Transparent PNG file on background in Matlab GUI

Hello,

For my GUI i have set a background image and now I want to put a (partly) transparent PNG image on a pushbutton on top of this background image, while keeping the background visible in the parts where the button image is transparent. How do I do this?

Thanks in advance!

Answer 1

[Robert Cumming]

I know this is an old post - but if anyone comes across this you may be interested in this blog on undocumentedmatlab

Answer 2

[Klont]

Simply make a matrix of size height (pixels) x width (pixels) x 3 (rgb) of doubles between 0 and 1, with NaN indicating transparency.

[Serge]

This works if you are happy with alpha being all or nothing...

Answer 3

[Walter Roberson]

You cannot do this directly. The uicontrol() that can have images placed on them (such as pushbutton) have a CData property that must be truecolor (RGB) values, but there is no alpha property, so there is no way to define transparency for such images.

The work-around is to figure out which part of the background image the button would be covering up, and to store the corresponding pixel values into the image array locations that should be transparent, and then to use that as the CData.

Ideally you would re-do this when the figure was resized; that would have to be triggered off of the figure resize callback, as uicontrol do not have a resize callback.

[Frank]

Thanks for the tip!

I tried the following code:

bg_image=imread('bg.png'); %background

pos_button=[500,500];

[button,button_map]=imread('button.png.'); %button image

[w,l,d]=size(button);

bg_button=bg_image(500:500+w,500:500+l); % part of background that is behind button

button = uicontrol('style','pushbutton','position',[pos_button,[w,l]]);

button_image=button;

for i=1:w

for j=1:l

for k=1:d

if button_image(i,j,k)==255 % places where background should be seen

button_image(i,j,k)=bg_button(i,j,k); %replace with background values

end

end

end

end

button_image_RGB=ind2RGB(button_image,button_off_map);

set(button,'cdata',button_image_RGB);

This works, but the colors of the background are not the colors they should be, but grey.

Any ideas why?

[Walter Roberson]

I suspect bg_image is 3 dimensional (truecolor, RGB) rather than 2 dimensional. You probably need to add a third index when extracting bg_button