Image Metadata

Metadata is data about an image or image file. It is not the pixel colour data itself: it does not store what colour each pixel is. It stores extra information that describes the image and the file, so software and people can understand and use it correctly.

Typical metadata includes the image width and height in pixels, the colour depth, the file format, the date the image was created or changed, and sometimes a location. For example, a photo of a skatepark saved as skatepark.png might have metadata saying it is 420 pixels wide, 260 pixels high, uses a colour depth of 8 bits, is a PNG file, was saved on 12 June and was taken in Leeds. None of these values is the colour of a pixel: they all describe the image or the file.

Why some metadata is essential

The width, height and colour depth in metadata are needed to interpret the pixel data correctly. The colour depth tells the computer how many bits make up each pixel's colour code. The width tells it how many pixel codes are in each row, so it knows where each row ends and the next one begins. The height tells it how many rows there are. Without these values, the same stream of bits could be split into pixels in the wrong-sized groups or arranged into the wrong-shaped grid.

Effect on file size and quality

Metadata adds a small amount to the file size, because the extra values and text must be stored too. It does not affect image quality, because it does not change the pixel colour data. A file-size calculation may ignore metadata and count only the pixel data, or it may include a small allowance for it. Either way, metadata is not what makes an image look sharper or more detailed.