base:backface_culling

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+ | ==== Backface Culling ==== | ||

+ | by Bitbreaker/ | ||

+ | |||

+ | An easy way to find out if a face faces towards the viewer or away is to check whether the area covered by a face is positive (frontface) or negative (backface). | ||

+ | This done by taking the first 3 vertices (already transformed into 2D) of that face and calculating the following: | ||

+ | |||

+ | (v1.y - v0.y) * (v2.x - v1.x) - (v1.x - v0.x) * (v2.y - v1.y) | ||

+ | |||

+ | If the result is positive, the face is visible and rendered, else it is discarded. | ||

+ | |||

+ | < | ||

+ | ; | ||

+ | ldx faces+0,y | ||

+ | lda vertices_x, | ||

+ | sta verticebuf_x+0 | ||

+ | lda vertices_y, | ||

+ | sta verticebuf_y+0 | ||

+ | |||

+ | ldx faces+1,y | ||

+ | lda vertices_x, | ||

+ | sta verticebuf_x+1 | ||

+ | sec | ||

+ | sbc verticebuf_x+0 | ||

+ | |||

+ | ;set up first factor | ||

+ | sta z1_+1 | ||

+ | eor #$ff | ||

+ | sta z2_+1 | ||

+ | |||

+ | lda vertices_y, | ||

+ | sta verticebuf_y+1 | ||

+ | sec | ||

+ | sbc verticebuf_y+0 | ||

+ | |||

+ | ;set up second factor | ||

+ | sta z3_+1 | ||

+ | eor #$ff | ||

+ | sta z4_+1 | ||

+ | |||

+ | ldx faces+2,y | ||

+ | lda vertices_x, | ||

+ | sta verticebuf_x+2 | ||

+ | sec | ||

+ | sbc verticebuf_x+1 | ||

+ | |||

+ | ; | ||

+ | tay | ||

+ | |||

+ | lda vertices_y, | ||

+ | sta verticebuf_y+2 | ||

+ | sec | ||

+ | sbc verticebuf_y+1 | ||

+ | |||

+ | ; | ||

+ | tax | ||

+ | |||

+ | z3_ lda tmath1,y | ||

+ | sec | ||

+ | z4_ sbc tmath2,y | ||

+ | sec | ||

+ | z1_ sbc tmath1,x | ||

+ | clc | ||

+ | z2_ adc tmath2,x | ||

+ | |||

+ | ;skip if negative | ||

+ | bmi + | ||

+ | jsr drawface | ||

+ | + | ||

+ | </ | ||

+ | |||

+ | Though things are fast, all this can be a bit unprecise when faces get small, so it is smart to let the filler test if x1 > x2 when drawing a line of a face and thus stopping filling if this happens, else you might end up in slight glitches. | ||

+ | |||

+ | ==== Backface Culling - Alot faster method ==== | ||

+ | |||

+ | by JackAsser / Booze Design | ||

+ | |||

+ | Please see the attached PDF. {{: |

base/backface_culling.txt ยท Last modified: 2015-04-17 04:30 (external edit)