A second scaling method is to divide the image into groups of three scanlines. For each group, one white output scanline is generated. Each scanline of the group is then taken in turn and for each horizontal position in that scanline that is black, black is put in the corresponding position in the output scanline. This ensures that lines are preserved in the reduced image. However, this method takes much longer, because each position of each scanline is inspected to produce the final image.

When the original line drawing is like a blueprint with white lines on a black background, you would want to preserve the white (the foreground) in each scanline. You would use preserve foreground as the scaling option in this case. Also, the image may not be black and white, but green and red, or any other color combination.

When the image region is defined, foreground and background colors are established, either by specifying them or by default. You can then specify either preserve foreground or preserve background to indicate which color ESL should preserve. So if the line drawing of a house is rendered white on blue in the original, with the foreground white and the background blue, simply specify preserve foreground in the image region definition.