There are many brands of graphics card for the Amiga, all claiming advantages over Commodore’s built- in chips and giving higher resolution and more colours. The .Amiga native displays are limited to about a third of a million pixels before they start to flicker. A decade ago this was reasonable, making the .Amiga well suited to TV and video work. But now computers use custom-made monitor displays, the .Amiga does not make best use of the new screen modes. Graphics cards offer thousands or millions of colours, so that several programs can share a screen without competing for a limited number of colour ‘pens’. ‘Chunky’ display modes store all the bits for each pixel together, which makes life easy for 3D games, PC and Mac emulation. It’s not all good news. When you switch to a graphics card you forfeit Amiga specialities like overlapping sprites and dual playfield (overlaid) displays - but you gain speed, resolution, colours and reduce flicker. For many graphics enthusiasts this is a worthwhile tradeoff, even if it means icons and screens move a little less smoothly. Whatever the resolution, a graphics card is useless without software. The first cards came with custom code, but as cards proliferated programmers looked for a standard way to drive them, regardless of manufacturer, and Retargetable Graphics (RTG) was born. The block diagram shows, in a simplified way, how RTG fits in with existing Amiga software. HIT THE METAL Old programs may use the Amiga graphics library or access the display hardware directly. RTG utilities like ChangeScreen intercept calls to the libraries, patching in the screen mode requestor to divert displays to the RTG system. This only works if the software does all its work with system calls, not ‘hitting the metal’ directly like many games. Retargetable software is not optimised for a particular screen format, as old Amiga software often was. New ‘system friendly’ Amiga programs support the screen mode requestor directly, so they can use either Amiga or Zorro graphics. When a graphics card mode is selected, system calls are directed to the RTG library'. Programs like CyberWindows and PicassoPhoto only work with a graphics card. These ‘retargetable applications' expect colourful public screens and use a new RTG API interface. This allows software for, say, CyberGraphX to work with Picasso96, even though the RTG library and drivers differ. You can only use one RTG scheme at a time, and your Amiga will get upset if you have two configured simultaneously.