A 16-bit game on a 32-bit surface shows a doubled picture, then wrong colours

Symptoms

Each row of the picture shows twice, side by side, in the left half of the window. After a first fix, the colours are wrong. Wine may log "Failed to blit".

Cause

The game writes 16-bit pixels into a surface that has the screen's format, which is 32-bit today. Each 32-bit pixel holds two of the game's pixels, so a row fills half the width. The game also reads the screen's pixel format to choose between RGB565 and RGB555. Wine's software blitter converts 565 to a 32-bit screen. It does not convert 555 (measured in POD).

Evidence

The evidence is screenshots and the game's own pixel-format query. In that query the green mask 0x7e0 means 565. The mask 0x3e0 means 555.

What to do

Give the back surface an explicit RGB565 pixel format (DDSD_PIXELFORMAT, 16 bits, masks 0xf800, 0x7e0, 0x1f). Make the game read its pixel format from that surface instead of the screen. Wine then converts each frame to the screen.

Tried

• RGB555: Wine fails to blit it to a 32-bit screen.