Scaling

Digital image processing often involves the need to reduce the size of images stored on disk, and there are two primary methods for achieving this: scaling and image compression. Scaling is a straightforward technique that directly reduces the image's dimensions. On the other hand, image compression utilizes advanced algorithms to decrease the space occupied by the image while preserving most of its quality. However, the intricacies of these algorithms are beyond the scope of our current discussion

Downscaling

To scale an image down, we take X and Y scaling factors, and using their quotients against the width and height, we derive the new "reduced" dimensions. By looping through these dimensions, we extract corresponding values from the full-size image, multiplying the pixel coordinates by the scaling factors. Let's take a closer look.

Implementation

implementation can be found at src/transformations/scale.c

void deimos_apply_scaling(Image * img, int factorX, int factorY) {
  int newWidth = img->width / factorX,
      newHeight = img->height / factorY;

    /* ... */

    for (int i = 0; i < newHeight; i++) {
      for (int j = 0; j < newWidth; j++) {
        for (int c = 0; c < img->channels; c++) {

          int original_x = j * factorX;
          int original_y = i * factorY;

          scaled_img->bytes[(i * newWidth + j) * img->channels + c] = img->bytes[(original_y * img->width + original_x) * img->channels + c];
        }
      }
    }
  
  /* ... */
}

In the image scaling process, we iterate through each pixel of the smaller image. To extract the pixel value from the original image, we multiply the coordinates of the new image pixel by the scaling factor. This approach works because it essentially involves 'skipping' pixels in between, resulting in a shorter image at the expense of losing some details. If you think about it, a scaling factor of 2 means we are skipping every other pixel in the image.

Result

.\bin\deimos scale --sigma -2 .\data\img1.jpg .\docs\.gitbook\assets\scale-3.jpg

NOTE: the sigma above is just the scaling factor

Upscaling

Similar to downscaling, while upscaling we take X and Y factors, and set the dimensions of the new image via their product against the original dimensions, and we literally repeat the same loop, so a scaling factor of +2 would mean we are instead of skipping, now repeating each pixel twice.

This method of upscaling is calleed nearest-neighbour interpolation since we are just repeating the nearest neighour to the current pixel in the smaller image N times. This method is simple, at the cost of the image quality it outputs, which is why this method isn't really used much in production.

Implementation

implementation can be found at src/transformations/scale.c

void deimos_apply_scaling(Image * img, int factorX, int factorY) {
  int newWidth = img->width * factorX,
      newHeight = img->height * factorY;

    /* ... */

}

It is identical to the previous method except we mulitply the factors with the width instead of dividing them.

Result

.\bin\deimos scale --sigma 2 .\data\img1.jpg .\docs\.gitbook\assets\scale-plus-2.jpg

NOTE: the sigma above is just the scaling factor

PreviousAffine Transformations

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