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Rasters

1,975 bytes added, 14:13, 3 January 2012
typo
When programming, the raster "Raster" is often referred a demo term which refers to as the act of modifying the setting colours while the picture is being drawn by the display (TV or monitor). This is done to display more than the usual [[Video modes]] number of inks between 2 scanlines, in interrupt modecolours.
This is a way to The display more than draws one line every 64microseconds, with the usual whole image drawn at a frequency of 50Hz.The [[Video modesCPU]] number ([[Z80]]) runs at about 4 MHz. Most instructions are multiples of coloursa microsecond and this makes timing for making rasters easy.
The time between the display of 2 Scanlines traditional raster is made by changing the CRTC allow the CPU to order the Gate array to modify the setting colour for one pen once each line creating a horizontal bar of inkscolour.
The screen is updated with a frequency But changing the colours can be used to have different areas of 50Hzthe screen in different colours, e.g..The [[CPU]] ([[Z80]]) runs at about 4 MHzfor a status panel and a game area.
So It has enough cycles between 2 scanlines is also a good technic to order thisdisplay very coloured layers.
You can also use ==Raster== Here I will refer to the traditional raster, where the colour for one pen is changed once each line. The final result is usually a horizontal bar of colour that covers the whole width of the screen. ==Split Rasters== Split rasters are made by changing the colour for one pen more than once on each line. The final result is usually more than one horizontal bar of colour side by side covering the whole width of the screen. ==Raster scroll== This is a form of split raster to "actually" display 2 different screens, sometimes with different Video Modesbut where the position of the first raster is offset each frame. The effect is that the entire split raster moves in the horizontal. If you then move the bars accross the screen then you can make the bars scroll. Ultimately, this can be used to form letters and a horizontal scrolling message. ==Diagonal rasters== This is a form of traditional raster, but instead of timing to ensure that colours are always changed at the same time on each successive line, the timing is altered so that the colours occur at successively earlier positions on succeeding lines. The effect can be done by ensuring each line does not take 64 cycles, but instead 63 or less cycles depending on the effect wanted.  ==Examples of Raster effect in Games==*Some GX4000 games are CPC games which include Rasters effect to add colours.**Crazy Car 2 : extra colours layer in the sky.**Fire and Forget 2 : Idem.**Switchblade : also allows a Graphic heavy good Red layer. *[[HUDStriker in the Crypts of Trogan]] : one of the rare games in Mode 1 with no interference with "massive" use of in-game rasters (not only in the [[ScrollingHUD]]actually). This games looks a lot like Switchblade indeed...It is a mod of the Switchblade engine.Yet there is a pseudo Colour attribute clash : when monster pass from one raster to another, they change colour==Examples of Raster Effect in Demos==
==Links==
*[http://en.wikipedia.org/wiki/Raster Wikipedia link to Raster concepts]
*[http://en.wikipedia.org/wiki/Raster_interrupt Wikipedia on Raster-interrupt]
==CPCWiki Links==
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