Background
✅ verified · a person never chooses this one: the background is not a group, and both routes lay it · code: appendix B
How an artist paints it
Lay the whole background first and paint the figure over it – the background is never cut to the figure’s shape. A flat or blurred-out background takes a few passes of the largest soft brush; a background with sky, buildings or trees is each its own part (their own pages).
Note the measure in “a few passes”. No artist makes ten thousand marks for something with no edges and no detail – there was never that much to say about it. What this page is really about is how to write “a few passes” as numbers.
What it looks like in the source
The worked example’s background is a ring of light around the subject, darkening outward, with no pixels at all where the subject covers it.
Our method ✅
A low-resolution colour field, not twelve thousand strokes.
First make the reference: cut the subject out to black and extrapolate back into it from the pixels outside (a normalized convolution at sigma = 24). Without that extrapolation the area is black, and the moment the silhouette and a part’s outline differ even slightly, a black rim shows. Then reduce the reference to one thirty-second of the source’s width (FIELD_DIV) and save it as a small PNG; when painting, it is scaled back to full size with bicubic interpolation, fills the canvas and goes on the bottom layer – the subject’s area is covered by the part groups above, so the subject need not be cut out and no unfilled gap is left around the silhouette.
The resolution was measured rather than guessed:
| The method | description numbers | the background area against the source |
|---|---|---|
| three flat passes plus one soft pass (the old way) | on the order of twelve thousand strokes | the baseline |
| a field at 1/64 | 1/111 of the old way’s | level with the old way |
| a field at 1/32 (what it does now) | 1/28 of the old way’s | better than the old way |
| a field at 1/8 | 6/10 of the old way’s | a little better still |
(The exact readings are in ch. 19.) 1/32 is the point at which it is both closer than the brush version and explicable; going finer costs ten times the numbers for a little more, which is re-encoding the source in dots (ch. 7).
It writes pixels, and it says why. BG_Base filling black is three numbers describing the whole canvas; BG_Field is a small image, and the criterion accounts for how many pixels that image itself has, not how much it covers once stretched. Both pixel writes carry a why in the painting journal. The old criterion “at most 2 pixel writes” is precisely what this page overturned: it forced something blurred out to be laid with a brush (ch. 7).
Layers
01_BG/: BG_Base (filling black) -> BG_Field (the colour field). Below every part group.
Knobs
FIELD_DIV (32 by default): what fraction of the source’s width the field is. The extrapolation at sigma = 24 is fixed in the method itself. A background of sky, buildings or trees is a different part (its own page) rather than a different knob.
Criteria
No black rim around the silhouette in composite-1x.png; zero bare pixel writes in the painting journal; a darkest pixel of (0,0,0) across the image; and the background’s description count should be among the smallest in the picture.
Where it breaks
- Not extrapolating the background: the slightest difference between the silhouette and an outline shows a black rim.
- Too fine a field: it saves no numbers and starts copying the source. Too coarse: the gradation in the ring of light around the subject disappears.
- Laying a gradient with a brush: error-driven
softstrokes alone leave black patches (once the error falls below the threshold no stroke starts, and the ground shows between strokes); adding flat passes covers them, but that is spending twelve thousand strokes on something one blur does.
On origin
Ch. 18, “The layered route”: the field’s resolution, its description count, and the readings over the background.