SDL Image Filtering

[alhanz]

Hey guys I'm trying to do an image filtering program using SDL library. The problem is, I'm able to extract out every pixels and the RGB to form the image. I tried to sharpen it however it doesn't change anything.

Supposingly, by using these matrix, it will sharpen the image.
int sharpen[3][3] = { {0,-1,0}, {-1, 5,-1}, {0,-1,0}};

This is what I've done.

int rsum = 0;
    int gsum = 0;
    int bsum = 0;
    int psum = 0;

    // Need to lock the surface before playing with the content
    SDL_LockSurface(source);
    for (int i = 0; i < img_size; i++) {
        // Get the content of the pixel
        // source -> pixel hold the address of the 1st pixel
        // *(source -> pixels) get the content of the pixel
        pixel = *((Uint32*) source -> pixels + i);

        // Get Red component
        temp = pixel & fmt->Rmask; /* Isolate red component */
        temp = temp >> fmt->Rshift; /* Shift it down to 8-bit */
        temp = temp << fmt->Rloss; /* Expand to a full 8-bit number */
        R[i] = (Uint8) temp;

        // Get Green component
        temp = pixel & fmt->Gmask; /* Isolate green component */
        temp = temp >> fmt->Gshift; /* Shift it down to 8-bit */
        temp = temp << fmt->Gloss; /* Expand to a full 8-bit number */
        G[i] = (Uint8) temp;

        // Get Blue component */
        temp = pixel & fmt->Bmask; /* Isolate blue component */
        temp = temp >> fmt->Bshift; /* Shift it down to 8-bit */
        temp = temp << fmt->Bloss; /* Expand to a full 8-bit number */
        B[i] = (Uint8) temp;

        for (int x = 0; x < img_height; x++)
        {
            for (int y = 0; y < img_width; y++)
            {
                for (int row = -1; row <= 1; row++)
                {
                    for (int col = -1; col <= 1; col++)
                    {                             
                                rsum += filter[row + 1][col + 1] * R[i]; // Red
                                gsum += filter[row + 1][col + 1] * G[i]; // Green
                                bsum += filter[row + 1][col + 1] * B[i]; // Blue
                                
                    }
                }
	    }		
	}
    }

    SDL_UnlockSurface(source);

So what I did is to loop every single pixels then try to multiply it. It suppose to sharpen the image. But it didn't do anything. Any solution to this?

[Peter87]

I don't understand what you mean by "sharpen the image". Is filter the same as sharpen? You never write anything back to the pixel data so that's why the image doesn't change.