Re: [OpenCV4Android] What is a better way to read an image in a downsampling way, and then get the brightness value based

[Alexander Smorkalov]

Hello Linda Li,

You can use color conversions from OpenCV Java API. You can convert rgba frame to gray scale by function Imgproc.cvtColor(mRgba, mGray, Imgproc.COLOR_RGBA2GRAY). It is the preferred way for most CV algorithms.

[Andrey Pavlenko]

Mat imread(const string& filename, int flags=1);

function can automatically convert the image to gray-scale when the `CV_LOAD_IMAGE_GRAYSCALE` flag is specified;

see http://docs.opencv.org/modules/highgui/doc/reading_and_writing_images_and_video.html#imread

as for downscaling - you can call 

void resize(InputArray src, OutputArray dst, Size dsize, double fx=0, double fy=0, int interpolation=INTER_LINEAR )

see http://docs.opencv.org/modules/imgproc/doc/geometric_transformations.html#resize