/\
/ \
+/----\---+
/ \ |
/| \ |
\| / |
\ / |
|\ / |
| \ / |
| \/ |
| |
+---------+
Sum with clipping :
+/----\---+
/ \ |
| \ |
| / |
\ / |
|\ / |
| \ / |
| \/ |
| |
+---------+
In order to cut the parts outside the screen it is needed to take into consideration each face's line separately, to check the co-ordinates of the two points and, in case that only one point comes out from the screen, to calculate the intersections whit the screen.
In some cases it is useless to draw a line:
* both points are out of the screen
* the line is horizontal
In case that the line is horizontal it is useless to draw, here is an example:
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The blitter fills a memory's area starting when it finds a pixel and finishing when it finds another one. In our example the face is perfectly filled only thanks to the two vertical line.
Here is the source's code in assembler for 68020+
move.l #X0,d0 move.l #Y0,d2 move.l #X1,d1 move.l #Y1,d3 * START CLIPPING Clip: cmp.l d2,d3 ; se y1 = y2 -> DON'T DRAW THE LINE bne.s Continua NO_LINEA: rts * X0 < X1 Continua: cmp.l d0,d1 bgt.s X0MinoreX1 exg.l d0,d1 exg.l d2,d3 X0MinoreX1: * CHECK OF THE LEFT SIZE tst.l d0 bpl.s NOClippingSinistro ; if d0 positive than NOClippingSinistro tst.l d1 ; d0 < 0 , d1 < 0 outside the screen bmi.s NO_LINEA ** LEFT CLIPPING ClipSinistro: moveq #0,d4 moveq #0,d5 move.w d1,d4 ; d4 = x1 sub.w d0,d4 ; d4 = x1 - x0 move.w d3,d5 ; d5 = y1 sub.w d2,d5 ; d5 = y1 - y0 swap d5 ; d5 << 16 divs.l d4,d5 ; WARNING : 68020+ ; d5 = d5 / d4 = pre ogni X, quante Y si sposta ? neg.l d0 ; d0 = d0 negato mulu.l d0,d5 ; WARNING : 68020+ ; d0 = d0 * d5 swap d5 ; d5 >> 16 add.w d5,d2 ; New Y -> X = 0 moveq #0,d0 NOClippingSinistro: * CHECK OF THE RIGHT SIZE cmp.l #320,d1 ; if d1 < 320 than NoClippingDestro blt.s NoClippingDestro cmp.l #320,d0 ;