IGP texture filtering quality
Among discrete GPUs, anisotropic filtering comparisons have become somewhat superfluous. Today's solutions apply the same level of filtering quality with the same mipmap transitions at all polygon angles, which yields generally consistent results across different GPU makes and generations.
In the integrated world, though, things aren't quite as rosy. We witnessed that first-hand when comparing Llano to Sandy Bridge last year. While Llano's IGP had a nice, consistent filtering pattern, Sandy's HD 3000 integrated graphics exhibited huge variations in filtering quality at different angles.
Happily, though, things have improved quite a bit with Ivy Bridge. Take a look:
The patterns above are the output of our Direct3D AF Tester. In case you're not familiar with it, here's our explanation from last year's Llano review:
In the images above, you're peering down a 3D-rendered cylinder or tube, and the inside surface of that tube has been covered with a simple texture map. The colored bands are what are known as mip maps, or increasingly lower resolution copies of the base texture mapped to the walls of the cylinder. The further you move from the camera, the lower the resolution of the mip level used. In the pictures above, the different colors show different mip levels. (Of course, mip maps don't normally come in different colors. They look very much like one another and like the base texture. This test app colors them in order to make them easily visible.) Mip maps are a helpful tool in texture filtering because sampling from a single copy of the original, high-res texture can be work-intensive and, in a constrained grid of pixels, can produce excessive high-frequency noise, which is visually disruptive. In other words, a little bit of blurring and blending in the right places can be beneficial to the final result.
Alongside mip mapping, we're layering on a couple of additional techniques to improve image quality. We're using trilinear filtering to blend between mip levels, so that we don't see abrupt transitions or banding. That's why the different colors transition gradually from one to another. We're also using anisotropic filtering, grabbing more samples for textures that exist at certain angles on the Z or depth axis—typically on surfaces stretching away from the camera, like floors, walls, and ceilings—in order to preserve sharpness that simple mip mapping would destroy. All of these things we take for granted in modern GPUs, which have custom hardware onboard to perform these functions.
In a nutshell, we want the color patterns to map consistently to the geometry (so, in this case, we want them to be perfectly circular), and we want the transitions between each color to be smooth. Trinity's Radeon HD 7760G integrated graphics has no trouble with either task. Ivy Bridge's HD 4000 IGP also manages mostly circular patterns with smooth transitions, but if you look closely, you'll see jagged lines where the red fades into the background checkerboard pattern. As for Sandy Bridge, well, the image speaks for itself.
In a real-world example, the differences are plainly visible. Trinity and Ivy Bridge both give us nice, sharp textures at off-axis angles of inclination, while Sandy Bridge fails in a very noticeable way. Those textures only look sharp on Sandy's IGP if we rotate the viewport to align the wall with the edge of the screen.
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