imwrite saving image incorrectly

I am trying to use app designer to create an app where you can load an image from your desktop, compress or decompress the image and then save the image. I have been able to load the image without any issues however when I save the image it removes the background or makes the whole image black. Heres my code:

[file, path] = uiputfile('.png');
image = imread(app.imagePath);
imwrite(image,fullfile(path,file))

app.imagePath is the path to the original image that I want to save

Answer 1

[Ameer Hamza]

Does your png image have transparent components? Try saving with the alpha channel.

[file, path] = uiputfile('.png');
[image, ~, amap] = imread(app.imagePath);
imwrite(image,fullfile(path,file), 'Alpha', amap)

[Harry West]

I added this feture to the code but it gave this error:

Error using writepng>parseInputs (line 349)

The value of 'alpha' is invalid. Expected input to be of size 16x16, but it is of size 0x0.

[Ameer Hamza]

That shows your png images do not have an alpha mask. But in that case, the background issues shouldn't happen. Can you attach one of your images?

[Image Analyst]

And, Harry, don't use "image" as the name of your variable because it's an important built-in function that you just blew away.

[Harry West]

The images with alpha seem to have been fixed however this image also appear to turn black when saved. What issue would this be?

[Ameer Hamza]

Ok, the issue is not the alpha map; rather, the png file is saved as an indexed image.

Either convert it to rgb image before using imwrite()

[img, cmap] = imread('block.png');
img_rgb = ind2rgb(img, cmap);
imwrite(img_rgb, 'test.png')

or specify colormap in imwrite()

[img, cmap] = imread('block.png');
imwrite(img, cmap, 'test.png')