Re: [Matplotlib-users] The meaning of the figure(dpi=xxx) parameter

[John H. <jdh...@ac...>]

>>>>> "Al" == Al Schapira <a.d...@wo...> writes:

    Al> In order to get the size of the overall plot window to
    Al> anywhere near full screen, I've had to use something like

    Al> 	figure(1, figsize=(18,12), dpi=72) or figure(1,
    Al> figsize=(9,6), dpi=144)

    Al> Both produce the same overall size plot window (11.25" wide by
    Al> 8" tall), but in the latter case, the text size is much larger
    Al> than in the latter case. Likewise the linewidths.

    Al> Can you please explain the interaction among the dpi value,
    Al> the font sizes displayed, and the overall plot size.

Hi Al, 

This is a complicated issue and I don't have a full answer for you.
The problem is compounded by the fact that I am trying to make the DPI
parameter produce figures that look the same across the backends, and
different backends have often have an additional parameter that makes
assumptions about the number of pixels per inch on your display - eg
gd assumes 96, and these are not under my control.  

The total figure width in pixels is figure width in inches * dpi;
ditto for height.  If you set dpi to your device PIXELS_PER_INCH, the
width should be correct if you measure it on the screen with a ruler.
You will probably need to set PIXELS_PER_INCH for the backend you are
using to be correct for your display.  This is a parameter the
respective backend files, eg, in matplotlib/backends/backend_gtk.py.
For backend_agg, you will have to change it both in
src/_backend_agg.cpp and matplotlib/backends/backend_agg.py.  This is
something I would like to rationalize and perhaps move to the rc file.
I suggest setting it to your display width / pixels width, eg, 116 in
your example.  Then

  figure( (8,6), dpi=116)

should produce a an 8 inch by 6 inch figure.  Let me know.  Then
default dpi can be set in the rc file.

None of this matters, of course, for PS, which is resolution
independent.

dpi is not really suitably named because of this additional parameter
PIXELS_PER_INCH.  dpi is really a resolution parameter.  When you
increase dpi, the relative sizes of everything in your image should
increase proportionately (line widths, text sizes, etc) but you have
more pixels of resolution.  When you increase figure size for a give
dpi, everything should appear smaller because the physical size of the
objects in your canvas haven't changed, but your canvas has increased.

I'm open to suggestions on how to make this better.

JDH