Question

Problem : The image loses its transparency when plot using surf

I have already figured out how to create a png file with a transparent background as described in numerous other threads.here

However, when plot using surf.m, the image is not transparent

Here is the code that I have so far:

img = imread('image.png');

A1 = ones(size(img));A2 = ones(size(img));A3 = ones(size(img));
A1(img(:,:,1)==0)=0;A2(img(:,:,2)==0)=0;A3(img(:,:,3)==0)=0;

A = A1+A2+A3;
A= A(:,:,1);

imwrite(img,'test.png','alpha',A);
[img,map,alpha] = imread('test.png');

ximage = [-.5,.5;-.5,.5];
yimage = [0,0;0,0];
zimage = [.5,.5;-.5,-.5];

surf(ximage,yimage,zimage,'Cdata',img,'Facecolor','texturemap','Edgecolor','none','alphadata',alpha);
axis vis3d

The code converts image.png (blue square) to a test.png with a transparent background (get rid of the black background). test.png is then used to generate a surf plot which turns out to be not transparent. Any idea what I am doing wrong?

Answer 1

There is a function called alpha in MATLAB which sets transparency for the objects in current axes. I suggest to change the variable named alpha to another name by replacing the following line

[img,map,alpha] = imread('test.png');

to

[img,map,alphaChannel] = imread('test.png');

Now, after running surf, you can set the transparency for your plot through alpha function.

Using alpha function

surf(ximage,yimage,zimage,'Cdata',img,'Facecolor','texturemap','Edgecolor', ...
                          'none','alphadata',alpha);
alpha(0.5); %# line added
axis vis3d

Using surf function

If you want to set the transparency through the surf function, you need to add 'FaceAlpha' parameter:

surf(ximage,yimage,zimage,'Cdata',img,'Facecolor','texturemap','Edgecolor', ... 
                          'none','AlphaData',alphaChannel,'FaceAlpha',0.5);

Result

More information about alpha and surf functions.

---

Setting a matrix transparency

But those functions above sets the transparency for the entire plot. If you want to set your original matrix of transparency, you need to pass 'FaceAlpha','texture' parameter to surf:

handler = surf( ximage , yimage , zimage , 'Cdata', img , ...
    'FaceColor','texturemap',                             ...
    'EdgeColor','none',                                   ... 
    'FaceAlpha','texture',                                ...
    'AlphaData', alphaChannel);
axis vis3d

Result

More details. Hope it helps!