🚀 go-pugleaf

Hi all.

A quick question to the elite, from a PostScript novice:

I have a vague recollection that someone (possibly on this forum many moons ago, in which case my apologies) told me that PostScript RIPs will downsample non-bilevel images to fit the line screen frequency of the device.

If I didn't imagine it, could I ask whether that's standard behaviour, or the behaviour of a particular RIP?

I'm interested because one could have, for example, an image containing both grey scale and purely black-and-white elements, and if the above is true, the bilevel elements would only be rendered at (double) the line frequency of the device, rather than the full RIP resolution - and might also have grey level interpolation. Correspondingly, a sharp-edged grey circle on a white background at high resolution in an image could either look like a sharp circular cut-out of the corresponding grey halftone screen, or like a grey circle downsampled to double the line frequency (which isn't the same thing); it could even be something more complicated if there's some low-level image processing going on to compensate for the sharp edge and printer effects.

I'd be interested in whether the same question applies for non-images - is a PostScript grey circle actually a cookie-cutter slice from a grey screen, or mapped onto a line-screen frequency set of blocks? (In this case, I'd expect it to be a pure cut-out, since the algorithm for per-pixel colour mapping to halftones is well-known, but the image case is less clear to me.)

If I imagined it, the guideline not to provide images at more than twice the target line frequency has a distinct exception in this case (although obviously it's *usually* sensible).

I realise I could find an experimental answer, but I'd really like to know what the official position, if anything, is before I make any sweeping statements on the subject to anyone. I may be missing it, but I couldn't find a detailed description of the sampling mechanism in the Red Book.

I'd be most grateful for any inside information.