Is texture filtering an anti-aliasing technique?

Question

I know of anti-aliasing techniques with MSAA, but MSAA can only handle edges, but Shader sampling can handle internal pixels.

What's the difference between texture filtering and shader sampling, and is it also an anti-aliasing technique, such as Bilinear, Trilinear, which makes the image look smoother?

Answer 1

Multisample Anti-aliasing (MSAA) is designed specifically for frame buffers and solve one problem - aliasing issues on geometry corners (pixelated / jagged triangle or line boundaries). MSAA texture cannot be used (in a straightforward way) for texturing 3D object.

Texture filtering is a more general thing designed to solve aliasing and blurring issues while mapping 2D image onto 3D geometry. Trilinear and anisotropic filters use mip-maps (cascade of downsampled images).

Texture filtering normally produces anti-aliased results within the textured geometry, but aliasing on geometry corners has different origin and cannot be eliminated by using some better texture filtering algorithm - that's why MSAA and texture filtering are applied independently in 3D engines.

Apart from MSAA, frame buffer anti-alising can be done by Super-Sampling Anti-aliasing (SSAA) technique - which basically renders scene into offscreen frame buffer of larger resolution and then resized to screen resolution using Bilinear texture filter (apparently, Trilinear filtering is of no use for stretching 2D rectangle) or another downsampling filter. Offscreen buffer might be also of smaller resolution than screen resolution, in which case Bilinear texture filter can be applied for upscaling image (producing blurry result).

MSAA outwins SSAA in performance with a nasty trick - while SSAA truly computes several samples per fragment, MSAA evaluates the Fragment Shader stage only once per fragment. So that each sample within the fragment has initially the same color (including the one, that was retrieved from texture in a Fragment Shader), but depth values will vary per sample and each sample will be tested/blended individually with corresponding samples in MSAA frame buffer. This is why MSAA helps with anti-aliasing of polygon edges, but has no effect on polygon's interior. Fragment Shader with discard keyword does not benefit from extra samples in MSAA buffer either, as all samples are discarded or not discarded at once (producing jagged edges).

Modern APIs have an option called Sample Shading (sampleShadingEnable in Vulkan, GL_SAMPLE_SHADING in OpenGL 4.0+) when rendering into MSAA frame buffer. This option asks renderer to truly execute the Fragment Shader within MSAA samples, practically making SSAA from MSAA. minSampleShadingFactor option controls the ratio between truly computed and masked samples, so that 1.0 means all samples computed (like SSAA) and smaller ratio would mix SSAA and MSAA techniques - as a compromise in performance (e.g. 8 MSAA with 0.5 ratio = 4 truly computed samples making anti-aliasing of polygon interior and exterior and 4 more samples used only for anti-aliasing jagged edges).