How to change dpi in PostScript

问题:

I am a newcomer in PostScript language. Now I am using ps to print a scaled image of dots. The default dpi in ps is 72dpi, I am wondering how to change it to 600dpi.

The code i write as follows, it is a square of dots.

%%Dot_unit %%position setting 100 200 translate %%size setting     52 49 scale %%parameter setting:width height bit matrix 52 49 1 [52 0 0 -49 0 49] %%dot unit discribe {< fff7ff7ff7ff7f  ffffffffffffff ffffffffffffff ffffffffffffff fffff7ffff7fff ffffffffffffff ffffffffffffff ffffffffffffff ffffffffffffff fdfffffdffffff ffffffffffffff ffffffffffffff 7ff7ff7ff7ff7f ffffffffffffff ffffffffffffff ffffffffffffff ffff7ffffff7ff ffffffffffffff ffbfffffbfffff ffffffffffffff ffffffffffffff ffffffffffffff ffffffffffffff ffffffffffffff 7ff7ff7ff7ff7f ffffffffffffff ffffffffffffff ffffffffffffff fffffff7ffffff ffffffffffffff fffffbffffffff ffffffffffffff ff7fffffffffff ffffffffffdfff ffffffffffffff ffffffffffffff 7ff7ff7ff7ff7f ffffffffffffff ffffffffffffff ffffffffffffff ffffffff7fffff ffffffffffffff ffffffffffffff ffffffffffffff ff7fffffffffff 7fffdfffffdfff ffffffffffffff ffffffffffffff fff7ff7ff7ff7f >} image showpage

回答1:

There is no default dpi in PostScript. User space is set up so that the co-ordinate system is 72 units to the inch, but that's not actually the same thing. If you want a different user space then you alter the CTM (Current Transformation Matrix). All co-ordinate system alterations rely on changes to a matrix, often the CTM.

In the case of images, the image operator maps the image into a 1 unit square area, which is then passed through the image matrix to render to the canvas, using the CTM. In the case of the example above the image is scaled by a factor of 52 in the x direction and 49 in the Y direction. If you change the scale or the image matrix, you will get different results.

The area covered by the image is device-independent, when you print it the user space objects are rendered into device space (using the device matrix). So if your printer is 600 dpi or 2400 dpi, the image will still cover the same area on the output page.

All you need to worry about is the area you want covered, you don't need to be concerned with the resoltuion.

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