Bilinear and trilinear filtering
Texture filtering deals with how a texture – a 2D image (and
other data) – is displayed on a 3D model. A pixel on a 3D
model won’t necessarily correspond directly to one pixel on its
texture (called a ‘texel’ for clarity), because you can view the
model at any distance and angle. So, when we want to know
the color of a pixel, we find the
point on the texture it
corresponds to, take a few
samples from nearby texels, and
average them. The simplest
method of texture filtering is
bilinear filtering, and that’s all it
does: when a pixel falls between
texels, it samples the four nearest texelstofinditscolour.
Introduce mipmapping, and you have a new problem. Say
the ground you’re standing on is made of cracked concrete. If
you look straight down, you’re seeing a big, detailed concretetexture. But when you look way off into the distance, where
this road recedes toward the horizon, it wouldn’t make sense
to sample from a high-resolution texture when we’re only
seeing a few pixels of road. To improve performance (and
prevent aliasing, Austin notes) without losing much or any
quality, the game uses a
lower-resolution texture – called
a mipmap – for distant objects.
When looking down this
concrete road, we don’t want to
see where one mipmap ends
and another begins, because
there would be a clear jump in
quality.Bilinearfiltering doesn’t interpolate between
mipmaps, so the jump is visible. This is solved with trilinear
filtering, which smooths the transition between mipmaps by
taking samples from both.WEDON’TWANTTOSEE
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