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I need to download an image and measure it's contrast. I have some Python code which does what I need, but now I need to convert it to C, so that I can use it from other languages which do not have OpenCV bindings. This is the Python version:

import cv2
import numpy as np

nparr = np.asarray(bytearray(image_file_in_memory), dtype=np.uint8)
img = cv2.imdecode(nparr, 0)

height, width, depth = img.shape
roi = img[0:bottom, 0:right]

edges = cv2.Sobel(roi, -1, 1, 1)

count = np.sum(edges);

This is what I have so far. But I can't get the pixel count to tally properly. It always returns the value height x width, instead of only the count of white pixels, like the Python version does.

int count = 0;
IplImage *img, *tmp;
CvMat *mat = cvCreateMat(1, src_len, CV_32SC3);
/* src is the in-memory image */
mat->data.ptr = (unsigned char *)src;

img = cvDecodeImage(mat, -1);
if (! img) return -1;
cvReleaseMat(&mat);

cvSetImageROI(img, cvRect(0, 0, right, bottom));
tmp = cvCloneImage(img);

cvSobel(img, tmp, 1, 1, 3);

count = cvCountNonZero(tmp);
/* or */
count = cvSum(tmp).val[0];

I need to download an image and measure it's contrast. I have some Python code which does what I need, but now I need to convert it to C, so that I can use it from other languages which do not have OpenCV bindings. This is the Python version:

import cv2
import numpy as np

nparr = np.asarray(bytearray(image_file_in_memory), dtype=np.uint8)
img = cv2.imdecode(nparr, 0)

height, width, depth = img.shape
roi = img[0:bottom, 0:right]

edges = cv2.Sobel(roi, -1, 1, 1)

count = np.sum(edges);

This is what I have so far. But I can't get the pixel count to tally properly. It always returns the value height x width, instead of only the count of white pixels, like the Python version does.

int count = 0;
IplImage *img, *tmp;
CvMat *mat = cvCreateMat(1, src_len, CV_32SC3);
CV_8SC1);
/* src is the in-memory image */
mat->data.ptr = (unsigned char *)src;

img = cvDecodeImage(mat, -1);
if (! img) return -1;
cvReleaseMat(&mat);

cvSetImageROI(img, cvRect(0, 0, right, bottom));
tmp = cvCloneImage(img);

cvSobel(img, tmp, 1, 1, 3);

count = cvCountNonZero(tmp);
/* or */
count = cvSum(tmp).val[0];