6.5. Scale Image

Overview

The Scale Image function enlarges or reduces the image. It changes the scale of the image content and resizes the canvas so that the whole image remains visible. It also gives you informations about the size and the resolution your image will have when printed.

It acts on the whole image. If your image has layers with different sizes, it is possible that making the image smaller will shrink some of them completely away. If this happens you will be warned before the operation is applied.

If you want to scale a particular layer, use the function Scale Layer

Pixel Dimensions

You must remember that an image can be located in four places: In the image file, in the RAM after loading, on your screen when displayed, on paper after printing.

This section deals with making your image bigger or smaller, what dimensions will be in RAM and then in file after saving.

When you open the dialog, the displayed dimensions are that of the original image. On screen, all pixels are displayed and the dimensions depend on the screen resolution and on the Dot dor Dot option you have choosen.

You can set there the Width and the Height you want to give to your image by adding or removing pixels, resulting in a variation of dimensions. Click on arrowheads to change the value in the text box. If you have clicked in this box, you can use the Up and Down arrow keys for precise setting. You can act directly on linear dimensions in the unit you want, and that will add or remove pixels. These linear dimensions in memory are not much useful: they depend on resolution and are different from that on screen. Only pixels are important, from which the section name.

Ratio is the ratio between the original dimension and the new dimension, for each X and Y axis. Default is 1. You can change it. If the adjacent Chain is intact both axis will develop jointly. If you break it by clicking on it, then you can set every axis separately: this will result in deforming the image.

You can enlarge an image safely only if the original has a high resolution, pixels enough to avoid adding new pixels. Else, missing pixels are calculated by interpolation but no new detail is added and the more enlarged the more blurred, with aliasing. You can improve the result by using the filter Sharpen (after scaling) but the best method is to use a high resolution when scanning an image you plan to enlarge.

Reducing your image may be necessary if you intend it to a Web page. You have to take in account that Internet users still have 15' screens and that your big image cann't be completely displayed on them. Most of screens work in 800x600 and 1024x768 resolution.

Adding or removing pixels is called "Resampling".

Print Size & Display Unit

This section deals with the size and resolution your image will have when printed. Remember that all the pixels of the image will be printed and that there will be no splitting.

When you launch the Scale function GIMP displays the dimensions of the original image that will allow printing without deformation.

As for screen, you can enlarge or reduce the printed image with the same risks.

Resolution deals with ouput resolution, that is the number of pixels that will be used in every inch of the printed image. You must not mistake the output resolution with the printer resolution which is expressed in dpi (dot per inch): several dots are necessary to print a pixel.

The resolution proposed by GIMP is the resolution of the original image. If you increase the output resolution the printed image will be smaller since more pixels are used for an inch. Conversely and for the same reason resizing modifies resolution.

Increasing resolution results in increasing the printed image sharpness. This is quite different from simply reducing the image size since no pixels are removed.

Modifying resolution also acts for the image file. It is not visible at the pixel level but it is at the dimension level (inch for example). By reducing resolution you decrease the dimensions in the image file. By combining resolution reduction and pixel reduction and by restablishing the original dimensions of your image you can reduce the weight of your image file if PNG compression is not enough.

Interpolation Type

Here you can select the method used to interpolate the color of added pixels, valid for screen display and printing.

  • None: no interpolation is used. Pixels are simply enlarged as they are when zooming.

  • Linear:

  • Cubic: The best method but needing much calculation.

[Note] Note

To prepare your image for printing in a particular format use the Crop tool, with inch for unit: The displayed size is the printing size.

[Note] Note

If scaling would produce an image larger than the "Maximum new image size" set in the Environment page of the Preferences dialog, you are warned and asked to confirm that you really want to do it. Saying yes will not necessarily have bad consequences, but you should give it a moment of thought, because very large images consume a lot of resources, and outrageously large images may consume more resources than you have, causing GIMP to crash or otherwise behave unpleasantly.