Mipmap is a small pre-filtering variant of a texture image that represents the different levels of detail (LOD) of the texture. By using the mipmap's reduced filter mode, the Graphics Core can be set to automatically calculate which LOD is closest to mapping the mipmap's texels to the pixels in the render target and texture it with the correct mipmap.
There are two important advantages to using Mipmap:
• Improve performance by dramatically increasing texture cache efficiency, especially in the case of strong shrinks.
• Improve image quality by eliminating aliasing caused by texture sampling without mipmapping.
The single limitation of mipmapping is that each image requires approximately one-third of the texture memory. Depending on the situation, this cost may be small compared to the benefits of rendering speed and image quality. In some cases, you should not use a Mipmap. In particular, mipmapping should not be used where filtering cannot be reasonably applied, such as textures containing non-image data such as indexes or depth textures. Textures that are never reduced should also be avoided. For example, texture elements in UI elements are always mapped one-to-one with pixels.
Ideally, mipmaps should be created offline using tools such as PVRTexTool, which is available as part of the PowerVR Graphics SDK. Mipmaps can be generated at runtime, which is very useful for updating mipmaps that render texture targets. In OpenGL ES, you can do this using the function glGenerateMipmap. There are no such built-in functions in Vulkan, developers must manually generate them. This does not apply to PVRTC textures, it must generate mipmaps offline. It must be decided which cost is most appropriate, and the storage cost generated offline or the runtime cost (increasing code complexity in the case of Vulkan) generates a mipmap at runtime.
Finally, it should be noted that the lack of filtering between mipmap levels may result in visible seams at the mipmap transition, a form of artifact called "mipmap banding." Trilinear filtering in OpenGL ES can be achieved by using the filter mode GL_LINEAR_MIPMAP_LINEAR. In Vulkan, the filtering mode should be set to VK_SAMPLER_MIPMAP_MODE_LINEAR for trilinear filtering. This effectively eliminates these seams for higher image quality.
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