Blur
The three blur effects — Gaussian, Motion, and Radial — their kernels, and what each one does to alpha.
Reference
- Panel
- Effect Controls
- Section
- Add effect ▸ Blur
The Blur family holds 3 effects. All three blur premultiplied colour, which means alpha is blurred along with colour — a blurred layer's silhouette softens rather than staying crisp. Blurring premultiplied colour is what keeps a dark halo from creeping in around the edges of the artwork.
The sections below follow the order of the Blur submenu.
The blur effects
Gaussian Blur
A true Gaussian blur, run as two passes — one horizontal, one vertical. The kernel's standard deviation is derived from Radius at half its value, with a floor of 1. Samples at the edge of the frame are clamped to the edge pixel.
The kernel's half-width. Fractional slider positions are rounded to a whole number when the effect renders.
At Radius 0 the effect skips its render pass entirely, making it a true no-op rather than a pass that costs time and returns the same image.
Motion Blur
Smears the layer along a straight line by averaging evenly weighted samples either side of each pixel. The taps are unweighted — this is a box blur along the line, not a Gaussian one, so the smear has hard ends rather than fading out.
The direction of the smear. At 0 the blur is horizontal.
The half-length of the smear. Samples are taken this far to either side, so the full smear spans a little over twice this value.
At Length 0 the effect returns the layer unchanged. Sample positions are snapped to whole pixels, so shallow angles can produce visible stair-stepping along the smear.
Radial Blur
A zoom blur radiating from the centre of the frame. Sixteen samples are taken along the line between each pixel and the centre, and averaged.
The length of the zoom streak. At 0 every sample lands on the same pixel and the effect is a no-op.
Two things are worth knowing before reaching for this effect:
- There is no spin mode. Despite the family's name, Radial Blur only zooms. It cannot produce a rotational blur around the centre.
- There is no centre control. The origin is fixed to the centre of the frame and cannot be moved to a subject that sits off-centre.
The samples are taken on one side of each pixel only — toward the centre — so alongside the streak the image also reads as pulled slightly inward as Amount rises.