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Blank png: x.png does not set the alpha/transparency info correctly ! #848

jypeter opened this Issue Oct 27, 2014 · 3 comments


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jypeter commented Oct 27, 2014

I have finally understood why I was getting completely blank png images, as reported in #845 after trying to open the png in different programs: it seems that somehow the image is there, but the alpha channel value is set to 0 for all the pixels (according to gimp's color picker tool), which make the image seem completely transparent

Am I the only person having this problem ???

@doutriaux1 , could you check the properties of the saved png you generate?

When I run the following commands

import numpy as np, vcs
id100 = np.identity(100)
x = vcs.init()

I get the following completely blank/transparent image!
If I remove the alpha/transparency with ImageMagick (, I get a png with the visible image I want! Of course, I'd like to have directly a png I can see :)

convert -alpha off test_default_png.png test_default_png-noalpha.png



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doutriaux1 commented Oct 27, 2014

@jypeter we changed the default back to have a non-transparent backaground, you can control this in your version via the foloowing parameter:
x.png(bla, draw_white_background = True)
x.png(bla, ignore_alpha=True)

@doutriaux1 doutriaux1 closed this Oct 27, 2014

@doutriaux1 doutriaux1 added this to the 2.1 milestone Oct 27, 2014

@doutriaux1 doutriaux1 self-assigned this Oct 27, 2014


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jypeter commented Oct 27, 2014

Pfff, how am I supposed to guess that, it's not even in help(x.png) :/

I have tried the 4 different combinations of draw_white_background set to True or False and ignore_alpha set to True or False and

  • I confirm I get an image with ignore_alpha=True. It has a white background, regardless of the value of draw_white_background
  • I never get an image with ignore_alpha=False, regardless of the value of draw_white_background

draw_white_background does not seem to have any effect and I have not been able to get an image on a transparent background

I'm using the 2.0.0 tag UV-CDAT


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doutriaux1 commented Oct 27, 2014

not in your version but in master draw_white_background works.

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