How to transform canvas getImageData() to single channel matrix?

[nwheatle]

Hi I'm looking to use the image processing features of your library.
The sobel_derivatives documentation says it uses "single channel only".
jsfeat.imgproc.sobel_derivatives(source:matrix_t, dest:matrix_t);
I don't know what is meant by single channel image.

How can I convert grayscaled getImageData() output ( [r0, g0, b0, a0, r1, g1, b0...] ) to a single channel matrix_t input for jsfeat.imgproc.sobel_derivatives?

Thank you for your help,
Nicole