Game Boy Advance Dev: Blending Pixels

The Game Boy Advance has some simple pixel blending abilities. Here's how to do them all.

Target audience: GBA devs

First I tried to write this for a wide audience, but it quickly grew into a book. If you're not familiar with GBA development, there might not be much for you here.

With that out of the way, let's dig into the three blending techniques available on the GBA.

Fade to black

This allows you to fade a scene to black, and then defade a scene from black, great for transitions. This one is simple.

Set up REG_BLDCNT

Tell REG_BLDCNT you want to fade to black

Here I am telling the GBA I want everything on screen to fade to black by passing in all background layers, the backdrop and sprites (BLD_OBJ). From what I can tell, libtonc does not include a constant for FADE_BLACK that is compatible with BLD_BUILD (maybe I missed it?), so I just defined my own.

Set the fade level

Then set the fade level in REG_BLDY. This register dedicated 5 bits to this value, so the value can be between 0 (no black at all), to 16 (completely black), with all values in between providing the fade. Ramping this up and down over time is how to achieve your fade.

The fade level values

5 bits should mean the maximum value is 31, right? The value is in 1.4 fixed format, and must be between 0 and 1 (inclusive). So that means anything 16 and above is just black.

Technically the fixed values mean the fade is 0.5, 0.75, etc. But for me, I just consider it between 0 and 16 and that works fine for me.

Not fading sprites

A neat little effect is to leave sprites out of the fading. This accomplishes a "spotlight" effect, kind of like in a play. Considering it's so cheap to do, it's a nice trick.

Fade to white

Fading to white is identical to fading to black, except REG_BLDCNT gets a different flag to indicate white.

Blending backgrounds

This time instead of blending the scene with black/white, we are now blending different layers together, to accomplish a half transparency effect (often called "alpha').

Set up REG_BLDCNT

This is similar to fading to black/white, except there are two differences.

First we need to indicate we want layer blending.

Which is just like before. But this time since we are fading between layers, we need to tell the GBA what constitutes a "top" layer and what is the "bottom". Now in BLD_BUILD, that zero will go away and be replaced with what we want the bottom to be.

Here we are blending background 0 into background 1.

And here we are blending sprites and background 3 into the backdrop.

One important caveat is the blending only happens if a pixel from a "top" layer is actually above (z-order wise) a pixel from a "bottom" layer. So you need to make sure you have background and sprite priorities/ordering set up properly.

Another caveat is when we indicate BLD_OBJ, that means all sprites will blend. We'll talk about individual sprite blending down below.

Setup the blend values

Now that we have told the GBA what we want to blend, we need to tell it how they should blending using REG_BLDALPHA.

Just like in fade to black/white, there are 5 bits for each layer that allow placing a 1.4 fixed value number. So just like before, 16 is the max, and technically these are fixed point numbers but I just think of them as 0 through 16.

By specifying 8 for eva (the top layer's blending) and 8 for evb (the bottom layer's blending), we essentially get the two evenly blended together. A pretty typical "transparency" result.

Changing these numbers can change the effect drastically. For example setting them both to one means they both only contribute a tiny portion of their color to the result, so the end result is very close to black. Setting it to something like BLDA_BUILD(14, 1) means the top layer contributes almost all of its color, and the bottom layer just a little bit, so the end result is heavily biased towards the top layer. You will probably have to play with these numbers to get a feel for what they do.

Blending individual sprites

And finally the best blending technique in my opinion, selectively choosing which sprites blend.

Tell a sprite it should blend

First set the blend flag on the sprite in OAM. Something like this.

You're just flipping the two blending bits to alpha on OAM attribute 0.

Tell the whole scene to blend

Just like above in normal alpha blending, we need to tell REG_BLDCNT to set up an alpha blend.

But this time, sprites that have their blend flag set participate in this setup regardless.

Let's say we want background 0 to blend into background 1.

This means background 0 will blend into 1 as expected. But it also means any sprites that have their blend flag set will also blend into background 1, assuming their priority places them above.

So if you just want individual sprites to blend and nothing else, leave the top layer blank.

Now only sprites that have their blend flag set will blend into background 1, and nothing else.

For truly transparent sprites, have them blend into everything.

Sprite to sprite blending is not possible.

I left sprites out (BLD_OBJ) because doing blending between sprites is not possible. The GBA will take all the pixels from all of the sprites and only consider the highest one (z-order wise).

gbatek explains it like this:

Before special effects are applied, the display controller computes the OBJ priority ordering, and isolates the top-most OBJ pixel. In result, only the top-most OBJ pixel is recursed at the time when processing special effects. Ie. alpha blending and semi-transparency can be used for OBJ-to-BG or BG-to-OBJ , but not for OBJ-to-OBJ.

One caveat

Since all blending is done through a single register, that means only one type of blending is possible at any given time.

All is not lost though, if you really want alpha blending and fading to black/white, the latter can be accomplished through palettes, yet another future blog post.