when giving a polygon a vertex with W=0: it goes through the clipping stage normally

BUT

when that vertex has a negative Y, something weird happens

like

when the segment made of it and the previous vertex is clipped

the new vertex gets a positive Y, when it should be negative

only happens with W=0. as if it were trying to use -W somewhere and it shat itself due to 0 remaining positive, causing something to get the wrong sign


observation on large W values:

W=0x1000000: polygon is 'skipped'. it's still stored in RAM, but it isn't rendered. might be W getting truncated to 24 bits later on and triggering some W=0 safeguard.

W=0x1000001: rendered, but color interpolation is unusual. value is getting truncated in some way before getting to the rasterizer.



it's probably causing overflow somewhere in the viewport transform.

W=0xFFFFFF is normal. W=0x1FFFFFF is fucked. so W is not getting truncated.

larger power-of-two values result in nothing showing up, too.

0x7FFFFFFF fucks up even more. causes bogus clipping (extra vertices stored).

0x80000001 is clipped/culled out (nothing stored).

0x2000001 to 0x40000001 are same as 0x1000001. coordinates are fine.

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Kuribo64