matplotlib-users

[Matplotlib-users] removing Figure background (or, making it transparent)

[Andrew S. <str...@as...>]

Hi all,

I'd like use matplotlib to draw shapes on a transparent background using 
the Agg backend to save .png files. Is this possible?

I've tried

fig._figurePatch.set_alpha(0)

This seems to get halfway, but there's still (opaque) white drawn in the 
background.

Is there a "recommended way", and, if so, what is it?  Otherwise, can we 
improve matplotlib to allow drawing on transparent backgrounds?

Cheers!
Andrew

Re: [Matplotlib-users] removing Figure background (or, making it transparent)

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

>>>>> "Andrew" == Andrew Straw <str...@as...> writes:

    Andrew> Hi all, I'd like use matplotlib to draw shapes on a
    Andrew> transparent background using the Agg backend to save .png
    Andrew> files. Is this possible?

    Andrew> I've tried

    Andrew> fig._figurePatch.set_alpha(0)

There is a vestigial limitation from the bad old days when we didn't
have an alpha channel.  For polyons, there is no place holder for a
separate alpha channel for the edge and face.  The way to do this
right is to simply replace all rgb instances throughout the library
with rgba, but this is a fair amount of work that touches the entire
library.  What I am doing currently is sharing the alpha attribute
between edge and face for polygons.

So setting alpha=0 *should work. What backend and version are you
using?  Rectangle calls gc.set_alpha before calling draw_polygon in
patches.Patch.  In _backend_agg.RendererAgg.draw_polygon, the alpha
attribute for the face is taken from the edge which gets it from the
gc (around line 204 in _backend_agg).  I say all this just because
from my read of the code *it should* work, and indeed, in my tests

  f = figure(1, facecolor='r')
  f._figurePatch.set_alpha(0.0)

does work for me using Agg and the latest matplotlib.

What backend and version are you using?

That said, the best way to do this is to add a frameon attribute for
figures, because this will be respected across backends including
those who don't understand alpha.  This way you can say

f = figure(1, frameon=False)

I checked this into CVS.

JDH





    Andrew> This seems to get halfway, but there's still (opaque)
    Andrew> white drawn in the background.

    Andrew> Is there a "recommended way", and, if so, what is it?
    Andrew> Otherwise, can we improve matplotlib to allow drawing on
    Andrew> transparent backgrounds?

    Andrew> Cheers!  Andrew


    Andrew> -------------------------------------------------------
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Re: [Matplotlib-users] removing Figure background (or, making it transparent)

[Andrew S. <str...@as...>]

John Hunter wrote:

>>>>>>"Andrew" == Andrew Straw <str...@as...> writes:
>>>>>>            
>>>>>>
>
>    Andrew> Hi all, I'd like use matplotlib to draw shapes on a
>    Andrew> transparent background using the Agg backend to save .png
>    Andrew> files. Is this possible?
>
>    Andrew> I've tried
>
>    Andrew> fig._figurePatch.set_alpha(0)
>
>There is a vestigial limitation from the bad old days when we didn't
>have an alpha channel.  For polyons, there is no place holder for a
>separate alpha channel for the edge and face.  The way to do this
>right is to simply replace all rgb instances throughout the library
>with rgba, but this is a fair amount of work that touches the entire
>library.  What I am doing currently is sharing the alpha attribute
>between edge and face for polygons.
>
>So setting alpha=0 *should work. What backend and version are you
>using?  Rectangle calls gc.set_alpha before calling draw_polygon in
>patches.Patch.  In _backend_agg.RendererAgg.draw_polygon, the alpha
>attribute for the face is taken from the edge which gets it from the
>gc (around line 204 in _backend_agg).  I say all this just because
>from my read of the code *it should* work, and indeed, in my tests
>
>  f = figure(1, facecolor='r')
>  f._figurePatch.set_alpha(0.0)
>
>does work for me using Agg and the latest matplotlib.
>  
>

By "working for you", do you mean that you don't have a red face?  Yes, 
that's fine, but if you read my initial email, I said "there's still 
(opaque) white drawn in the background" for this case.  What do you 
get?  I predict that you do.

>What backend and version are you using?
>  
>
Agg, PS, CVS HEAD

>That said, the best way to do this is to add a frameon attribute for
>figures, because this will be respected across backends including
>those who don't understand alpha.  This way you can say
>
>f = figure(1, frameon=False)
>
>I checked this into CVS.
>
>JDH
>  
>

OK, here's a sample program (modified slightly from scatter_demo2.py:

#!/usr/bin/env python
import sys
from matplotlib.matlab import *
from data_helper import get_daily_data

fig = figure(1,frameon=False)

intc, msft = get_daily_data()

delta1 = diff(intc.open)/intc.open[0]

volume = (10*intc.volume[:-2]/intc.volume[0])**2
close = 0.003*intc.close[:-2]/0.003*intc.open[:-2]
p = scatter(delta1[:-1], delta1[1:], c=close, s=volume)
set(p, 'alpha', 0.75)
set(gca(), 'xticks', arange(-0.06, 0.061, 0.02))
set(gca(), 'yticks', arange(-0.06, 0.061, 0.02))
xlabel(r'$\Delta_i$', fontsize='x-large')
ylabel(r'$\Delta_{i+1}$', fontsize='x-large',
       verticalalignment='center',
       horizontalalignment='right',
       rotation='horizontal'
       )
title(r'Volume and percent change')
grid(True)
gca().set_frame_on(0)
fig._figurePatch.set_alpha(0.0) # doesn't affect output
savefig('scatter_demo3',facecolor='b',edgecolor='g')
#show()

Now, with the Agg backend (saving to PNG), I still get a non-transparent 
white background. (The face and edgecolors were specified just to test 
if they show up, which they don't.)  Also, with the PS backend, I get a 
white rectangle at figure size.

So, it looks like there's another white rectangle creeping in there 
somewhere...  Can we find it and get rid of it?

Cheers!
Andrew

Re: [Matplotlib-users] removing Figure background (or, making it transparent)

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

>>>>> "Andrew" == Andrew Straw <str...@as...> writes:

    Andrew> By "working for you", do you mean that you don't have a
    Andrew> red face?  Yes, that's fine, but if you read my initial
    Andrew> email, I said "there's still (opaque) white drawn in the
    Andrew> background" for this case.  What do you get?  I predict
    Andrew> that you do.

Oh, I did misread.  Yes, I was getting rid of the red but didn't
notice the white background of the canvas.

This does, as you noted, show up whether or not you set alpha to zero
and whether or not you draw the figure frame.  The reason is, when I
initialize the agg buffer (src/_backend_agg.cpp) I do

  rendererBase->clear(agg::rgba(1, 1, 1));

which sets a default background of white with a default alpha of 1.  I
just tested the following change to line 46:

  rendererBase->clear(agg::rgba(1, 1, 1, 0));

Ie, manually set the alpha of the background color to zero, and it
worked as expected with your example.  Now the background is truly
transparent.  I also checked this in CVS but you can just change the
one line to get the same result.

JDH

Re: [Matplotlib-users] removing Figure background (or, making it transparent)

[Andrew S. <str...@as...>]

>Ie, manually set the alpha of the background color to zero, and it
>worked as expected with your example.  Now the background is truly
>transparent.  I also checked this in CVS but you can just change the
>one line to get the same result.
>
>  
>
Thanks!  It works great!

Matplotlib rocks!

Andrew