This service is more advanced with JavaScript available. Encyclopedia of Multimedia Edition. Editors: Borko Furht. Contents Search. Converting to gray-scale will equalize all the channels in the image, before they are dithered. But as each channel is dithered independent of each other and in a random way, the result is not a bitmap image as you would expect.
Instead you will get a splatter of color pixels, especially for mid-tone colors. Here is the correct way to generate a proper random dithered bitmap image. As a special feature for this operator, IM will ensure a bitmap image is generated in the special " -channels " option of ' All ' is used.
The best results can be obtained by using a slightly smaller range, just as you get using the " -monochrome " operator. The ' o8x8 ' ordered dither was part of the IM core code for a long time, but was not used. It was only added as an option to the " -ordered-dither " operator IM v6. At this time the maps were given more definitive names to allow further expansion of the " -ordered-dither " operator, though the older backward compatible 'tile size' names, were retained as aliases to the new names.
Also the 'maps' that produced the 'o3x3' and 'o4x4' were completely revised to produce a better 'diffused pixel' dither pattern. Before this the maps produced distinct 'clumps' of pixels. See Ordered Dither Upgrade notes page for examples of the old patterns before they were fixed, as well as other changes made during the development for the official release of the upgrades in IM v6.
Before the release of IM v6. With the Re-Development of Ordered Dither this limitation was lifted, better naming selected, and extra halftone screens orthogonal forms was added see example arguments below. This produces enough colors to be only slightly smaller than the GIF color table limits. With the increase in the number of colors the result looks a lot better that the results of a simple standard uniform colormap.
The use of " -virtual-pixel " by the " -fx " operator using calculated indexes like this was broken before IM version 6. As you can see this widened the range of semi-transparent pixels that use pure horizontal lines as a dither pattern. This gives a better better shadow effect, though it probably should only be used with a less fuzzy shadow than the example used here. Note however that this type of change to a threshold is very uncommon.
Though justified for the intended use in this case. Basically it does not properly define a gradient, or allow for lighter and darker shades of patterns. For that we need to make a much more complex threshold map, with more pixels, and more patterns. DIY Horizontal Line Dither Here I expanded the simple horizontal lines dither pattern I created above into a set of patterns, to produce a smoother gradient from 'off' to 'on'.
Legal Notices Online Privacy Policy. Dithering in web images Search. About dithering. When optimizing images, keep in mind that two kinds of dithering can occur:. Control dither in web images. This may be acceptable for some visual elements on the page, but if your basic navigation buttons and background graphics are dithered, parts of the page will be hard to read and the overall effect will be amateurish:.
Now that the vast majority of Web users have more sophisticated display screens most designers choose to use GIF graphics with custom colors or full-color JPEG graphics and just accept that they will dither on some small percentage of display screens that are still set to colors.
One compromise is to mix navigation graphics done in browser-safe colors with full-color JPEG graphics. The full-color images will be dithered on color screens, but the navigation buttons will look the same on all screens.
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