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$2131 - Windowing Information

The masking windows are pretty simple. The windows can be used to mask off a
portion of any BG on the scanline. With HDMA, they can be adjusted per
scanline. They can be combined in various ways, per BG. Each can be used to
select either the region of the BG to keep, or the region of the BG to hide,
per BG. All that's left is to see the registers above and the section
"RENDERING THE SCREEN" below for details.

The Color Window
----------------

The color window is rather different. The color window itself can be set
to clip the colors of pixels to black (before math, so it's almost the same
effect you'd get by setting all entries in the palette to black, then fixing
them before you do subscreen addition--the only difference is that half math
will not occur), and to prevent all color math effects from occurring. These
can be applied never, always, inside the "clip" windows specified for the color
window, or outside the "clip" window.

Bits 6-7 of register $2130 controls whether the pixel colors (and half-math)
will be clipped inside the window, outside the window, never, or always. Bits
4-5 do the same for preventing color math.

Consider the main screen set up so BGs 1 and 2 are visible in an 8x8
checkerboard pattern, with all the BG1 pixels red and all the BG2 pixels blue.
The subscreen is filled with a green BG, and color math is enabled on BG 1
only. You'll end up with a yellow and blue checkerboard. Turn on the color
window to clip colors, and you'll get a green and black checkerboard since
the subscreen is only added (to a black pixel) where BG1 would be visible. If
you clip math instead, you'll get the same display you'd get with color math
disabled on all BGs.

In hires modes, we use the previous main-screen pixel to determine whether the
color window effect should be applied to a subscreen pixel. See "Color Math"
below for details.