Idea Digital Imaging

Why do we need to sharpen images?

Creating a digital image, whether scanned or photographed, involves converting a real object into a representation made up of rows of square pixels. Detail that is finer than the pixel widths will be lost. Detail that falls between the sensor’s sample points will also be lost. Therefore, even the highest resolution scans on the finest scanners will require some sharpening.

You may have also noticed that working with pixels causes some strange illusions. Very high resolution scans often appear to be softer than lower resolution scans of the same image. This is usually caused by the sleight of hand required to show the image on a monitor where we view yet another representation of our image that has been re-sampled by your graphics card to a different pixel resolution.

Print sharpening

Images that are reproduced on a printing press are created from one or more monotone (single colour) plates. The impression of tonal values other than 0 or 100 per cent is created by using dots of varying size. Dividing the image into a fine mesh of dots causes loss of detail and a perceptual softening of the image.

The coarser the dot, the softer the image

Newspaper printing uses an extremely coarse dot with a frequency of between 65 and 85 lines per inch. An ordinary black and white image usually turns to mush on newsprint unless it has been sharpened robustly. You will notice that it is common for advertisers to spend a lot of time and money reinforcing and enhancing the detail of images in mono newspaper ads. The most expensive and clearest are actually “line & tone combines” where the image has been changed to an illustration made from a composite of a screened image and a line illustration.

For normal litho or web printing we don’t need to do that much work, but sharpening is still required to prevent the images looking soft.

Images from digital camera

Most of the sharpening techniques discussed here were designed and/or invented for use with images that were scanned from transparency, negative or print. Scanned images have distinctive grain patterns and noise that you don’t find in digital images. Sharpening this type of image is all about getting maximum sharpness without enhancing the grain.?

Images from digital cameras don’t have problems with grain; they have problems with noise. The noise can usually be seen lurking in one of the RGB channels and is caused by the CCD generating it’s own image information rather than capturing the image that the camera was pointed at.?

The problem with CCD noise is that it isn’t as regular as the grain in a scanned image and it has a colour. You’ll also find that working without grain causes other problems like banding and contouring in flat areas of your images. You may find yourself adding noise or grain effects to remove this.