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how and when do i invert sprites vertically ? (sprite_attribute & 0x80)

matt

When the sprite is evaluated -- the attributes are fetched first to see if they're to be flipped vertically.

The 'normal' CHR is determined by the subtraction of the current scanline and the sprite Y coord (also used to determine whether or not the sprite is in-range). Ex:



To flip vertically -- simply XOR is_in_range with $7 (8x8 sprites) or $F (8x16 sprites) after the sprite is found to be in range.

In case anyone misses it, Disch is using a trick of unsigned values in the comparison with 8. These are equivalent:

This works because a negative value converted to unsigned becomes a large positive value. It's a good trick to learn.

There's a detail which I handle inside the own rendering core: the PPU clipping. So, instead of ">= 0" I use ">= ppu_clip_sprites", 0 or 8.

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Zepper
RockNES developer

I think you're mixing up sprite clipping with in-range evaluations.

I guess so...
Anyway, thanks for the unsigned tip.

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Zepper
RockNES developer