A capture's last rows and columns can repeat their neighbours

Symptoms

A port's screen matches a captured frame everywhere but a few dim pixels along the bottom or right edge. The differences show only on some screens: where the picture differs from its neighbouring row or column there. A pointer that reaches the bottom edge seems to repeat its last row.

Cause

The display that was captured did not show the game's frame pixel for pixel near the edge. In POD Gold's captures (Wine in the emulated PC, the window cropped to its client area), row 478 repeats row 477, and columns 636 to 638 show columns 635 to 637. Row 479 and column 639 are the window's border. Inferred: the display scales the window by a pixel or so.

What to do

Compare the capture against the frame as that display would show it. In POD this is pod_ui::check::presented: it copies row 477 to 478 and moves columns 635 to 637 right by one. Do not change the port's drawing to fit the edge. A rule fitted to the edge, such as a pointer that repeats its last row when clipped, explains one screen and breaks on the next.

To find the mapping, take a screen whose background varies along the edge. For each differing pixel, find which pixel of our frame equals the capture there. A whole row or column with the same offset is presentation, not drawing.