Noise Texture

What is a noise texture?

A noise texture is an image filled with random values, used as an ingredient in another image rather than shown by itself. Designers lay it over flat color as grain, 3D artists plug it into bump and roughness maps, and shader writers read from it to scatter stars or break up a repeating tile.

Several quite different patterns go by the name. White noise sets every pixel independently, like the static on an untuned TV. Smooth noise, such as Perlin's, varies gradually and looks like clouds. Blue noise is just as random as white, but it avoids putting similar values side by side, so it looks evenly sprinkled with no clumps or gaps.

Why do flat gradients look better with a little noise?

An 8-bit image has 256 levels per channel. A dark gradient from #202020 to #303030 across a 1,920-pixel banner contains only 17 distinct grays, so each one becomes a stripe about 113 pixels wide, and the eye picks out every step. That's color banding.

Adding noise of about one level before the image is rounded to 8 bits scatters those boundaries. Pixels near an edge fall to one side or the other by chance, the stripes turn into a fine speckle, and the eye averages the speckle back into a smooth ramp. This is dithering, and sound engineers do the same thing when they add faint hiss before cutting a recording down to 16 bits.

White noise or blue noise?

White noise has energy at every scale, so it contains chance clumps and holes. In the late 1980s Robert Ulichney showed that dots placed with blue noise, which keeps its energy at the finest scales only, look smoother than any other random arrangement at the same density, since the eye is least sensitive to the finest detail.

Games and renderers now ship small tileable blue noise textures, often 64 or 128 pixels square, and use them to jitter shadows and reflections. The leftover error then looks like fine sand instead of blotches. Christoph Peters publishes a free set that many projects use.

Is film grain the same as digital noise?

They look alike and have different causes. Film grain comes from the silver halide crystals in the emulsion, each of which either develops or doesn't, and in color film from the dye clouds left behind. Faster film has bigger crystals, so a 3200 ISO stock looks much coarser than a 100. Noise in a digital photo comes from too few photons and from the electronics, and it is worst in the shadows.

Grain is costly to compress, because it's fresh random detail in every frame. The AV1 video codec can remove grain before encoding, send a short description of it, and have the player draw new grain on top during playback.

Adding a noise texture to your own picture

Film Grain Overlay lays animated grain over a photo or clip. Its Fine preset, Amount 0.12, breaks up banding in a sky or a gradient without reading as texture. Mono switches RGB off so each grain is gray, closer to black-and-white stock; with RGB on, every channel gets its own grain, as on color film. Dusty uses the Multiply blend at 0.4, which only darkens, like dirt on a print.

A noise field can move pixels as well as tint them. Displacement Map's Crinkle preset reads a fine noise field, Scale 40, and nudges each pixel a little way along it, like looking through rippled glass.

Further reading

• Free blue noise textures (Christoph Peters)