2D motion blur with velocity map

Blurring just the moving areas is easy, you're just taking a handful of samples starting from the current pixel along the motion vector. If there's no motion, all the samples end up on the same pixel, and thus there's no blur. (however, you can try to early-out in this case; I leave that as an exercise)

Trails, on the other hand, are bloody murder. The most common solution I've seen is basically a smart blur that smears the motion vector from adjacent pixels if said vector happens to intersect the pixel being shaded. The problem with this is that adjacent pixels with differing velocity can cancel each other out entirely (this can happen if, say, two objects that overlap onscreen are moving in the opposite direction, and is more common in practice than you'd think) which is pretty much impossible to solve nicely

Thanks. So, as I understand blurring is simple anought - I just need to sample fragments along velocity vector. Adding trail is much harder and it can't be done as postprocess - I need to draw object several times 'between' frames.

if you want to blur the box, not the background, you could use compute shader/opencl/cuda. in your velocity map, you'd read the pixel color of the box and the velocity map to figure out the position of the last frame for this pixel, and you'd blend the pixel with the existing image.
two important points:
-blending would need to be done with atomics
-blending the whole line per pixel might be too expensive, you might want to use some stochastic writing of the pixel (random positions inbetween)

What i found to work well was to only consider points for blurring that have similar velocity.

Thanks. So, as I understand blurring is simple anought - I just need to sample fragments along velocity vector. Adding trail is much harder and it can't be done as postprocess - I need to draw object several times 'between' frames.

Not necessarily, just that dilation is an expensive and error-prone process. (at least for a high-quality implementation)

Everything above happens in post.