Bloom can be implemented with a single Gaussian blur. For better quality, we combine multiple Gaussian blurs with different radius. For better performance, we do the very wide blurs
in much lower resolution. In UE3, we had 3 Gaussian blurs in the resolution 1/4, 1/8, and 1/16. We now have multiple blurs name Blur1 to 5 in the resolution 1/2 (Blur1) to 1/32 (Blur5).
We combine the blurs differently to get more control and higher quality. For best performance, the high resolution blurs (small number) should be small and wide blurs should mostly make
use of the low resolution blurs (large number).
Scales the color of the whole bloom effect (linear). Possible uses: fade in or out over time, darken.
Defines how many luminance units a color needs to have to affect bloom. In addition to the threshold, there is a linear part (one unit wide) where the color only partly affects the bloom. To have all scene colors contributing to the bloom, a volume of -1 needs to be used. Possible uses: tweak for some not real HDR content, dream sequence.
Modifies the brightness and color of each bloom. Using a black color will not make this pass faster but that can be done.
The size in percent of the screen width. Is clamped by some number. If you need a larger number, use the next lower resolution blur instead (higher number).