Silhouettes
Tracing from an alpha channel
Where the silhouette comes from, in priority order, and what to do when the alpha channel is lying to you.
4 min read
Everything the tracer does starts with one question: which pixels are artwork? A file with real transparency has already answered it. Everything else is an inference.
Three sources, in this order
| Source | How good | Note |
|---|---|---|
| PDF transparency | Exact | Read from the file's own transparency. A white background rectangle is often sitting under the image, so going by what the page looks like would report the whole thing opaque. |
| Raster alpha channel | Exact | A PNG, a lossless WebP, or a TIFF saved with an alpha channel. The channel says what is artwork and nothing is guessed. A lossy WebP has no alpha channel at all, and nor does a CMYK TIFF. |
| Background threshold | A guess | Used only when there is no alpha at all — a JPEG, a flattened TIFF, a lossy WebP. The background colour is sampled from the four corners. |
If a file has no alpha, the honest move is to ask for a better source before shipping a threshold trace. It will often produce something that looks right and is a guess.
Alpha cut: how transparent still counts
An alpha channel is not a yes or no, it is 0 to 255, and the blade is. Alpha cut is where the line falls, and it ships at 128.
Raise it to shed semi-transparent pixels. The symptom is a cutline that bulges around a soft glow, a drop shadow or watercolour spatter that has been welded onto the silhouette. On the sample file that is kept for exactly this, raising it to 240 dropped the translucent blobs and left the figure.
Blobs that overlap the subject at full opacity stay welded whatever you set. That needs a clean source, not a slider.
Minimum area: dropping what is detached
Dust, specks and a detached element traced as separate cut paths are dropped by minimum area, which is a fraction of the largest component rather than an absolute size. The default is 0.02.
It runs before the closing step, and that order matters: the other way round, the closing would weld nearby specks onto the artwork first and there would be nothing left to see as separate.
When the alpha is lying
The whole page traced as a rectangle is the signature. It means the file carries an alpha channel that is uniformly opaque — every pixel reads as artwork, so the silhouette is the full frame. Some export settings write one whether or not it means anything.
The fix is to strip the alpha channel so the file falls through to threshold detection. This happened for real on a sign that came back traced as its own page border.
When there is no alpha at all
Background tolerance is the distance from the sampled background colour that still counts as artwork, and it is used only in this case. It ships at 16.
Two things to know about it. The background is sampled from the four corners, so artwork touching a corner poisons the estimate. And a subject close in tone to what sits behind it — near-black on black — has almost no gap to find: on a JPEG the edge ends up being read off compression ringing, which is a guess dressed as a silhouette.
Soft edges, fur and watercolour are their own problem, and have their own guide.
What the tracer does with it afterwards
The transparency is flattened over white, and the finished PDF carries one flat, opaque picture of the artwork. That is worth knowing for one reason: by the time the file exists, nothing in it can say which white was background any more. Anything that needs to know that — cutting the gaps a silhouette encloses, for one — has to be pointed back at the original artwork.
Try it on your own artwork
Upload a file and read the preflight before you quote.