Hardware Heritage
Radial blur, often referred to as "zoom blur" or "god rays," was a holy grail of 90s demoscene graphics. It simulated the effect of a camera moving rapidly toward or away from a light source, creating a sense of speed and dynamism that pushed hardware to its absolute limits.
PC Buffer Mix
Software radial blur on PC was a late-era 486 trick. It required blending the current frame with a scaled-up version of the previous frame. Without alpha-blending, this was a slow process of averaging 64,000 bytes per frame.
Amiga Feedback
Amiga coders simulated blur by not clearing bitplanes or by using the Blitter to "zoom" the background with a 50% transparency mask--an iconic look seen in high-end AGA chipset intros.
Modern Multi-Sample
This implementation takes multiple samples of the image along a radial vector and averages them. This provides a perfectly smooth result without the artifacts of buffer-based feedback.
Radial Blur
Simulated motion blur and volumetric god-rays.
Legacy C (Software)
// Feedback loop
for (i=0; i < 64000; i++) {
screen[i] = (screen[i] +
last_frame_zoomed[i]) >> 1;
}
Modern GLSL
for(float i=0.0; i < 12.0; i++) {
float scale = 1.0 - (i/12.0) * 0.2;
col += texture(tex, uv * scale);
}