manipulating luma in YUV color space

Question

I wont to set contrast / brightnes on image witch is in form byte[]. The image is in YCbCr_420 color space (android camera). I am geting luma value this way :

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Answer 1

The little that I know from this API is that the values for the 3 channels are concatenated in a byte array. So, likewise in Windows working with RGB, I guess here you have the same, I mean, every 3 bytes representing one pixel. So I GUESS, you could access it jumping every 3 positions to access only one single channel (Luma in your case). I just don't know if the luma is represented by the first, second or third byte.

Second, if I understand you just want to change bright/contrast (increase/decrease) is that correct? Because if so, contrast is just multiplication and brightness is addition.

For instance - pseudo code assuming you are working with 8bits channel:

luma[y] = luma[y] * 1.10; //Increases contrast

or you can have a more generic form:

luma[y] = luma[y] + (luma[y] * contrast); //where contrast ranges from -1.0 to 1.0

Similarly you can do the same with brightness:

luma[y] = luma[y] + bright; //Where bright can range from -255 to 255

In both cases you have to be careful with overflows and underflows before you assign the final pixel result to your image.