Image Cropping and Resizing

Cropping an image refers to the process of cutting out a portion of the image to leave a smaller image that better captures (we hope) what you want in the photograph. Resizing referes to the process of changing the physical size of an image. Cropping loses some of the information from the original because a portion of the photograph is eliminated. Resizing at the same proportions keeps the information, but may lose some of the detail if pixels need to be combined to create the new size.

Cropping

Why crop a photograph? There are three reasons:

Composition

We will only look at a couple elements of composition of a photograph. This is a topic for a course on photography more than image processing. However, some aspects of composition can be enhanced through image processing, in particular cropping.

Rule of thirds

One common rule in composition of a photograph is to not have your subject centered; to not have a horizontal line, such as an horizon, or a vertical line, that divieds the photograph in half. Instead, the center of interest should be placed 1/3 of the width and/or height from the sides. A main horizontal line should be place 1/3 of the way from the top or bottom of the photograph, not halfway. If we fail to get these elements right when the picture is taken, we can change them with cropping. In the example below that has a grid indicating the thirds, we can see the subject is almost centered. This suggests the we should crop to get a better composition.

Thirds

Eliminating unwanted elements

Another aspect of composition is to make sure that the photograph does not have distracting elements that are not part of the intended scene.One way to do this is to crop so that the unwanted element is eliminated. Other ways are to use light or focus adjustments. The example below has a distracting purple of another thistle in the lower right (it would be worse if it were in focus). We could get rid of this and improve the composition by cropping.

Bee on thistle

 

Resizing

Resizing means that we change the size of an image without necessarily cropping it. If the proportions of the new size do not match the proportions of the original, then some cropping would also be involved. If we resize and image so that the image pixel size is changed, then the image is resampled to create a new array of pixels conforming to the new pixel size. The effects differ depending on whether we make the image smaller or larger.

If we make the image smaller, several pixels or parts of pixels in the old image will contribute to one pixel in the new image. This means that we will lose details of the image. However, we are presumably resizing the image to use it at that smaller size, so that we will not see the loss of detail. For example, the images on this page have been resized to fit comfortably on the web page. If we then re-enlarged such an image, the loss of detail would be very evident.

If we make an image larger in pixel size, then we must in effect expand pixels. The color in a pixel in the new image may derive from only one in the original, or may derive from two or three if it falls near a boundary. The image will suffer loss of clarity and if enlarged too much will become pixelated, where we can see the individual pixels from the original. You can see this by zooming in on an image. Enlarging the pixel size of an image should be minimized.