how to get two-dimensional matrix from an generated image using frame2im

[image, Map] = frame2im(m(1));
points = detectSURFFeatures(image);

There is an error about the incorrect input for the detectSURFFeatures function. image contains 367x575x3 uint8 but the function only accept m by n matrix as input, how can I only get 367x575 from image?

Thanks