ben-reilly/wpf-dash-render-test

Demonstrating a bug in WPF rendering of dashes at small scales

WPF Dash Render Test

It appears that WPF cannot render dashes smaller then 0.05 pixels before scaling, even if they are later scaled up beyond that limit.

This project demonstrates the bug by drawing groups of lines at different scales.

• Each line is the same length after scaling.
• Lines are first grouped by the value given to the DashStyle, scaled 1x, 10x, 100x, and 1000x.
• These groups are then clustered by the decimal log value of the base dash length, grouped by 1x, 2x, 4.5x, and 5x.
• Clusters have base dash lengths of 0.1px, 1px, 10px, and 100px.

The result is that within each group of four lines, all four lines should be drawn the same, and across groups in each cluster of 16 lines, the dashes should get progressively longer. (See the last cluster for a mostly correct example.)

Colour reference

Lines with dashes that are 0.05px or longer before scaling are rendered green to highlight the correct rendering.

Lines with dashes that are shorter than 0.05px before scaling are rendered red to highlight the incorrect rendering.

Gray vertical lines are spaced 10px apart to help with eyeball measurements.

Bugs

There are at least three bugs here:

1. Dashes are rendered a minimum of 0.05 pixels before scaling.
2. When scrolling horizontally, the last dash in a line may blink in and out.
3. The last dash in a line may be longer than the other dashes in the same line (e.g. cluster 4, group 3, line 4).