Resolution and File Size
Rules are different for halftone and stochastic screening.
- For halftone screens the rule of thumb is that the sampling frequency should be twice the target screen frequency. See examples in Fig. 6-6. Mathematically this is s = 2 l f where s is the sampling frequency in samples per unit length, l is the screen frequency in lines per unit length and f is the factor by which the image is to be scaled.
- For stochastic screens, more image information can be retained at a given resolution.
- Thus, larger scale factors can be used without loss of image quality.
- When using stochastic screens the scanned resolution should be tuned to the actual output device (e.g. digital printer, imagesetter) and the targeted viewing distance.
Sharpness - Visual perception exaggerates the effect of edges.
Unsharp masking - (USM) uses this feature to sharpen the edges. It generates an edge shape like the response by adding a multiple of the negative of the original signal to the signal itself. This generates a signal similar to the response above.
- On drum scanners with PMTs the scanner does USM with a separate PMT for the USM.
- For desktop scanners USM must be done in an image editing program. We illustrate this with Photoshop. Other sharpen tools in Photoshop are Sharpen, Sharpen More and Sharpen Edges.