How to save figure with transparent background - image

Question

I have a plot in Matlab and am setting the background to transparent by:
set(gcf, 'Color', 'None');
set(gca, 'Color', 'None');
When I try to save the image (from the viewer), I save as a ".png", but it saves with a white background. How can I save it with the transparent background?

Answer 1

It is disappointing but, MATLAB's default saveas and print commands cannot deal with transparent things very well. You'll have to save it with some background and then convert it either through imread/imwrite or some other tool.
There are some tools that might be helpful:
Export fig http://www.mathworks.com/matlabcentral/fileexchange/23629
svg export http://www.mathworks.com/matlabcentral/fileexchange/7401-scalable-vector-graphics-svg-export-of-figures
I prefer vector graphics, so use svg exports when transparency is needed. If indeed you have a bitmap, use imwrite(bitmapData, 'a.png', 'png', 'transparency', backgroundColor).

Answer 2

Things have changed since the MATLAB 2014b release. The newly implemented graphics system (so called HG2, for Handle Graphics version 2) does much better in terms of transparency.
Now it saves transparency correctly to SVG at least!

Answer 3

So I still wanted something simple that did not require me to install anything else (corporate pc not allowed :/). I stumbled upon this link, stating:
All you have to do is the following
1) In matlab file add the commands to format your figure with transparent background
set(gcf, 'color', 'none');
set(gca, 'color', 'none');
and save or export the figure generated in eps format. (say Bspline.eps)
2) Open Bspline.eps in NotePad
3) Look at the first line. For example %!PS-Adobe-3.0 EPSF-3.0. The last number 3.0 indicates the Postscript level. For level 3, search the string rf. You will find in one line like this (four numbers followed by rf)
0 0 3025 2593 rf %Comment that line using %.
(For level 2 search for string pr instead of rf)
Save the file.
Now you can use the eps file or you can convert it to pdf and then use it.
Anyway it will have transparent background
Extra
For me it was two lines with re and two lines, despite me having %!PS-Adobe-3.0 EPSF-3.0 just after each other. But the result was the Figure was now transparent.

Answer 4

As an addition to the answer by Memming. Matlab 2020 with exportgraphics supports transparent background but only for vectorized output (with doesn't work with rendered content). For exporting rendered data with transparency you can still not save it with transparency. However you can get the transparency by saving rendered data with two different background colors (white and black for example) and then loading both temporary images, solving a simple equation system and thereby retrieving the transparency and the original color data and then saving that all into a RGBA png file.
The key is to realize that the saved rendered output of Matlab is transparency times image data + (1 - transparency) times background color. From a single output, image data and transparency cannot be restored but from two outputs with different background colors it can. It would be easier if Matlab would support transparent background colors in rendered outputs, but this way works too.
Example script:
% create example data
g = -10:0.1:10;
[x, y, z] = ndgrid(g, g, g);
v = (x.^2 + y.^2 - 10).^2 + (z.^2 - 5).^2;
% render in 3D with transparency
fig = figure();
% one surface fully opaque
fv = isosurface(x, y, z, v, 20);
p = patch(fv, 'FaceColor', [1, 0, 0], 'FaceAlpha', 1, 'EdgeColor', 'none');
% another surface with transparency
fv = isosurface(x, y, z, v, 80);
p = patch(fv, 'FaceColor', [0, 1, 1], 'FaceAlpha', 0.5, 'EdgeColor', 'none');
fig.Children.Color = 'none'; % transparent background of figure axis
view([40 40]);
pbaspect([1,1,1]);
camlight;
lighting('gouraud');
% save figure in different ways
% save as vector format, doesn't produce nice output see for example https://stackoverflow.com/questions/61631063
exportgraphics(fig, 'test.pdf', 'ContentType', 'vector', 'BackGroundColor', 'none');
% prints warning: Warning: Vectorized content might take a long time to create,
% or it might contain unexpected results. Set 'ContentType' to 'image' for better performance.
% save as rendered output with transparent background not work
exportgraphics(fig, 'test.png', 'ContentType', 'image', 'BackGroundColor', 'none');
% prints warning: Warning: Background transparency is not supported; using white instead.
% now our solution, export with two background colors
exportgraphics(fig, 'test1.png', 'ContentType', 'image', 'BackgroundColor', 'k'); % black background
exportgraphics(fig, 'test2.png', 'ContentType', 'image', 'BackgroundColor', 'w'); % white background
% load exported images back in and scale to [0,1]
u = imread('test1.png');
u = double(u) / 255;
v = imread('test2.png');
v = double(v) / 255;
% recover transparency as a
a = 1 - v + u;
a = mean(a, 3);
a = max(0, min(1, a));
m = a > eps;
% recover rgb
c = zeros(size(u));
for i = 1 : 3
ui = u(:, :, i);
ci = c(:, :, i);
ci(m) = ui(m) ./ a(m);
c(:, :, i) = ci;
end
c = max(0, min(1, c));
% store again
imwrite(uint8(c*255), 'test.transparent.png', 'Alpha', a);
% temporary files test1.png and test2.png can now be deleted
These are the two temporary images with white and black background.
This is the resulting transparent image. To see that save the image and look at it with a suitable viewer.
One last comment: For high quality production figures you probably want to switch off (set the Visible property to off of) all rendered parts with transparency and save axes and labels as vectorized output, then reverse the visibility, i.e. switch off all axes and labels and only show the rendered parts with transparency and save them with two different background colors, then reconstruct the transparency and overlay both, the vectorized axes and labels as well as the rendered part with transparency and combine them. As of Matlab 2020b, Matlab is not capable of doing that on its own.

Answer 5

You can do it like this, Matlab2020a or a later version is valid!
x = 0:.1:2*pi;
y = sin(x);
figure;
plot(x,y,'LineWidth',4);
% save to transparented image
set(gcf, 'color', 'none');
set(gca, 'color', 'none');
exportgraphics(gcf,'transparent.emf',... % since R2020a
'ContentType','vector',...
'BackgroundColor','none')

Answer 6

According to Matlab staff exportgraphics(gca,'plot.png','BackgroundColor','none')
and similar should work in 2020a version, but when you try, you get beautifull errors in most cases:
Warning: Background transparency is not supported; using white instead.
I also got Warning: 'ContentType' parameter is ignored when it is set to 'vector' for image output. trying xing cui code.