[deimos]

Originally Posted by a/b

OK, i see. Not a typical poly renderer.

Option 1:
Assuming you clear the entire bitplane beforehand, and the horizon is a simple straight line: find min_y and max_y (screen coords) of the plane while drawing sky/plane lines and then only fill between them.

Option 2:
How about a simple post-processing loop to clear bytes that are equal to 1 (only the rightmost pixel is set), something like:

char  table[256] = { 0, 0, 2, 3, 4, ..., 255 };  // 1 is mapped to 0, the rest is unchanged
char* p_plane = sky_plane_bitplane+width/8-1; // last byte of the first line
for (int h = 0;  h < height;  ++h)
  { *p_plane = table[*p_plane];  p_plane += width/8; }

I think option 1 is conceptually similar to my idea of splitting the blit.

I'm not understanding your option 2 though. Exactly why would we want to do that?

Edit: Oh, post-process the result...

Edit 2: I could just draw the line again to do that, I know where it starts and ends. I'll investigate this further. Adding the line is a kludge, I don't have a problem adding a second kludge to unkludge it.