Scaling the Print Size of Images An Example for Converting Resolution and Size

When you open an image imported from your digital camera, GIMP tells you what the image dimensions are in the Image > Print Size window. For example, it might read as follows: 1200 pixels X 1600 pixels = 16.667 inches X 22.222 inches with a resolution of 72 pixels/inch (the figures of this example refer to the size and resolution of a 2 megapixel image from an elder camera). If you want to reduce the image to print size of 5 X 7 inches, for instance, you must set the value for Height to 7 inches. The image size will be recalculated to a size of 7 inches X 5.25 inches. The program calculates a new value of approximately 228 pixels/inch (exactly 228.571 pixels/inch) for the image's resolution. The Print Size function does not change the number of pixels. Therefore, the file size and the information content remain the same. This process does not require a recalculation of the pixels. The quality (= image information, amount of image data, number of pixels) is not changed, just the size of the pixel dots. Depending on whether the pixels are enlarged or shrunk, the dimensions of the image get bigger or smaller. The image is ready for print at a high quality. The canvas now has to be adjusted to the correct image size (the Image > Canvas Size option is described in section 2.3.8).

To size your image for use on the Internet, to send via email, or to use on a web page, select Image > Scale Image. Simply leave the resolution at 72 pixels/ inch (or 96 pixels/inch) and change the dimensions in inches or millimeters. This will reduce the number of pixels, making a recalculation of the image necessary. The result is a new number of pixels in the image: 378 pixels X 504 pixels = 5.25 inches X 7.00 inches with a resolution of 72 pixels/inch. Therefore, select Image > Scale Image > Interpolation: Cubic (Best) to create the highest-quality image.

d^J Set Image Print Resolution d^J Set Image Print Resolution

Figure 2.25

The Set Image Print Resolution window changing of the image size before printing

Figure 2.25

The Set Image Print Resolution window changing of the image size before printing

The following representation shows how resolution, image size, and quality interrelate:

The original as acquired from the camera.

1200 px x 1600 px = 16.667 in x 22.222 in at 72 ppi

Reversible M-►

The image as set to print size:

5.25 in x 7.00 in at 228.571ppi

Not reversible -►

The image as recalculated for the Internet:

378 px x 504 px = 5.25 in x 7.00 in at 72 ppi

Without recalculation (print size):

Number of pixels remains the same, quality (image information) and file size remain the same.

Number of pixels are reduced, quality (image information) and file size are reduced.

If you choose to enlarge an image, you must reduce the resolution by the factor by which you want to enlarge it. In this case, the entire number of pixels remains the same. The resolution and thus the print quality is reduced. To enlarge the image choose Image > Print Size. When printing, you should consider the fact that a resolution of less than 150 ppi will often produce poor results, even on a modern ink-jet printer. Generally, a resolution of 220 ppi is considered the bottom line for a good print.

There is an option to artificially enlarge an image using interpolation to increase both the size and resolution. This process calculates new image dots and adds them to the image. But if you enlarge an image beyond a certain size, it will usually become spongy and blurred. The existing image information is simply enlarged, and no additional details can be added later through calculation. Existing faults in the image are enlarged too, such as edges that appear from sharpening the image in GIMP. Nevertheless, there are qualitative differences depending on the interpolation method. Tests have shown that results with enlargements of a factor of 16x were still satisfactory. My own experience has shown that image editing programs such as GIMP can produce satisfactory results with a factor of 8x to 10x. The source of the image affects the quality. If it has a lot of contrast, sharpness with lots of detail, you can enlarge it more than if it were faint and blurred. You can enlarge as well as reduce images by selecting Image > Scale Image.

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