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C.2.1 When converting or viewing PostScript goes awry

Here are some notorious pitfalls with ghostscript (and other rendering programs for that matter).

Rendering.
  When you are converting to a raster format, make sure you use a high enough resolution so that the pixels do not show when it is enlarged onto a screen or using a projector. The right choice of resolution depends on the application, but do not feel limited to the default 72 dpi (dots-per-inch) that is offered by most converters.

Image compression.
  There are lossy and non-lossy compressions. A compression algorithm is called ``lossy'' when information is lost in the conversion: there is no way back to get the full original. The effect can be seen when there are sharp color transitions in your image: the edges will get blurry in order to allow a more efficient compression. JPEG uses a lossy compression, PNG is non-lossy, and TIFF generally does not use compression at all. We therefore recommend you convert to PNG if you need to rasterize your plot, and leave JPEG to photographs.

Embedded image compression.
  When your GMT plot includes objects produced by grdimage, psimage or pslegend, they are seen as ``images''. The default options of ghostscript will use a lossy compression (similar to JPEG) on those images when converting them to PDF objects. This can be avoided, however, by inhibiting the compression altogether, or using the non-lossy flate compression, similar to the one used in the old compress program. This compression is fully reversible, so that your image does not suffer any loss.

Auto-rotation.
  The ghostscript engine has the annoying habit to automatically rotate an image produced with portrait orientation (using the -P option) so that the height is always larger than the width. So if you have an image that was printed in portrait mode but happens to have a width larger than height (for example a global map), it would suddenly get rotated. Again, this function needs to be switched off. Apple's Preview uses the ghostscript engine and suffers from the same annoying habit. Oddly enough, ghostscript does not force landscape plots to be ``horizontal''.

Anti-aliasing.
  This is not something to worry about when converting to PDF, but certainly when producing raster images (discussed below). Anti-aliasing in this context means that the rendering tries to avoid aliasing, for example, sampling only the blacks in a black-and-white hachure. It does so by first oversampling the image and then using ``gray-shades'' when a target pixel is only partially white or black.

  Clearly, this can lead to some unwanted results. First, all edges and lines get blurry and second, the assumption of a white background causes the gray shades to stand out when transferring the image to background with a different color (like the popular sleep-inducing blue in PowerPoint presentations). A more surprising effect of anti-aliasing is that the seams between tiles that make up the land mask when using pscoast will become visible. The anti-aliasing somehow decides to blur the edges of all polygons, even when they are seamlessly connected to other polygons.

  It is therefore wise to overrule the default anti-aliasing option and over-sample the image yourself by choosing a higher resolution.

Including fonts.
  When you are producing print-ready copy to publishers, they will often (and justifiably) ask that you include all fonts in your PDF document. Again, ghostscript (and all converters relying on that engine) will not do so by default.

Next: C.2.2 Using ps2raster Up: C.2 Converting GMT PostScript Previous: C.2 Converting GMT PostScript Contents Index