Does imdecode return modified image?

edit

I don't think you're using imdecode right. First of all, you should almost never do a new Mat(). Certainly not as a parameter to imdecode.

Second, what format did you save the image as?

is that java ?

and yes, jpeg compression will modify your color ramp ,(it's not really imdecode related)

if you're concerned about this, avoid jpgs, and rather use png

I call it from Java, yes. I tried it with PNG first and got my described results. Then I tried JPG, BMP and even TIF all giving me the same results. I understand, that some algorithm (like JPG for example) does some fancy stuff internally, i.e. when storing the image. But I wouldn't expect it from BMP or TIF.

@all Thank you for your comments @Tetragramm Why should you not call imdecode the way I described? Btw. I am not calling with new Mat() as parameter but new Mat(bytes) where bytes is an array of bytes (byte[]), just to make sure I understood you correctly. I found this code in a library called DataVec which can be found here: github.com/deeplearning4j/DataVec The lines of code where the magic happens is in a class called NativeImageLoader:

public INDArray asMatrix(InputStream is) throws IOException {
    byte[] bytes = IOUtils.toByteArray(is);
    Mat image = opencv_imgcodecs.imdecode(new Mat(bytes), 6);

I will also file this question in the forum of DataVec but as DataVec directly calls opencv I thought this forum is the right place.

Any idea why this happens?

My apologies, I thought it was C++, not Java.