Brightness

There is no per-point brightness on an XY display and there never was. What there is instead is better: brightness is time, and time is something you already control.

The mechanism

The display draws with one moving dot. Every pass deposits a little light. The longer the beam dwells somewhere, the brighter that place is.

That is the whole thing. Two consequences follow, and they are the only two real controls:

1. How much path you ask the beam to cover — density.
2. How fast it travels each part — speed.

Path density: the coarse control

A short figure gets redrawn constantly, so every stroke is hit again and again and reads bright. The same beam asked to cover ten times the distance in the same period gives each stroke a tenth of the attention, and the whole image sinks toward dim.

A short path is bright. A long path is dim. Wherever a path crowds against itself, it is bright again.

That last clause is free contrast. Where strokes converge — the centre of a rosette, the rim of a dense figure, the overlap in a wireframe — several passes land on the same place and it brightens without you doing anything. Composing so those crossings land where you want the eye is one of the cheapest tools available.

Edge count is therefore a light budget, decided before anything else. A model with thousands of edges will be dim no matter what you do afterwards, which is one of several reasons to remove hidden lines — see 3D.

Beam speed: the expressive control

Speed is where deliberate brightness lives.

You can see it in any ordinary figure without setting anything up. On a Lissajous curve the turning points at the outer edges are always the brightest part — that is where the motion reverses, so that is where the beam slows. The middles of long straight runs are always faintest.

Do it on purpose and you get a bright region that travels around a figure which never changes shape. Because it is pure timing it works on any drawing — a face, a wireframe, a logo — with no special preparation.

The controls

Phase-rate weighting is in the Shape Morph panel:

Start with Zones at 1 and raise Dwell. One bright patch appears and the rest dims to compensate: there is no extra light, only light moved from somewhere else. Drag Hotspot and the patch walks around a perfectly stationary drawing.

Then raise Zones. Around three or four the effect stops reading as a highlight and starts reading as a texture — and the sound changes with it.

Why the same control does two jobs

Changing how fast the beam moves changes the sound. The picture is made of the audio, so the timing of the path is part of the waveform.

Speed up and slow down around a shape and the shape stays identical while the tone changes completely. At one zone you perceive it as brightness; at several you perceive it as timbre. It is one operator, and which one you are "using" is a question about your attention, not about the signal.

The figure is provably unchanged throughout — the beam visits exactly the same points, on a different schedule. This is why the effect is safe to apply to anything.

There is a hard limit built in: past a certain depth the beam would have to run backwards to slow down that much, which retraces part of the path and genuinely changes the drawing. The control stops before that point.

The Z axis, and why released work avoids it

Plenty of bench scopes have a third input that modulates beam intensity directly. It is the closest thing to an actual brightness knob, and it has four problems:

1. Narrow usable range. Only a small slice of the input does anything visible.
2. It drifts. Analog path on analog hardware — yesterday's setting is not today's, and the intensity knob moves the range under you.
3. Bright costs sharpness. Harder beam, thicker line, so detailed images hit a ceiling before they turn to mush.
4. It cannot travel. Stereo carries X and Y. A third channel does not fit in a stereo release, so a piece that depends on Z plays back on nobody's scope but yours.

Perception is logarithmic too, so linear Z control looks wrong even when it works — the curve has to be shaped for the change to appear even.

None of this makes Z useless on your own bench. It just is not shippable, which is why the best-travelled work in this form solves brightness in the path instead.

Display controls are a different job

Intensity, gain, persistence and afterglow change the whole picture at once. A long-persistence look makes a fast figure read much brighter overall — but it lifts every stroke, not the one you chose.

Use the display controls to set the character of the instrument. Use path and speed to set the contrast inside the drawing. See Beam effects.

Comparison

Related

• Controlling brightness with no brightness control — the illustrated version
• Phase — the operator family this belongs to
• 3D — hidden-line removal as a brightness decision
• Beam effects — the display side