An image file such as a JPEG or PNG file contains two types of data. The image essence and metadata.
The image essence is what is usually thought of as the image, i.e. it is colours and the position of those colors in the picture. When an image file is viewed, the image essence is unpacked into a two-dimensional array of color values in memory. Typically each pixel is represented as a four-value tuple: (red, green, blue, alpha). Each element in the tuple can have a value from 0 to 255 and the “alpha” value indicates the level of transparency for the pixel.
The metadata can hold information such as a description of the photo, copyright information, camera details, and other fields. Additionally, the metadata contains information about the dimensions of the image. The dimensions are represented in two different formats. These formats are referred to as the “Pixel Dimensions” and the “Document Size” in Photoshop.
The “Pixel Dimensions” is the size of the image measured in pixels in the X and Y directions. These pixel-based dimensions allow the image essence to be mapped from a linear array (such as a file) to a two-dimensional array in memory. The Pixel Dimensions size determines the size of the image when viewed in the majority of software because software is typically written to show one image pixel for every screen pixel.
The “Document Size” dimensions format consists of three values: (width, height, resolution). The width and height are be represented in Photoshop as linear units (mm, inches, etc) or as relative units (percentage of original). The resolution value is measured in pixels/inch (or pixels/cm). The Document Size dimensions are typically most important when printing an image. If the pixels/inch value matches the resolution of your printer (dots/inch) then the image on the printout will be width by height in size. However, if the resolution of your printer is twice that of your image you are likely to get a printed image that is half the height and half the width of the “Document Size” values. The reason is that that without any specific request for scaling, the printer will map one image pixel to one dot of output.
To resize an image in photoshop, open the image and select the “Image Size” tool from the “Image menu. In Photoshop CS6 this produces the dialog shown below:
Photoshop CS6 Image Size Dialog
Changing the Pixel Dimensions will change the size of the image as measured in pixels. The checkboxes at the bottom of this dialog control which numeric values are locked to the same relationship as in the original image.
Resolution values specific to the output or viewing device. Typical values for various devices are:
- old monitor: 72 pixels/inch (common old publishing assumption)
- average monitor: ~120 pixels/inch
- low-end printer: 300 dots/inch
- typical printer: 600 dots/inch
- Retina laptop: 220 pixels/inch
As with most Photoshop dialogs, holding down the option-key will change the “Cancel” button to a “Reset” button. The reset button will revert the settings in the dialog back to the original values from the image.
The Image Size dialog is best used to change the size of an image without adding or removing any image content[1. Other posts describe the related “Canvas Tool” (for adding to images without scaling) and the “Cropping Tool” (to remove parts of the image.)]. That is, the Image Size tool is used to scale an image up or down.