My ribbons program, developed on an SGI, makes pretty pictures of molecules with the OpenGL graphics API. Just draws a lot of polygons (mostly triangles) and lines. Image files simply capture the resulting pixels painted on screen. With the typical 1280x1024 high-res monitor, 1224x978 RGB pixels are saved. An image file 1224 pixels wide printed out to picture 13cm wide works out to be about 240 dpi -- but what if you wanted 600 dpi?
SGI has a buggy 'off-screen rendering' extension that in some cases allows you to double this (however, the process screws up the bitmapped text). See the notes under Image Panel help.
One can simply scale (supersample) this image by 2.5 using standard image tools (on sgi: imgworks, imgcopy; elsewhere GIMP, PhotoShop), that also allow interconversion between image file formats, eg, SGI and TIFF. Have done this before and it works ok. Explicit SGI example follows, to make the journals happy, try this:
First, be sure to use full-screen antialiasing when saving at full size.
Run the sgi image processing program:
imgworks ribbonsimage.rgb
Under enhance, choose sharpen and apply about 10% to taste'
(i really like the look of it).
Then under transform, choose scale and enlarge to the journal's specs.
Save your image, overwrite, or, eg, save as new file 'big.rgb'.
Then use the new sgi convertor:
imgcopy big.rgb big.tiff
We have such things with very satisfactory results. The scaling (another kind of supersampling) of course blurs the image, which is why the sharpening first helps. You can probably do better if you are an expert in PhotoShop or the GIMP. A recipe for these packages would be greatly appreciated.
And there is "ray-tracing" (which you cannot do with OpenGL) that can create non-interactive "photo-realistic" images at arbitrary resolution. Our last artistic cover for Acta Cryst (Acta Cryst D56:266-271), shown on ribbons web site) used the POV-Ray program to render the geometry output by ribbons to a 3000x3000 pixel image, to give the highest possible quality the editors desire. I have just added the ability to render the "line drawing" features, so you don't loose the feel of a good technical illustration. This came out in ribbons 3.16.
Or you could create a resolution-independent PostScript file - not a PostScript bitmap image, but the actual set of triangles, lines, and text strings output in the correct order by ribbons. Have recently added code derived from the SGI developers toolkit to do this in ribbons. There are a few little problems with the code, arising from the tricks you have to play with the OpenGL feedback buffer and the back-to-front sorting of graphics primitives you must do to solve the hidden surface problem (of course, OpenGL does this for you via Z-buffering in the interactive version, but I fear this is too much detail..). This is how we provided the cover image: Acta Cryst D56:937-944. See the notes in Image Panel help.
Ribbons User Manual / UAB-CBSE / carson@uab.edu