for the colors, plus 16x2 bits for the indices, giving a total of 64 bits for each texel, which equates to 4 bits per pixel. The two representative colors are stored as 16-bit RGB values (5:6:5). The sixteen pixels of the texel are then assigned a 2-bit index (0-3) that maps them to the color range. For every texel, DXT selects two colors from the texel, each determining one end of a color range of 4 colors. DXT sees images as collections of 4x4 blocks of pixels called "texels".
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