Re: pngcairo and resolution

[Ethan M. <merritt@u.washington.edu>]

On Saturday, 03 September 2011, Mojca Miklavec wrote:
> Dear list,
> 
> The pngcairo terminal behaves a bit weird when setting the size:
> 
> gnuplot> set term pngcairo size 10cm,10
> Options are ' fontscale 1.0 size 10.00cm, 10.00cm '
> 
> gnuplot> set term pngcairo size 10,10cm
> Options are ' fontscale 1.0 size 0.35cm, 10.00cm '

What's weird about that? The default unit is pixels.
Unless you force it otherwise, both the y dimension
is are assumed to be in the same units as the x dimension.

> On top of that I do find it a bit annoying that I'm not able to check
> how many pixels will be the final image size.

Specifiying the size in pixels is the default.
Just stick with that and you'll be fine.
Specifying in inches or cm doesn't really make any sense, since it
depends on the resolution of the output device.  The fact that the
terminal recognizes sizes in "in" or "cm" is largely an artifact
of sharing the code between pngcairo and pdfcairo.

> Also, the following note is probably not exact:
> 
>  The width of all lines in the plot can be modified by the factor <lw>
>  specified in `linewidth`. The default linewidth is 0.25 points.
>  (1 "PostScript" point = 1/72 inch = 0.353 mm)
> 
> >From what I understand the line width is one pixel (which makes it 1
> point at 1 dpi).

True. That sentence is left over from the pdf documentation.
It should not appear also in the png documentation.

> I would also like to be able to change resolution, but it seems that
> this is currently not possible.

A bitmap image (png, gif, bmp, ...) does not have a resolution.
Only the output device has a resolution.
It is true that some programs use the comment field of a png image
to store a suggested output resolution, but that does not actually
change anything in the image itself.