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fix(video): ignore enable bits for backgrounds a mode lacks

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DISPCNT enable bits for BG3 in Mode 1, BG0/BG1 in Mode 2, and every background
but BG2 in Modes 3-5 failed the whole frame with UnsupportedLayers, and the
window turned that into a stopped session. Hardware has no such backgrounds and
ignores the bits, so the renderer now masks them. The error variant is removed.

Prohibited OBJ mode 3 and shape 3 stay diagnostics: their hardware output is
not established here.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU
This commit is contained in:
0xrsydn 2026-10-14 08:35:32 +07:00
commit 0ef2719c6b
5 changed files with 31 additions and 36 deletions
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19
crates/gba-core/tests/graphics.rs
View file

@ -149,7 +149,7 @@ fn forced_blank_is_white_and_disabled_bg2_uses_palette_backdrop() {
}
#[test]
fn unsupported_modes_and_layers_report_errors_without_changing_output() {
fn unsupported_modes_report_errors_and_absent_layer_bits_are_ignored() {
let mut memory = mode3();
let mut frame = Framebuffer::default();
frame.clear(0x001f);
@ -160,14 +160,17 @@ fn unsupported_modes_and_layers_report_errors_without_changing_output() {
Err(VideoError::UnsupportedMode(mode as u8))
);
}
for layer in [0x100, 0x200, 0x800] {
memory.write16(DISPCNT, 0x403 | layer).unwrap();
assert_eq!(
memory.render_frame(&mut frame),
Err(VideoError::UnsupportedLayers(0x403 | layer))
);
}
assert!(frame.pixels().iter().all(|&pixel| pixel == 0xff0000));
// Mode 3 has only BG2. Stale BG0, BG1, and BG3 enables do not change the image.
memory.write16(VRAM_START, 0x03e0).unwrap();
memory.write16(DISPCNT, 0x403).unwrap();
let mut expected = Framebuffer::default();
memory.render_frame(&mut expected).unwrap();
for layer in [0x100, 0x200, 0x800, 0xb00] {
memory.write16(DISPCNT, 0x403 | layer).unwrap();
memory.render_frame(&mut frame).unwrap();
assert_eq!(frame.pixels(), expected.pixels());
}
}
#[test]