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Copying pixel data

5 posts in this topic

I'm trying to write some code to copy a smaller section of a bigger image to a new image for use in my game. I'm using SFML to load the image from the file. I know the format of the pixels are RGBA and are stored in a block of memory sized as width*height*4. I assume this means that each pixel is 4 bytes(1 bit per color).

[source]Texture* AssetLoader::getSubTexture(const std::string &name, int x, int y, int w, int h) {
sf::Image image;

if(!image.loadFromFile(name)) {
return NULL;

// Get the pointer for the image's pixels
const unsigned char *pixels = image.getPixelsPtr();

// Create new buffer for our new image
unsigned char *newImage = (unsigned char*)malloc(w * h * 4);
unsigned int imageWidth = image.getSize().x; // Pixels per row
unsigned int imageHeight = image.getSize().y; // Pixels per column

// Copy the pixels into the new image
for(unsigned int row = y; row <= y+h; row++) {

unsigned int index = x + (row*imageWidth);

// Copy the entire row of pixels
memcpy(&newImage[(row-y)*w], &pixels[index], w*96);

Texture* result = new Texture((const void*)newImage, w, h);
delete[] newImage;
return result;

This code works for my current testing situation. The image I'm testing on is 32x32. The portion of the image I'm copying with this method is (0,0,32,32).

[code]memcpy(&newImage[row*w], &pixels[index], w*96);[/code]

With this code, I'm trying to copy an entire row of pixels into the new buffer. I would assume that, to copy an entire row of pixels, the size of memory I'm copying would be widthOfImage*4bytes. However, this only copies the fist few lines of the image and stops. When I do w*96 as in my example, the entire image gets copied. Of course, this breaks down if I try to copy a different size image.

My question is, what is happening here and how do I properly copy the pixels? I can't seem to figure out where the number 96*32 comes from that makes the image load fully.

32x32x4 = 4096
96*32 = 3072
1024 Edited by CirdanValen

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This is wrong:
[source lang="cpp"]// Copy the entire row of pixels
memcpy(&newImage[(row-y)*w], &pixels[index], w*96);[/source]
1. you have 4 bytes, not 96.
2. it is y-row not row-y, this gets negative, this is the reason you copy only the first line. Increasing 4 to 96 is a hack to copy more, but still a bug.

The correct version:
[source lang="cpp"]// Copy the entire row of pixels
memcpy(&newImage[(y-row)*w], &pixels[index], w*4);[/source]

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I just wanted to add, I don't think it's a good idea to mix malloc/delete. You should use 'new' to allocate your memory.

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