how do I Change the opacity level of mat?

[Khawar khan]

Hi, I'm newbee and I want to scale the image and then change the transparency level as well, I've successfully scaled the mat doing this

     Size size = new Size(width, height);

      // Scales image

      Imgproc.resize(cvImage, scaledImage, size);

Now I don't know how to change the tranparency level, I've searched for this but people are doing in opencv c++, while I can't find a way to do the same in opencv4android.

Thanks in advance.

[Alexander Smorkalov]

Hi,

For RGBA mat you can change transparency by multiplying the alpha channel of each mat element on scalar. You can do something like this:

Core.multiply(inputFrame, new Scalar(1,1,1,0.1), mIntermediateMat);

The last element of RGBA point is alpha channel.

[Khawar khan]

Sorry for late reply, Sir, I tried provided method but actually its not changing image's opacity, btw I've replaced inputFrame and mIntermediateMat 
with one mat which actually needs its opacity, am I doing wrong?

[Alexander Smorkalov]

Do you render your image as RGBA8888? If you use other pixel format alpha channel may be truncated.

[Khawar khan]

Well I'm rendering just by file name, not using any color type i.e. Highgui.imread("mnt/sdcard/image.png");, I'll give a try when I get free and will let you know, thanks any way.. I'm glad you're helping me out. :)

[Alexander Smorkalov]

The problem is in the imread/imwrite parameters. By default the functions read and write only RGB data cropping alpha channel. You need to set falgs parameter in the proper value to solve the issue. See docs here.
BTW, jpeg format does not support alpha channel by design. Use png only.