New Z value when Scanline++

As we move across a scanline (increment X), the new Z value is simply

Z_new = Z_old+ dz/dx

However, when we change scanlines, the new Z value will be

Z_new = Z_old + dz/dy + (leftX_new - leftX_old)*dz/dx

where Z_old is the starting Z of the previous scanline. Note that:

Eleft.dxdy = leftX_new - leftX_old

so

Z_new = Z_old + dz/dy + Eleft.dxdy * dz/dx

Finally, let:

Zdscan = dz/dy + Eleft.dxdy * dz/dx

Then each scanline increment, we compute the new Z as:

Z_new = Z_old + Zdscan

Note that Zdscan has to be computed only once for each subtriangle, at the same time that dz/dx and dz/dy are computed.

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