Applying a layout scale to Slate line elements results in different thicknesses for AA vs non-AA lines.
The following screenshots show a green antialiased line and a blue non-antialiased line, drawn with the same endpoints and thickness.
At 1:1 layout scale with DPI scale 1, AA lines are the same thickness as non-AA lines (as expected):
When the editor is zoomed in (which changes the layout scale), the lines are different thicknesses:
This happens for any layout scale that zooms in on the element, not just when using the editor to zoom in. It affects lines drawn by C++ calls to FSlateDrawElement::MakeLine(s) and those drawn from UMG using the widget blueprint library’s DrawLine(s) functions.
Repro steps
- Make a new blueprint project in UE4.27
- Create a new widget blueprint and open it
- Switch to “Graph” view and override the “OnPaint” function
- Add two calls to Draw Line: one antialiased and the other not, same endpoints, same thickness (must be >1)
eg:
- Compile the blueprint
- Switch to the “Designer” view and ensure DPI Scale is 1 (I used
Screen Size → Monitors → 21.5-24" monitor)
Observe:
At 1:1 zoom with DPI scale 1 the two lines appear to have the same thickness. When zoomed in the non-antialiased line appears thicker than the antialiased line
Expected:
At any layout scale the non-antialiased line appears the same thickness as the antialiased line (AA filter width excluded)