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A mipmap is a set of pre-calculated versions of the same image progressively decreasing in size until the image reaches a dimension of 1x1. But an object seen in the distance needs much less resolution to look reasonable, and smaller resolutions require less resources. Game/simulation performance matters far more.Īn object seen up close needs a relatively high-resolution texture so that it doesn't look pixelated. But file size is a relatively small consideration these days. For example, a 1024x1024 image with an alpha channel and mipmaps will result in a 1.37MB file. But many graphic applications in games and sims don't require tight control over the colors, and good choice of color palette can eliminate any visible results of this DXT effect.įile size on disk can be large for DDS. In short, you lose much of the original color range. Where formats like JPG or PNG use 8 bits per channel per pixel (RGB 8:8:8) for a 24 bit color depth (32 bits with PNGs having an alpha channel), DXT reduces the spectrum down to 5:6:5 bits using an interpolation algorithm to arrive at the new color values. DXT generates images using a 16 bit color depth. It is not a good format where retention of the true color is critical, particularly in those situations where fine differences may have a large impact. The compression degrades the original colors. Never use DDS files for editing and archiving. For this reason DXT can be problematic when used with normal maps, though there are work-arounds. 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.
#Nvidia dds plugin for gimp 64 bits
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.
#Nvidia dds plugin for gimp plus
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.
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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".