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This section contains information on how to create efficient textures and explains how to control aspects of texture rendering, like the format they are uploaded to the graphics hardware.
3D hardware (but also software) can render textures more efficiently if the dimensions are powers of two (abbreviated as "PO2") (e.g. 256x256, 512x128...), to the extent that hardware and graphics APIs (e.g. OpenGL) require textures to have PO2 dimensions. So does CrystalSpace for all textures, 2D and 3D. While you can feed non-PO2 textures into CS, they will be resized internally to a PO2 size (e.g. 640x480 will become 512x512). The resizing isn't very good, though: the texture will end up looking rather ugly when rendered.
Although most modern hardware supports non-PO2 textures, there is still hardware that doesn't, and non-PO2 textures have limitations (e.g. no mipmapping, no wrapping around on edges) on all but high-end (as of time of this writing - e.g. NVidia GeForce 6800) hardware.
Bottom line: Use power-of-two sized textures.
Another facet is what size actually to use; it all depends on the content, of course. Some points to consider:
A texture has to be physically stored somewhere. CrystalSpace supports a number of image formats to load textures from, common ones such as PNG, TGA, BMP, JPG, DDS, GIF, and less common to exotic ones like MNG and JNG.
What format you want depends on considerations like whether you need an alpha channel or not, can accept lossy compression, and the disk space taken up. The color depth is pretty much unimportant, CS uses truecolor textures when rendering with OpenGL, so at runtime e.g. using paletted textures gains you exactly nothing over truecolor images.
Alpha is supported by PNG, TGA, BMP, DDS, MNG, JNG, the compression is lossless for PNG, GIF, TGA, BMP, and can be lossless or lossy for MNG and DDS. MNG is a bit special as it is an animation format and hence is usually used if you want animated textures.
Commonly, PNG, TGA are used for textures with alpha and JPG for textures without. Those formats are commonly perceived as the best options, as they cover the commonly needed features and offer decent compression. However, the best format you can use is actually DDS.
To determine why, take a look at what happens if e.g. a PNG texture is loaded into CS:
Unobviously, step 3 actually contains a recompression. That is due the fact that CrystalSpace uses texture compression (which has a positive effect performance-wise), but the texture data is sent to OpenGL in RGB(A) format, which means that the driver needs to compress the texture - this costs some time.
DDS offers an advantage here, as, in DDS files,
That means that the steps 1 and 2 above are basically not needed, and so is the recompression in step 3, as the data that needs to be uploaded is precompressed. Getting rid of all that processing greatly improves load time.
Without alpha (DXT1 compression), 4 bits per pixel are needed, with alpha (DXT3, DXT5), 8 bits per pixel are needed. That is before any zip compression, though; the gross file size of a DDS file can rival those of PNG and JPG files.
DDS files can be created with e.g. a Photoshop plugin to export DDS files which is available from NVidia (http://developer.nvidia.com/object/nv_texture_tools.html) or GIMP DDS plugins which are also available on the web (http://nifelheim.dyndns.org/~cocidius/dds/). Last but not least, CS' DDS plugin is also able to save DDS files, in conjunction with the ‘csimagetool’ app you can have a simple DDS converter.
As mentioned above, textures in CS are compressed before being uploaded to the graphics hardware; while compressed textures are fast, they are sometimes undesirable (e.g. for normal maps - see http://developer.nvidia.com/object/bump_map_compression.html for an illustration of the problems).
CrystalSpace allows quality control here on a per-texture base through texture classes. Basically, a texture class is a collection of certain settings that control how a texture is uploaded to the graphics hardware. E.g. the ‘lookup’, ‘normalmap’ and ‘nocompress’ cause textures to be stored uncompressed on the hardware.
Additionally, texture classes also attach some "semantics" to textures -
useful for tools or humans that read the raw world file. The class of a
texture can be set by adding
<class>normalmap</class> or similar
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