Re: [Matplotlib-users] weird matplotlib imread question for png

[Fabrice S. <si...@lm...>]

Le vendredi 17 avril 2015, oyster a écrit :
> Two 8bpp(Gimp, xnview say so) graylevel png files can be downloaded
> The first (ramp-gray.png) which gives right array shape is
> http://bbs.blendercn.org/data/attachment/forum/201504/17/090627ejhixti8vdthdnnn.png
> The second one (python-gray.png) which gives 'wrong' array shape, at
> least to me, is
> http://bbs.blendercn.org/data/attachment/forum/201504/16/222351w3952n3o9968m9a5.png
> 
> [code]
> from pylab import *
> 
> imgRampPng=imread('ramp-gray.png')
> print (imgRampPng.shape)   #(1, 255), that is right
> print (imgRampPng.min(),imgRampPng.max()) #(0.0, 0.99607843)
> print ()
> 
> imgGrayPng=imread('python-gray.png')
> print (imgGrayPng.shape)    #(128, 128, 3), *but I suppose it should
> be (128, 128)*
> print (imgGrayPng.min(),imgGrayPng.max())   #(0.0, 0.98823529)
> print ()
> Ch1=imgGrayPng[:,:,0]
> Ch2=imgGrayPng[:,:,1]
> Ch3=imgGrayPng[:,:,2]
> print (Ch1.min(), Ch1.max())    #(0.0, 0.98823529)
> print (Ch2.min(), Ch2.max())    #(0.0, 0.98823529)
> print (Ch3.min(), Ch3.max())    #(0.0, 0.98823529)  #that is to say,
> Ch1/2/3 hold same data
> print ()
> [/code]

Did you read my answer from yesterday ? your python-gray.png uses a
color palette, the integer values stored in file are indexes to the
(RGB) values given in the palette. Matplotlib is doing right in
converting the indexes to the RGB values.
The color palette represents here the grayscale, that's why all channel
are equal. But the data is stored in the compact palette way.

How were the two files produced?

-- 
Fabrice