How to "smart resize" a displayed image to original aspect ratio

Question

I have an application in which end-users can size and position images in a designer. Since the spec calls for the image to be "stretched" to the containing control, the end user can end up with an awkwardly stretched image.

To help the user with image sizing I am thinking of implementing a smart resizer function which would allow the the user to easily fix the aspect ratio of the picture so that it no longer appears stretched.

The quick way to solve this is to actually provide two options: 1) scale from width 2) scale from height. The user chooses the method and the algorithm adjusts the size of the picture by using the original aspect ratio. For example: A picture is displayed as 200x200 on the designer but the original image is 1024x768 pixels. The user chooses "Smart Size from width" and the new size becomes ~200x150 since the original aspect ratio is ~1.333

That's OK, but how could I make the algorithm smarter and not bother the user by asking which dimension the recalculation should be based on?

Answer 1

If I'm interpreting your spec correctly, you want a result that is no larger than the one the end-user laid out originally; you want one of the two dimensions to shrink, and the other to stay the same. In other words, the new size should fill the designer space in one direction while shortening the size in the other direction to retain the original aspect ratio.

original_ratio = original_width / original_height
designer_ratio = designer_width / designer_height
if original_ratio > designer_ratio
    designer_height = designer_width / original_ratio
else
    designer_width = designer_height * original_ratio

Often you'll be working with integer coordinates, but the divisions to produce the ratios above need to be floating point. Here's a rearrangement of the formula to be more integer friendly. Make sure your integers have the range to handle the maximum width*height.

if original_width * designer_height > designer_width * original_height
    designer_height = (designer_width * original_height) / original_width
else
    designer_width = (designer_height * original_width) / original_height

Answer 2

Here's a solution I came up with when having to deal with this problem. Turned out pretty short and straightforward imo, just wanted to share it.

Handy "formulas": ratio = W / H → W = H * ratio → H = W / ratio

1. Calculate the ratio.
2. Calculate the height of the image if you would set the width to the new size width (maximum allowed width).
3. Calculate the width of the image if you would set the height to the new size height (maximum allowed height).
4. See which of the two sizes does not override the max size in any dimension. If the ratio is not 1 one of them will always be wrong and one will always be correct.

In Javascript

// Returns array with new width and height
function proportionalScale(originalSize, newSize)
{
    var ratio = originalSize[0] / originalSize[1];

    var maximizedToWidth = [newSize[0], newSize[0] / ratio];
    var maximizedToHeight = [newSize[1] * ratio, newSize[1]];

    if (maximizedToWidth[1] > newSize[1]) { return maximizedToHeight; }
    else { return maximizedToWidth; }
}

originalSize and newSize is an array [0] = width, [1] = height

Answer 3

I also wanted to know this and all I saw were endless examples of scaling width OR height but could leave the other overflowing.

• Resize width AND height without the need for a loop
• Doesn't exceed the images original dimensions

.

private void ResizeImage(Image img, double maxWidth, double maxHeight)
{
  double srcWidth = img.Source.Width;
  double srcHeight = img.Source.Height;

  double resizeWidth = srcWidth;
  double resizeHeight = srcHeight;

  double aspect = resizeWidth / resizeHeight;

  if (resizeWidth > maxWidth)
  {
    resizeWidth = maxWidth;
    resizeHeight = resizeWidth / aspect;
  }
  if (resizeHeight > maxHeight)
  {
    aspect = resizeWidth / resizeHeight;
    resizeHeight = maxHeight;
    resizeWidth = resizeHeight * aspect;
  }

  img.Width = resizeWidth;
  img.Height = resizeHeight;
}

Answer 4

Calculate the new dimensions for both variants ("scale by width" and "scale by height"), then use the one that fits in the display.

Alternatively you could also calculate the the aspect ratio of the "bounding box" and compare it against the aspect ratio of the original image. Depending on which aspect ratio is larger, the height or the width needs to be scaled.

You also could restrict the resize process so that in all cases "scale by width" is done. Then, to change the size of the image, the user always has to change its width. The height will always be adjusted automatically.

Answer 5

Took the suggestion above and made it scale up/down within max height / width. Here the python code for it and also added support for rotating things while keeping within limites:

def _resize(image, dimensions, rotate=None): """ Resizes an image to be as close as possible to specified dimensions. Image is a django image-model-field.

    Will both scale up and down the image to meet this while keeping the proportions
    in width and height

"""

if image and os.path.isfile(image.path):

    im = pil.open(image.path)
    logging.debug('resizing image from %s x %s --> %s x %s ' % (im.size[0], im.size[1], dimensions[0], dimensions[1]))

    if rotate:
        logging.debug('first rotating image %s' % rotate)
        im = im.rotate(90)

    srcWidth = Decimal(im.size[0])
    srcHeight = Decimal(im.size[1])

    resizeWidth = srcWidth
    resizeHeight = srcHeight

    aspect = resizeWidth / resizeHeight # Decimal

    logging.debug('resize aspect is %s' % aspect)

    if resizeWidth > dimensions[0] or resizeHeight > dimensions[1]:
        # if width or height is bigger we need to shrink things
        if resizeWidth > dimensions[0]:
            resizeWidth = Decimal(dimensions[0])
            resizeHeight = resizeWidth / aspect

        if resizeHeight > dimensions[1] :
            aspect = resizeWidth / resizeHeight
            resizeHeight = Decimal(dimensions[1])
            resizeWidth = resizeHeight * aspect

    else:
        # if both width and height are smaller we need to increase size
        if resizeWidth < dimensions[0]:
            resizeWidth = Decimal(dimensions[0])
            resizeHeight = resizeWidth / aspect

        if resizeHeight > dimensions[1] :
            aspect = resizeWidth / resizeHeight
            resizeHeight = Decimal(dimensions[1])
            resizeWidth = resizeHeight * aspect

    im = im.resize((resizeWidth, resizeHeight), pil.ANTIALIAS)

    logging.debug('resized image to %s %s' % im.size)
    im.save(image.path)

else:
    # no action, due to no image or no image in path
    pass

return image