Alpha blending sprites in Nintendo DS Homebrew

Question

I'm trying to alpha blend sprites and backgrounds with devkitPro (including libnds, libarm, etc).

Does anyone know how to do this?

Answer 1

As a generic reference, i once wrote a small blog entry about that issue. Basically, you first have to define which layer is alpha-blended against which other layer(s). Afaik,

• the source layer(s) must be over destination layer(s) to have some blending displayed. that means the priority of source layers should be numerically lower than the the priority of destination layers.
• the source layer is what is going to be translucent, the destination(s) is what is going to be seen through (and yes, i find this rather confusing).

For the sprites, specifically, you then have 3 ways to achieve alpha-blending depending on what you need and what you're "ready to pay" for it:

• You can make all the sprites have some alpha-blending by turning on BLEND_SRC_SPRITE in REG_BLDCNT[_SUB] ... not that useful.
• You can selectively turn on blending of some sprites by using ATTR0_TYPE_BLENDED. The blending level will be the same for all sprites (and layers)
• bitmap-type sprites use direct colors (bypassing the palettes), so the ATTR2_PALETTE() field of GBA sprites is useless and has been recycled into ATTR2_ALPHA.

Answer 2

Sprites on the DS can be alpha blended using the blend control registers. TONC gives the necessary information for getting blending working on the main screen because the register locations are the same. Alpha blending on the subscreen uses the same process with different registers at a 1000h offset.

The registers you'll be looking at are REG_BLDMOD, REG_COLV, and REG_COLY for the main screen and REG_BLDMOD_SUB, REG_COLV_SUB, and REG_COLY_SUB for the sub screen.

Also remember that you'll have to change the sprite's graphic mode to enable blending per sprite.