Each documented preset has an enhanced version (`enhanced: true` in
`imageTrace.make`, an Enhanced checkbox under the panel's preset): the preset
with Auto Grouping, and with Shapes in all but High Fidelity Photo. Gradients
stay an option. Flat Logo, a preset of our own, has none. The legacy presets,
and documents traced with them, are as they were.
So that an enhanced preset is no worse than its preset on that preset's inputs:
- A hole that one colour fills is the same shape as the path that fills it, so
the two still share one outline (a shape beside a fitted hole left slivers).
- A rectangle is as large as where its sides lie on average, not their farthest
points.
- Auto Grouping keeps the paths of a group in the order they were traced in. It
put a path first and what is inside it on top: a white counter traced under
its black letter was then painted over the letter's edge, a light seam
(Default with Auto Grouping, edge dE 10.24 -> 9.88; legacy 10.05).
The harness measures every enhanced preset beside its preset.
Create: Strokes drew every line with round caps: a square-ended line 10 px wide
had its corners 2.94 px outside the stroke.
- A line's ends are square when its ink reaches as far a third of the width to
either side of its middle as along it (a round end stops short there by a
seventh of the width); told from lines 3 px wide and more, all the ends of a
line together. The stroke then has square caps (`TracedPath::square_caps`).
- The last points of a wide line's skeleton can turn towards a corner of its
end. An end that has left the line the skeleton followed a width back (by a
quarter of the width, 1 px at least) is put back on it; and an end may be
pulled back as well as pushed out to where the ink ends.
Outline on the thin-line inputs: largest distance from the art 2.94 -> 0.88 px
(clean 2.94 -> 0.41; aliased 0.88 -> 0.81), mean 0.055 -> 0.050 px, anchors
14 -> 10.
- A sharp corner may fall up to 0.75 px short within 3 px of where it should be:
anti-aliasing and blur take that much off a corner (not on a thin bar, which
is all corners).
- A rounded rectangle gets the radius that fits its outline best, searched
around the one its area says, instead of that one alone.
- A rectangle's direction is tried along its longest straight side, the longer
axis of its area and the pixel grid: a blurred one may have no straight side.
- A regular polygon's sides, corners and size are read from the outline's shape
(how its distance from the middle goes round, and its area), not from how many
anchors the fitter gave it.
- An outline without anti-aliasing is known to half a pixel: it gets half as
much room again.
Plain shapes found with Shapes on, all sizes and variants: Default 57% -> 98%,
6 Colors 62% -> 100%. The harness counts false shapes: none on any input but one
thin line (a rectangle 2 px wide) whose trace is 0.70 px off with or without
Shapes.
Gradients (params `gradients`, a checkbox in the panel for Grayscale and Color;
off unless asked for): an area whose colour changes evenly along one direction
is traced as one path filled with a linear gradient, not as bands of flat colour.
Gradients are found before the palette is made, so no colour is spent on their
bands: a connected run of smoothly changing pixels is a gradient when a plane
fits each colour channel with all three sloping the same way, the colours along
that way are at most six stops with ramps between them, and painting them back
gives the area's own pixels. Flat colour, soft edges between flat colours and
shading that goes two ways are left as they were.
The harness gains two linear-gradient inputs with known lines and stops, and
measures whether each is found, the angle of its line and the colours of its ramp.
Auto Grouping (params `autoGrouping`, a checkbox in the panel; off unless asked
for): each traced path that has others inside its outer contour comes out in a
group with them, the path first, nested as deep as the picture is. The path that
covers the whole image is nobody's group.
The grouped result paints the same picture: where paths can overlap (stacked
layers, Flat Logo, strokes) a path only joins a path below it, and only if that
moves it under nothing that shares any of its box.
The harness measures three presets with the option on ("grouped %"), painting
them in the grouped order.
Shapes (params `shapes`, a checkbox in the panel; off unless asked for): a filled
outline that is a circle, an ellipse, a rectangle or a regular polygon comes out
as that live shape. A shape is only made when every point of the traced outline
lies within 0.5 px (0.2% of the shape's size, if more) of it; an ellipse whose
radii differ by less than twice that is a circle of its area.
The outline before fitting: a corner's own pixel no longer pulls the two edges
that meet there, and a one-pixel step between two long edges is the corner where
they meet. Sharp corners kept: Default 69% -> 76%, the colour presets 86% -> 91%.
The harness gains plain-shape inputs and "shapes %".
Create: Strokes (the Outline preset, or the option) traced lines along the pixel skeleton: right to a pixel,
with a width counted in pixels, and fitted by a separate, older fitter.
- Each skeleton point is moved to the middle of the line's ink, read from the image across the line: every
pixel counts as much ink as its colour is the line's rather than the paper's, the ink's centroid is the
middle and its sum the width (an anti-aliased or blurred line says both to a fraction of a pixel).
- The stroke width is the median of those widths, no longer a whole number of pixels.
- Each end goes where the ink ends, less the half width a round cap adds.
- Centre lines are fitted as outlines are: a straight line is two anchors, corners stay corners, Snap Curves To
Lines levels them. The old Douglas-Peucker and simplify fitter, and the crate's pathops dependency, go.
Outline on the thin-line inputs, all sizes and variants (docs/image-trace-quality.md), before -> after:
- painted outline 0.310 -> 0.055 px off on average; 81 -> 14 anchors for five lines
- centre line 0.295 -> 0.052 px from the true centre line; stroke width 0.13 -> 0.02 px off; 18.4 -> 2.7
anchors a stroke (blurred, noisy and scanned lines: 2.0 each; aliased: 4.6)
- largest distance 1.84 -> 2.94 px: the square ends of a 10 px line under a round cap
Presets without Create: Strokes are unchanged.
The documented option "replaces slightly curved lines with straight lines and snaps lines near 0 or 90 degrees
to absolute 0 or 90 degrees" (helpx, Image Trace panel options); ours did the first half only.
- With Snap Curves To Lines, a straight piece at least 2 px long and within 2.5 degrees of level or upright is
made exactly so, by moving its ends across it to their mean (the page gives no angle: 2.5 degrees is our
choice). The pieces next to a moved end follow it.
- A junction other regions share stays where it is, so abutting regions still have one curve along their edge;
a line between a junction and a free end is levelled to the junction.
- The harness gains askew inputs (a rectangle, a plus sign and two lines, 1 to 2 degrees off level and upright)
and an "upright %" column: the share of the outline length that is exactly level or upright.
On the clean askew inputs (docs/image-trace-quality.md): Black and White Logo and Technical Drawing, the presets
with the option on, go from 0% to 91% and 95% of their outline exactly level or upright. Their outlines are then
0.49 and 0.59 px from the art as drawn where they were 0.05 (4.2 px at the far end of a 300 px side turned 1.5
degrees): that distance is what the option is for. Presets without the option are unchanged.
Against the documented panel (https://helpx.adobe.com/illustrator/desktop/manage-objects/traces-mockups-symbols/image-trace-panel-options.html):
- Ignore Color: "Ignores a specific color while tracing. Use the color picker to selectively ignore a color."
The panel's Ignore White is now Ignore Color with a colour button beside it (white until another colour is
picked); `imageTrace.make` takes `ignoreColor: "#rrggbb"` beside `ignoreWhite`, and leaves out the palette
colours within about 30 levels of it. Documents saved before have no such setting and ignore white as they
did. Flat Logo mode leaves out white only.
- Auto Color ("Creates a posterized image from a photo or artwork"): Color mode with the Automatic palette.
- Outline ("Simplifies the image to black outlines"): Black and White with Create: Strokes and no fills.
- The Outline preset's name is translated as a preset's name, not as the Outline view's.
Measured (docs/image-trace-quality.md): Auto Color on flat art is 0.140 px off on average with 4.0 paths (16
Colors: 0.141, 4.2). Outline on the line inputs, the first measure of Create: Strokes: 0.310 px off on average,
1.84 at worst, 81 anchors for 5 lines (Line Art's filled outlines: 0.055, 0.96, 46).
crates/trace/tests/quality draws its inputs from known vector art (no image files), traces them with every
built-in preset and measures fidelity, distance from the true outlines, economy, artefacts, time and memory.
docs/image-trace-quality.md records the numbers on main before any change, beside the same measures of an
open tracer run as a black box. No change to the tracer.
bflatastic