Quote - "They are limited in the number of polygons they can handle, and the texture size. Since scanline needs a collision test for every polygon in every pixel, it looses its performance when the scene gets to large."
Zbuffer is something that is very very fast for culling the pixels of each polygon, the time that is used can be easily ignored in the whole time needed to render a pixel.

Again as far as I know you use the scanline algorytm to fill a Z- or W-Buffer.

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