Mode 1 BG3 high-priority overlay for HUD-style layering.
Demonstrates the Mode 1 BG3 priority bit, which promotes the normally lowest-priority 2bpp layer to render above BG1 and BG2. This hardware feature is how most SNES games implement status bars and HUDs that overlay the gameplay scene without using sprites. Three separate backgrounds are loaded: BG1 and BG2 provide the scene (4bpp, 16 colors each with separate palette banks), while BG3 (2bpp, 4 colors) draws on top of everything when BG3_MODE1_PRIORITY_HIGH is set in setMode().
Based on the PVSnesLib example by odelot. Backgrounds inspired by Streets of Rage 2.
- SNES Concepts
- BG3 priority bit (BG3_MODE1_PRIORITY_HIGH): promotes BG3 above BG1+BG2
- Mode 1 layer depths: normally BG1 > BG2 > BG3, but priority bit inverts BG3
- Multi-layer VRAM layout with separate tilemap and tile regions per BG
- Palette banking via bgInitTileSet() palette offset parameter (e.g., 2, 4 for 32/64-byte CGRAM offsets)
- VRAM layout: tilemaps at $0000/$0400/$0800, tiles at $2000/$3000/$4000
- What to Observe
- Three background layers visible simultaneously
- BG3 (the HUD/overlay layer) renders on top of BG1 and BG2
- Each layer uses a different palette bank (no color sharing between layers)
- The scene is static (no scrolling or animation)
- Modules Used
- console, sprite, dma, background
- See also
- background.h, dma.h, video.h
Entry point – load 3 BG layers and display with BG3 high priority.
Sets up a Mode 1 scene with three background layers. BG1 and BG2 are 4bpp (16 colors each) providing the main scene, while BG3 is 2bpp (4 colors) used as an overlay/HUD layer. The key feature is BG3_MODE1_PRIORITY_HIGH passed to setMode(), which promotes BG3 above BG1 and BG2 in the rendering order. Without this flag, BG3 would render behind the other layers (lowest priority by default in Mode 1).
Each layer loads into its own VRAM region and uses a separate CGRAM palette bank (controlled by the palette offset parameter in bgInitTileSet()). The palette offset is in units of 16 colors: offset 2 means CGRAM colors 32-47 (BG1), offset 4 means colors 64-79 (BG2), and offset 0 means colors 0-15 (BG3).
- Returns
- Never returns (infinite loop).
1.9.8