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Why is there Gap in this Matlab Polar presentation with export_fig?

Code which yields a gap at the line from origo (0,0) to (1,0), which seems to reach zero if C dimensions reaches infinity; however, I cannot get it small enough with any sizes of C so I think the artifact can be caused by Matlab figure internal features or image data itself (AnderBiguri) because it does not seem to occur for img=imread('peppers.png'). Code which makes the image, stores it by export_fig and maps it from Cartesian to Polar where the artifact occurs

close all; clear all; clc;

%% Make image
f1=figure;
hax=axes(f1);
x = rand(1,6);
y = exp(rand(1,181));
C = rand(3613,3613);
imagesc(hax, x, y, C); 
box(hax, 'off');
axis(hax, 'off');
set(hax, 'yTickLabel', []);
set(hax, 'xTickLabel', []); % for polar presentation
set(hax, 'Ticklength', [0 0]); % http://stackoverflow.com/a/15529630/54964
filename='/home/masi/Images/testi';
% by default, export_fig has cropping
[img, alpha] = export_fig(filename, '-png', '-native', '-q101', '-a1', '-m1', '-RGB', '-nofontswap', '-dpng', hax);
close all; 

%% Read Image
img=imread('/home/masi/Images/testi.png');
% http://stackoverflow.com/a/7586650/54964
[h,w,~] = size(img);
s = min(h,w)/2; % have here .../1, some phenomenon occurs
[rho,theta] = meshgrid(linspace(0,s-1,s), linspace(0,2*pi,s));
[x,y] = pol2cart(theta, rho);
z = zeros(size(x));
subplot(121), imshow(img)
subplot(122), warp(x, y, z, img)
view(2), axis square tight off

Fig. 1 Output when size(C)=[3613 3613], Fig. 2 Output when size(C)=[36130 36130]

enter image description here enter image description here

Matlab: 2016a
Export_fig: 08/08/16 version
OS: Debian 8.5 64 bit
Linux kernel: 4.6 of backports
Hardware: Asus Zenbook UX303UA

Solution

  • I had a quick look at this, and yeah, something strange is happening. The problem is that you have a border around your image, a border that does not seem to be there when plotting the image, because its the same color as the background!

    If you create your image with 

    C = rand(100,100);

    and then plot it after loading it with imagesc instead of imshow you can see this:

    enter image description here

    That is what it gets translated to an band missing in the polar plot. If you just remove the border around the image of size 1, you'll get a full plot.

    My best guess is that somewhere inside export_fig either your statement % by default, export_fig has cropping is false or the cropping has some minor bug and takes 1 pixel from the background.