How Generel Perspective projection works (-JG)?

Q&A
1

I make this map with a perspective projection (-JG).

gmt grdimage @earth_relief_15m -Rd -JG-58:26/-34:36/9000/0/0/0/90/60/13.4c -B0 -C -png map

mapa
mapa1025×686 178 KB

I don’t understand well how Width and Height parameters works (below I copy the definition). In the map I used 90 and 60 and I get an stretch map. Is it ok?

The Width and Height of the viewpoint in degrees. This number depends on whether you are looking with the naked eye (in which case the view is about 60° wide), or with binoculars, for example.

2

Yes, it is needlessly complicated. The general perspective projection was implemented in GMT 4 by a very motivated guy who no longer is involved. Hence he made decisions that we now live with, such as the very ugly -JG syntax with all those parameters. On our long list of things to do we can add:

  1. Refresh that syntax with modifier flags and default values (so all those 0/0/0 things can go away unless you really want twist, etc.
  2. Add a clear description of these parameters with a sketch and a few examples showing the effects.

I think (1) is long overdue and should probably happen first. I will give it a think. I am guessing the standard -JG syntax stays and then the extra parameters should be set via optional modifiers. We will of course support the original mess in backwards compatibility mode.