Page 55 - DCAP303_MULTIMEDIA_SYSTEMS

Unit 4: Image

figure 4.11: resolution of image notes

This is not the whole story when it comes to representing bitmaps on physical devices because
different devices have different colour depth capabilities.

4.1.3 Colour Depth Conversion
Very often it is necessary to represent a bitmap with one colour depth onto a device with different
colour depth capabilities. Of course, if the destination device has better colour than the bitmap
then there is no issue since the bitmap can be exactly represented. In the reverse situation where
the destination has different and lower capabilities, then the bitmap has to be converted into
something that gives the best possible representation.
As an example, consider the problem of representing greyscale images on monochrome (black and
white) devices. This is achieved by using a variable number of black and white pixels to represent a
grey level. Fortunately the black and white device usually has much higher resolution than the bitmap
so there are a number of pixels available to create the greyscale approximation. Consider a 75 DPI
greyscale bitmap (See Figure 4.12) to be displayed on a 300 DPI black and white printer. There is a
matrix of 4×4 black and white pixels that can be used to represent each greyscale pixel.
figure 4.12: 75Dpi Greyscale Bitmap

There are a number of techniques that can be used to form the corresponding arrangement of
black and white pixels, one technique is called dithering. Even using dithering, there are lots of
possible algorithms for deciding the dithered pixel arrangement. The following shows a grey
level ramp (See Figure 4.13) with the corresponding black and white dithered examples (greatly
enlarged) using pattern and diffusion dithering.

figure 4.13: Grey Level ramp

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