How to add alpha channel to the image and convert that image into png format?

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goldensona 2012-12-30

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评论： Walter Roberson 2018-3-7

Hi i want to combine image with the alpha channel having 100%opacity no color it. and after some processing i have to remove the aplha channel .how to remove it can anyone help me to do.

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Walter Roberson 2012-12-31

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在 MATLAB Online 中打开

imwrite(RGBarray, filename, 'png', 'Alpha', ones(size(RGBarray,1),size(RGBarray,2)) )

When you imread(), use

[RGBdata, map, alpha] = imread(filename, 'png');

Note: this is specific to PNG files. A couple of other image formats handle Alpha this way, but some of the other image formats handle Alpha in other ways.

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goldensona 2013-1-2

for image editing purpose, use superimposing or painting ,if i use photoshop to change it will destroy pixel value

Walter Roberson 2013-1-2

If you change the RGB array values of the image, then they are changed just like with photoshop.

You can display one image on top of another, but it does not sound like you want to do that.

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Image Analyst 2012-12-31

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I'm not sure exactly what you mean. Do you just want to put stuff in the overlay, like with functions plot(), line(), patch(), annotation(), etc. and be able to turn them on or off? Or do you want to save stuff you put in the overlay, say burned into the image and saved as a disk file with export_fig()? Or do you want to do what Walter suggested?

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Walter Roberson 2013-1-2

I do not know why MATLAB Image Processing Toolbox does not support RGBA arrays. It was a design decision. RGBA would complicate tools such as rgb2bw() and rgb2hsv().

"modification in image": are you talking about "brushing"? I doubt you are. The figure tools in MATLAB do not change the original data. For example, if you display an image, and rotate that image using the figure Rotate tool, then it is only the display that is changed, not the array that was copied to the display. If you build tools to allow the user to do things like paint, then you have complete control over which arrays get modified and how many "undo" you support and so on.

If you create an image in MATLAB and write it out in a file format that supports an alpha layer (such as PNG), and you take that file into (say) photoshop and modify it there, then which channels get modified are going to depend upon the program and the tools you use within the program.

Might I suggest that you discuss what you are really trying to do? It sounds like you are either attempting to watermark or do steganography.

goldensona 2013-1-27

can any one tell about tampering localization capability ?

Walter Roberson 2013-1-28

Maybe, if it were defined. But it sounds like something that should be analyzed through theory (which is outside the scope of this forum), or else through experimentation.

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Jo 2018-3-7

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编辑：Jo 2018-3-7

在 MATLAB Online 中打开

If you need using TIFF format to add alpha channel, you can refer to this page: https://www.mathworks.com/help/matlab/ref/tiff-class.html#btqyn4b-3

The second example can solve your problem:

 rgb = imread('example.tif');
numrows = size(rgb,1);
numcols = size(rgb,2);
alpha = 255*ones([numrows numcols], 'uint8');
data = cat(3,rgb,alpha);
t = Tiff('myfile.tif','w');
t.setTag('Photometric',Tiff.Photometric.RGB);
t.setTag('Compression',Tiff.Compression.None);
t.setTag('BitsPerSample',8);
t.setTag('SamplesPerPixel',4);
t.setTag('SampleFormat',Tiff.SampleFormat.UInt);
t.setTag('ExtraSamples',Tiff.ExtraSamples.Unspecified);
t.setTag('ImageLength',numrows);
t.setTag('ImageWidth',numcols);
t.setTag('TileLength',32);
t.setTag('TileWidth',32);
t.setTag('PlanarConfiguration',Tiff.PlanarConfiguration.Chunky);
t.write(data);
t.close();