Hezekiah M. Carty wrote:
> On Wed, Jun 3, 2009 at 4:36 PM, Hazen Babcock <hbabc...@mac.com> wrote:
>> Hezekiah M. Carty wrote:
>>> While writing the Cairo backend for plarc, I am having some trouble
>>> getting the transformation from world coordinates to Cairo coordinates
>>> correct. From what I understand, the proper method is:
>>>
>>> cairo_x = plP_wcpcx(world_x);
>>> cairo_y = plP_wcpcx(world_y);
>>> cairo_a = plP_wcpcx(world_x + world_a) - cairo_x;
>>> cairo_b = plP_wcpcy(world_y + world_b) - cairo_y;
>>>
>
> My apologies - I sent the original message with the wrong From: line,
> so my original post only made it to Hazen, not the list.
>
> My intent is/was to ask which transform to do in PLplot core. So
> given, in plot-world coordinates:
>
> point_x, point_y
>
> how would I translate that location to the values:
>
> intermediate_x, intermediate_y
>
> which would be passed to the underlying driver (Cairo, in this
> example)? Once they get to Cairo, my understanding from the code and
> your comment is that the final Cairo (x, y) would be:
>
> intermediate_x * aStream->downscale, intermediate_y * aStream->downscale
>
> I think my problem is that I am not calculating (intermediate_x,
> intermediate_y) appropriately. I thought that plP_wcpcx and plwcpcy
> would do this, but they give the erroneous results I mentioned in the
> original message. I have attached the example plots again for the
> list since they didn't make it to the list last time.
>
> Thank you for your help on this Hazen!
cairo_y = plP_wcpcx(world_y);
^^^
A typo perhaps?
I think you might also need to transform to device coordinates instead
of world coordinates, but I'm not sure. I was looking around in
src/plline.c but I didn't immediately see how plline() handled this
transformation.
-Hazen
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