Other examples included material that had been browned or yellowed by age. This was also a case of contrast deficiency, and correction was done by fixed thresholding. A final example boils down to the same thing, slight variability, but it is not significant. Fixed thresholding solves this problem as well. The microfilm equivalent is certainly legible, but it comes with dark areas. Though THOMA did not have a slide of the microfilm in this case, he did show the reproduced electronic image. When one has variable contrast over a page or the lighting over the page area varies, especially in the case where a bound volume has light shining on it, the image must be processed by a dynamic thresholding scheme. One scheme, dynamic averaging, allows the threshold level not to be fixed but to be recomputed for every pixel from the neighboring characteristics. The neighbors of a pixel determine where the threshold should be set for that pixel. THOMA showed an example of a page that had been made deficient by a variety of techniques, including a burn mark, coffee stains, and a yellow marker. Application of a fixed-thresholding scheme, THOMA argued, might take care of several deficiencies on the page but not all of them. Performing the calculation for a dynamic threshold setting, however, removes most of the deficiencies so that at least the text is legible. Another problem is representing a gray level with black-and-white pixels by a process known as dithering or electronic screening. But dithering does not provide good image quality for pure black-and-white textual material. THOMA illustrated this point with examples. Although its suitability for photoprint is the reason for electronic screening or dithering, it cannot be used for every compound image. In the document that was distributed by CXP, THOMA noticed that the dithered image of the IEEE test chart evinced some deterioration in the text. He presented an extreme example of deterioration in the text in which compounded documents had to be set right by other techniques. The technique illustrated by the present example was an image merge in which the page is scanned twice and the settings go from fixed threshold to the dithering matrix; the resulting images are merged to give the best results with each technique. THOMA illustrated how dithering is also used in nonphotographic or nonprint materials with an example of a grayish page from a medical text, which was reproduced to show all of the gray that appeared in the original. Dithering provided a reproduction of all the gray in the original of another example from the same text.