Difference between revisions of "Video modes"

Revision as of 13:31, 23 October 2009

Contains stuff from Wikipedia.

The standard video modes or Screen Modes.

Those are :

• Mode 0: 160×200 pixels with 16 colors (4 bpp)
• Mode 1: 320×200 pixels with 4 colors (2 bpp)
• Mode 2: 640×200 pixels with 2 colors (1 bpp)
• Mode 3: 160x200 pixels with 4 colors (2bpp) (this is not an official mode, but rather a side-effect of the hardware)

The Video modes are known to display pixels with different sizes.

Basically, the Amstrad CPC Video work like a CGA video card from a PC. But extra features like a 16 colours mode exist.

Colours

• 3-level RGB

The Amstrad CPC (old) palette features 27 colours, despite the fact that the Video system (Gate Array and CRTC) could actually handle 32 colours.

Because this is a 3-level RGB custom palette.

This palette is used by the Amstrad CPC old series only and is not found in any other computers (but clones of course).

The 3-level (not bits) RGB uses three level for every red, green and blue color components, resulting in a 3³ = 27 colours palette.

• 12-bit RGB

Used by the Amstrad Plus series.

It's like the good old Amiga 500 palette.

Systems with a 12-bit RGB palette use 4 bits (16 values) for each of the red, green, and blue color components. This results in a 16³ = 4096 colours palette.

Mode 0 graphics

Mode 0 graphics are blockier thanks to the "2x1" ratio.

Courtesy from Wikipedia.

Yet the good amount of colours allows to use AntiAliasing technics to soften the shapes.

It is the mode that made CPC a Colour Personnal computer.

Ditherings are sometimes used in this mode, yet the good number of colours allows better colour layers.

Dithering more offers a good "texture" or feel effect than real "new colours".

Mode 1 graphics

The pixels are square, but the only 4 colours a bit... few.

Courtesy from Wikipedia.

Yet a massive amount of horizontal "split rasters" allows to multiply the number of colours displayed on the screen.

It's like using the colour attribute specificity (as on ZX Spectrum) but with horizontal limitations.

Basically a CGA standard PC video display.

Pictures and graphic material have to make good use of ditherings in order to get a realistic look, yet the square pixels allow smooth ditherings.

Mode 2 graphics

The pixels are 1x2, extremly slim.

Courtesy from Wikipedia.

This mode is difficult to use in a coloured way.

Better used as a Text Mode, yet it can be included in a splitted screen (games).

Because of this Mode, the Amstrad CPC was perhaps one of the 8 bit computer able to display the highest screen resolution in Full Screen (the so called "overscan")...

You have to dither a lot to get a good picture as there are no real colours shades.

Border

The border allows 1 another additionnal colour to be displayed on the screen with no tricks.

Amstrad Plus

The Amstrad Plus display the same modes, but 15 more colours can naturally be displayed thanks to the Hardwired Sprites.

This means 32 colours per screen with no tricks (including Border).

Also it is worth noting that the resolution of those Sprites (16x16 pixels) can be displayed through "magnification".

The basic x1/x1 pixel is like a Mode2 Pixel (a x=1 and y=2 ratio...or X= 0.5 and Y=1).

This can be multiplied by x2 or x4 in both dimensions (X and Y).

A mode 1 like pixel must then be a x2/x1 pixel. A mode 0 like pixel x4/x1.

Related pages

• CRTC
• Gate Array
• ASIC

External Links

Wikipedia's pageon the different 8 Bit systems Hardware Palettes and Modes including amstrad CPC of course...

Wikipedia's page on RGB palette including Amstrad CPC specific one

Les Sucres en Morceaux the French site from supersly, shows many exemples and technics to use the different Video mode of the amstrad CPC.