I have a PostScript page that contains a single filled rectangle, and the size of the page and the rectangle are exactly the same. Why does PDF Converter make the rectangle a little smaller than the page, depending on the value of HWResolution (Hardware Resolution)? Or more generally, we find that line drawing objects may be smaller that it should be based on the PostScript description after it is converted to PDF using PDF Converter, depending on the value of HWResolution.
It is important to remember what PDFConverter is, and how it functions, to understand this issue.
PDF Converter is basically a copy of the Postscript RIP processor. It is fitted with an output driver that captures the rendering, and converts it to PDF, but it is basically converting Postscript to a raster image. One of the primary controls of this process is HWResolution (Hardware Resolution).
When the RIP draws a line, it treats the line as a raster image. So it is important to it to know the size of a pixel, and to always place the start and endpoint of a line on pixel boundaries. So while we may, in Postscript, say that we want to draw a line from 0.00137 to 26.23546, the PostScript engine will adjust that start point and the end point of that line to precise pixel bounds. This will generally result in the line always being exactly as long as requested (if both ends happen to be pixel bounds) or slightly shorter.
The default value for HWResolution is 600 Dots Per Inch, or 8.33 pixels per point. So some adjustment is almost always made for elements drawn to precise point boundaries. But at this resolution, the difference will generally be only 1/600th of an inch, which is not visible to most users. If the resolution is set lower, this difference can become visible.
HWResolution is used only in PDF Converter, and only to align the edges of elements. It has no other effect on the PDF produced, which can be rendered on a device of any resolution. But because PDF Converter can actually process with PostScript as if the software is producing a raster image, PDF Converter does that using Device Independent Postscript, and makes the values Device Dependent. Then, PDF Converter produces a Device Independent PDF document from the result.
There is some opinion that decreasing the HWResolution causes PDF Converter to convert pages more quickly. But this loss of resolution in the size/placement of lines is a direct result of the lowered resolution. This is a good reason, therefore, not to lower the resolution. In fact, for the highest fidelity of converting PostScript to PDF, this resolution should be set at the highest resolution expected for the output PDF document. That is, if we expect the PDF to be used only for screen display, we may safely lower the resolution. But if we expect to print the pages in a production environment, we should probably increase the resolution to a higher number. Preferably, the resolution should be a multiple of 72.