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Cell Transparency may be semi-transparent. The semi-transparent setting only makes sense for ray tracing with COLOR. The row and column position of the pixel are used to determine if the ray stops at the semi-transparent surface or proceeds through it. If the sum of the row and column are odd, the ray terminates and the pixel color and shade are determined by the surface. Otherwise, the ray continues and the pixel is set by whatever the ray strikes next.
Semi-transparent surfaces can only be rendered correctly in the Super resolution mode chosen in the Interpolation Box submenu of the Graphics menu or by the command
where a ray tracing is made for every pixel.
The ETCH algorithm looks for changes in surface from pixel to pixel. These changes are guaranteed at each pixel on a semi-transparent surface, leading to a filled-in black surface. Because the absolute pixel position is used, a semi-transparent surface behind a semi-transparent surface will not be seen, nor will the far side of a semi-transparent cell. These limitations could be overcome, and different semi-transparent algorithms introduced, with further work.