Pathfinder not aligning or cutting precisely

Question

Learning vector illustration I am having issues with the Pathfinder.

I am working on making an Alpaca out of basic shapes.

For the face, I use ellipses:

1. darker ellipse in the back,
2. brighter ellipses in the front to shape our the head.

I copy-pasted the darker ellipse in front of the other one, selected it with the 2 brighter ellipses and used Minus-Front.

The issue is that I get this outer edge from the darker ellipse below. They should be exactly at the same location so cutting them out should not give this outer edge.

Any clue?

Before the pathfinder:

After the pathfinder:

I recolored the shape to white when the ellipse in the background is darker.

Pierre

Answer 1

The outer edge has nothing to do with precision of the shapebuilder. It has to do with how the rendering engine works. Or rather how it does not work as you would expect.

The programmers are taking a shortcut, they are slapping each shape on top of each other. Then using the fractional coverage calculation as alpha. But alpha represents a transparent window pane not a fractional pixel. They have conflated coverage as alpha. This is an error! Its called a conflation artefact for more on this see:

Image looks embossed when converted to SVG

Yes photo editors do similar errors in photoshop all day long without much problems. But your having a very pathological case.

How much conflation you get depends on the rendering engine.

Answer 2

The example looks like a classic case of an atialiasing problem called conflation (as described by @joojaa), which is common with all vector image editors, not just Illustrator. It's caused by having the same shapes stacked on top of each other. There must be an underlying darker oval shape which is causing the problem.

If you construct this differently, you can avoid the problem.

Try it this way:

1. Draw all the shapes, and align as required

2. Copy and Paste in Place the larger oval

3. Select both circles and do Object > Compound Path > Make

4. Select the compound path (the circles) plus the oval, and then in the Pathfinder hit Intersect.

5. Fill the pieces with the required colours.

This will leave you with only three pieces, without extra underlying shapes, i.e. just the original oval, plus an extra two pieces which intersect with it.

Example