Motion streaks and the brightness you lose

Below about one shape-radius of travel per exposure, motion reads as motion and needs nothing. Past a couple of radii it starts wanting a streak. And past that, the streak stops being a stylistic choice and starts being an exposure problem.

The same emitter at three speeds, labelled on each panel in shape-radii traveled per exposure. The faint ring is the same shape unstreaked. The figure at the bottom is the peak brightness left after the light is spread along the smear.

The threshold, in radii and not in pixels

The useful unit is how far the shape moves relative to its own size, not how many pixels it covers, because a big slow object and a small fast one can travel the same pixels and look completely different.

The part that gets forgotten

A streak spreads a fixed amount of light over a longer area, so its peak brightness falls. Implementations that draw a streak at the original brightness make fast objects appear to glow harder, which is exactly backwards and is why fast neon often looks like it is gaining energy from nowhere. 

float area0 = PI*r*r;            // the shape at rest
float area1 = area0 + 2.0*r*travel;   // the swept capsule
float brightness = area0 / area1;     // conserve the light

At seven radii of travel that is roughly a fifth of the original peak. The third panel is genuinely much dimmer than the first, and that is correct rather than a bug.

Which is a trap for anything with a bloom threshold

If your bloom only fires above some brightness, a small bright element can streak itself below that threshold and stop blooming entirely, mid-motion. It reads as the glow breaking rather than as the object moving fast, and it appears only on the fastest beats, which is the worst possible place to discover it.

Either keep the peak above threshold by shortening the streak, or use a bloom without a hard threshold. This is the same trap the motion blur note ends on, arriving from a different direction.

Rules of thumb

  1. Measure speed in shape-radii per exposure, never in pixels.
  2. Below one radius, do not streak. Above five, the streak is the whole read.
  3. Conserve the light: divide peak brightness by the area ratio.
  4. Streak along the velocity, not along a fixed axis, or diagonal motion looks wrong.
  5. Watch for small bright elements dimming below a bloom threshold on the fastest beats.

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