Compare Z-buffer, A-buffer; scan-line

Z-buffer vs A-buffer

Scan-line algorithm to find visible surfaces

Data: edge table (end points, slope, polygon id), polygon table (plane coefficients A, B, C, D, colour, in/out flag), active edge list.

Initialise: build edge table and polygon table, set all flags to OUT.
for each scan line y from top to bottom:
    add edges that start at y to the active edge list;
    remove edges that end; update x of each edge (x = x - 1/m, since y decreases by 1);
    sort active edges by x;
    for each x-interval between consecutive edges:
        toggle (IN/OUT) the flag of the polygon owning the edge crossed
        count = number of polygons with flag IN in the interval
        if count == 0:
            fill interval with the background colour
        else if count == 1:
            fill interval with the colour of that polygon
        else:
            for each polygon IN, compute depth z = (-A*x - B*y - D)/C
            fill interval with the colour of the polygon of smallest z
    next scan line

Optimisation: if the polygons do not intersect, depth is computed only at the start of the interval, and scan-line coherence reuses the previous scan line's data.