Opencv... getting at the data in an IPLImage or CvMat

Question

I am doing some simple programs with opencv in python. I want to write a few algorithms myself, so need to get at the 'raw' image data inside an image. I can't just do image[i,j] for example, how can I get at the numbers?

Thanks

Answer 1

Quick example of using LoadImageM to load an image file directly into a cvmat:

import cv

path = 'stack.png'
mat = cv.LoadImageM(path, cv.CV_LOAD_IMAGE_UNCHANGED)
x, y = 42, 6
print type(mat)
print mat[y, x]

Output:

<type 'cv.cvmat'>
(21.0, 122.0, 254.0)

---

Quick example showing how to multiple one or more color channels by 0.5:

for x in xrange(mat.cols):
    for y in xrange(mat.rows):
        # multiply all 3 components by 0.5
        mat[y, x] = tuple(c*0.5 for c in mat[y, x])

        # or multiply only the red component by 0.5
        b, g, r = mat[y, x]
        mat[y, x] = (b, g, r * 0.5)