Scanline Fill Triangle - Computer Graphics Step by Step

Fill a triangle one horizontal scanline at a time by computing the integer span on each row.

scanline-fill-triangle A scanline fill finds the left and right edge intersections for a row, then fills the integer pixels between them.

Example

Fill a triangle one horizontal scanline at a time. This is exact integer rasterization; real rendering anti-aliases these edges.

highlighted = computed this step

Step 1 — Set up

Use scanlines across this triangle.

\triangle ABC:\ (2,1),\ (6,5),\ (1,5)

Step 2 — Scanline A

Fill the integer pixels on this scanline.

y=1,\ x_L=2,\ x_R=2,\ \text{fill }\hlmath{(1,2)}

Step 3 — Scanline B

Fill the integer pixels on this scanline.

y=2,\ x_L=\frac{7}{4},\ x_R=3,\ \text{fill }\hlmath{(2,2)},\ \hlmath{(2,3)}

Step 4 — Scanline C

Fill the integer pixels on this scanline.

y=3,\ x_L=\frac{3}{2},\ x_R=4,\ \text{fill }\hlmath{(3,2)},\ \hlmath{(3,3)},\ \hlmath{(3,4)}

Step 5 — Scanline D

Fill the integer pixels on this scanline.

y=4,\ x_L=\frac{5}{4},\ x_R=5,\ \text{fill }\hlmath{(4,2)},\ \hlmath{(4,3)},\ \hlmath{(4,4)},\ \hlmath{(4,5)}

Step 6 — Scanline E

Fill the integer pixels on this scanline.

y=5,\ x_L=1,\ x_R=6,\ \text{fill }\hlmath{(5,1)},\ \hlmath{(5,2)},\ \hlmath{(5,3)},\ \hlmath{(5,4)},\ \hlmath{(5,5)},\ \hlmath{(5,6)}

Step 7 — Result

The filled triangle pixel set is complete.

\text{triangle pixels complete}