Introduction to 24-Bit Graphics
The human eye has the ability to distinguish over 16 million different shades and colours.
A computer that can display this many colours is said to have true colour capability (or photo-realistic graphics). In technical terms, each pixel or dot on the computer screen needs to have 24-bits (BInary digiTs) to represent each colour. 24-bit computer graphics are the ultimate in representing photographs and images of the real world. Although the Amiga 1200 and 4000 do not possess a real 24-bit display they come very close with the HAM-8 (Hold and Modify - 8 bitplanes) display mode. This is used for the main Photogenics display to provide photo-realistic displays while editing images.
Image Processing
When scanning photographs into a computer, combining images into a collage, or putting together graphics for a presentation, it is often necessary to edit the source material into a form more sutable for the selected application. In it's simplest format this can be the conversion of a picture from a foreign graphics file format into another which can be loaded by the chosen application program. In a more complex situation we might need to sharpen a photograph or make it brighter.
Colour Space
Digital images are normally stored as three separate channels of information, Red, Green and Blue. Like the Red, Green and Blue dots on your television or computer monitor, these three primary colours can be combined to produce all the other colours and shades. This is called RGB colour space, and is the system that Photogenics uses for storing colour images. There are other systems that can be used for working with digital images. Examples are CMY, CMYK, HSV (or HSB), HLS, and YUV. Some of the special effects in Photogenics work by converting the image into one of these different systems, altering it, and converting it back again.
A brief description of the colour models follows. Of course you do not need to understand how these models work to use the program, this information is primarily provided for general interest.
RGB:
The standard colour system used by Photogenics and most image processing programs. Eight bits are stored for each colour (R,G and B), providing a total of 24-bits of colour information.
HSB (HSV):
HSB stands for Hue, Saturation and Brightness (or Hue, Saturation and Value for HSV) and is a different way of describing colours. Rather than mixing three primary colours, the colour is described as a combination of the colour (hue), the saturation (how white the colour is) and the brightness (how bright the colour is). The default colour system for selecting colours in Photogenics is HSB as it is a more logical way of describing colours than RGB.
HLS:
This is a variant of the HSB/V system: Hue, Luma and Saturation. With this system the Luma varies from 0 (black) to maximum (white), hue
and saturation work similarly to HSB/V above. This colour model is not used in the Photogenics colour selector, but HLS calculations provide some of the interesting paint modes (the HLS negative luma option, for example).
CMY:
When used for printing, colours are a mixture of the three primary ink colours, Cyan, Magenta and Yellow. The CMY option can be selected in the Palette requester to choose colours based on this system. It is the logial inverse of RGB.
CMYK:
Most professional printing systems actually mix the three primary ink colours (cyan, magenta and yellow) plus black.
YUV:
Colour television uses yet another system for colour encoding - YUV. With this system Y is the brightness (or grey scale component) of the image, and U and V describe the colour, relative to Y.
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