From char* array to two dimentional array and back algorithm goes wrong - objective-c

Question

I think my algorithm has flawed logic somewhere. Calling the two functions should return the same image however it doesn't! Can anyone see where my logic goes wrong?
These functions are used on PNG-images, I have found that they store colors as follows: ALPHA, RED, GREEN, BLUE. Repeatingly for the whole image. "pixels" is just a long array of those values (like a list).
My intent is to do a lowpass filter on the image, which is a lot easier logic if you instead use a two dimentional array / matrix of the image.
// loading pixels
UIImage *image = imageView.image;
CGImageRef imageRef = image.CGImage;
NSData *data = (NSData *)CGDataProviderCopyData(CGImageGetDataProvider(imageRef));
char *pixels = (char *)[data bytes];
// editing image
char** matrix = [self mallocMatrix:pixels withWidth:CGImageGetWidth(imageRef) andHeight:CGImageGetHeight(imageRef)];
char* newPixels = [self mallocMatrixToList:matrix withWidth:CGImageGetWidth(imageRef) andHeight:CGImageGetHeight(imageRef)];
pixels = newPixels;
and the functions looks like this:
- (char**)mallocMatrix:(char*)pixels withWidth:(int)width andHeight:(int)height {
char** matrix = malloc(sizeof(char*)*height);
int c = 0;
for (int h=0; h < height; h++) {
matrix[h] = malloc(sizeof(char)*width*4);
for (int w=0; w < (width*4); w++) {
matrix[h][w] = pixels[c];
c++;
}
}
return matrix;
}
- (char*)mallocMatrixToList:(char**)matrix withWidth:(int)width andHeight:(int)height {
char* pixels = malloc(sizeof(char)*height*width*4);
int c = 0;
for (int h=0; h < height; h++) {
for (int w=0; w < (width*4); w++) {
pixels[c] = matrix[h][w];
c++;
}
}
return pixels;
}
Edit: Fixed the malloc as posters pointed out. Simplified the algorithm a bit.

Answer 1

I have not tested your code but it appears you are allocating the incorrect size for your matrix and low pass filter as well as not moving to the next pixel correctly.
- (char**) mallocMatrix:(char*)pixels withWidth:(int)width andHeight:(int)height {
//When using Objective-C do not cast malloc (only do so with Objective-C++)
char** matrix = malloc(sizeof(char*)*height);
for (int h=0; h < height; h++) {
//Each row needs to malloc the sizeof(char) not char *
matrix[h] = malloc(sizeof(char)*width*4);
for (int w=0; w < width; w++) {
// Varje pixel har ARGB
for (int i=0; i < 4; i++) {
matrix[h][w+i] = pixels[h*w+i];
}
}
}
return matrix;
}
- (char*) mallocLowPassFilter:(char**)matrix withWidth:(int)width andHeight:(int)height
{
//Same as before only malloc sizeof(char)
char* pixels = malloc(sizeof(char)*height*width*4);
for (int h=0; h < height; h++) {
for (int w=0; w < width; w++) {
// Varje pixel har ARGB
for (int i=0; i < 4; i++) {
// TODO: Lowpass here
pixels[h*w+i] = matrix[h][w+i];
}
}
}
return pixels;
}
Note: This code, as you know, is limited to ARGB images. If you would like to support more image formats there are additional functions available to get more information about your image such as CGImageGetColorSpace to find the pixel format (ARGB, RGBA, RGB, etc...), and CGImageGetBytesPerRow to get the number of bytes per row (you wouldn't have to multiply width by channels per pixel).