Change white pixels to invisible

Question

Jacob L 2020-11-26

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https://ww2.mathworks.cn/matlabcentral/answers/662708-change-white-pixels-to-invisible

评论： Matt J 2020-11-26

Hi, I have a .png I'm trying to overlay on top of an existing figure in matlab. Most of the image is just a white background. Is it possible to overlay everything but the white background from the .png to the existing figure? In other words, can I make the white background transparent/invisible?

The image is RGB 'unit8'. When I do this:

alphachannel = all(image == 255, 3);
imwrite(image,'invisible.png','Alpha',alphachannel);

This error comes up:

Error using writepng>parseInputs (line 349)

The value of 'alpha' is invalid. Expected input to be one of these types:

double, uint8, uint16

Instead its type was logical.

When I do this:

alphachannel = double(all(image ~= 255, 3));
imwrite(image,'invisible.png','Alpha',alphachannel);

The .png file appears to have the invisible pixels instead of the white pixels, but ,when I try to overlay the new .png on top of the existing figure in matlab, the "invisible" background is black. Is there anything I can do? Any help is appreciated, thanks.

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Answer 1

Matt J 2020-11-26

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https://ww2.mathworks.cn/matlabcentral/answers/662708-change-white-pixels-to-invisible#answer_556378

编辑：Matt J 2020-11-26

在 MATLAB Online 中打开

Shouldn't you have

alphachannel = 1-double(all(image == 255, 3));

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Matt J 2020-11-26

I recommend you attach all images so we can play with them.

Jacob L 2020-11-26

在 MATLAB Online 中打开

Okay - here is a summary of what's going on:

Starting with the Teddy_Bear.png

Then using

teddyBearProcessed = padarray(image,[1000,1000],255,'post');

to get Teddy_Bear_Padded.png

Then using

shiftedTeddy = imtranslate(image,[150, 150],'FillValues',255);

to get Teddy_Bear_Shifted.png

Then using

alphachannel = 1-double(all(image == 255, 3));
imwrite(image,'invisible.png','Alpha',alphachannel);

to get Teddy_Bear_Invisible.png

In the meantime I have another figure open in MATLAB. The output of the figure is saved in the attached as Stationary_Teddy_Centroid.png.

Then using

    hold on
    imshow('Teddy_Bear_Invisible.png');

to get problem.png.

What I'm trying to do is move the teddy bear on the screen so I can see the the path it takes. Kind of like how the path of the arm is seen in Stationary_Teddy_Centroid.png.

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