Fast defocused point rendering

[im.thatoneguy]

I am needing to render a bunch of snow with shallow DOF so I created a Pflow data operator that scales a camera facing card per point to be pixel sized regardless of distance from camera and then am scaling it based on the Physical camera's focus point. By putting a Bokeh image in the texture slot it properly scales (and attenuates based on scale) the shape. I am able with this trick to render a full 1080p frame with clean 20,000 snow flakes or dust motes in about 3 minutes with motion blur and 'DOF'. No motion blur is about a 10-15 second rendering at 1080p. This also avoids any post DOF artifacts which is nice although it can have problems with hold-out mattes since the cards can intersect matte objects.

Next feature to add is 'proper' bokeh scaling based on focal length and F-Stop.

[Morne]

Nice trick, thanks for the share

[Laserschwert]

Can't the particles be scaled along their motion vector to fake motion blur as well? I know, it won't be "correct" motion blur, but it might be enough in some cases.

[im.thatoneguy]

I like the way you think Laserschwert. It's not a bad idea. It wouldn't work particularly well with this defocus kernel since it has defects in the bokeh that would just stretch and look obviously scaled but a more 'perfect' defocus kernel or gaussian blur kernel would look pretty good.

The two reasons I can think of though not to do it would be 1) It would be really really really hard to calculate the correct vector/length for complex camera motion blur, especially interacting with particle velocity. 1b) You would also obviously miss out on swirly multi-segment motion blur if the snow was moving very quickly. 2) less importantly the blur shape wouldn't really be correct. It should be a capsule shape vs an oval.