The most important decision which you must take when you are writing the graphics routines for a program is which graphics Mode to use. There are seven different display types available on the Electron, and an eighth is available on the BBC. Each display type has its own characteristics, and you must choose which is most suitable for your particular program. Some graphics modes, for example, allow you to have many different colours on screen, but poor detail, while others allow you to have very great detail but few colours. You select graphics modes by using the MODE x command. Details of the seven different available modes are given below:

Key:

My advice in choosing which mode to use would be to try, if at all possible, to use one of the 20K modes. The detail of Mode 1, and the colours of Mode 2, are both far superior to the quality of Mode 5. If you are an efficient programmer, you should find that it is perfectly possible to fit a program of a reasonable length into memory at the same time as 20K of screen. Shipwrecked, for example, used Mode 2 for its graphics, as does Shipwrecked 2 on EUG #35.

Fitting a program into memory as well as a large screen memory is fairly easy when you are using a tape or DFS filing system, but this can prove difficult with ADFS, because 4K of the memory normally used to store programs is taken away for the ADFS to use, leaving you only a mere 4.5K free for programs. Assuming that once you have loaded the program you do not intend to use the disk again, there is a way around this. If you switch over to a tape filing system, the ADFS memory becomes redundant and can be reclaimed for BASIC use. Once you have done this, of course, you cannot access disks without resetting the computer, so load and save options in your program are restricted to tape only. This technique will be explained in greater depth in a later article on memory management.

After you have decided which mode to use, and so you know how many colours are available, you must decide what colours these are going to be. In Mode 2, where you have sixteen colours, you have access to all of the colours available, and so this choice does not affect you. In four-colour modes, on the other hand, you only initially have access to black, red, yellow and white. These are a warm selection of colours, but you might want a colder selection in your program, or just simply a set of colours which go well with your screen design. To do this, you use the command:

      VDU 19,logical colour number,actual colour number;0;

The logical colour number is the number that you use with the colour command to select that particular colour. In a four colour mode this must be between 0 and 3, and in a two colour mode this must be either 0 or 1. The actual colour number is the number of the colour which you want to assign to that logical colour. The actual colour numbers are given below:

Looking at the table above, you might wonder what the difference is between flashing black/white and flashing white/black. If you look at them both separately, there is little difference, but if you look at them together, you see that when one is black the other is white, and vice versa. So, for example, if you put a logo on the screen with the logo in colour 8 and the background in colour 15, the screen flashes between white on black and black on white. To demonstrate this, turn on your Electron and type:

      >MODE 6:VDU 19,1,8;0;:VDU 19,0,15;0;         <RETURN>

You can take this further. Four is the maximum number of colours allowed on screen at any one time. Therefore for different screens you can have different selections of colours. The main menu screen might be in warm colours, and then subsequent menu screens in colder colours. The changing of these logical colours is called "Palette Switching". You can go even further and create simple animation using palette switching; an effect which is called "Colour Cycling". For example, you put a row of colour 3 above a row of colour 2, with a row of colour 1 below that, and colour 0 at the base. Initially, you set all logical colours to black. Then, if you set logical colour 0 to white, the base appears. Now set logical colour 1 to white as well, the bar moves up the screen. Repeat this with colours 2 and 3, and you have a simple animation. This is even more powerful in MODE 2, where you can go through 16 rows up. This is demonstrated in the demo that accompanies this tutorial.

The swirly effect on the title screen of this issue's title screen was achieved in this way. The colours black, blue, cyan and white are cycled through a pattern on the screen in order.

Having decided which graphics mode you are going to use, and which colours to have on the screen, you must decide how you are going to get your graphics onto the screen. There are many methods for producing graphics, and which you choose depends upon what you want the graphics for, and what you want them to do (and, of course, your programming ability). The most obvious is the PRINT command. This is a quick, easy way of putting text onto the screen. When used with the COLOUR command and the TAB(x,y) function, many interesting effects can be achieved.

These are, however, greatly limited by the fact that only letters can be displayed on the screen. This limitation may be overcome using user defined characters (see the Graphics chapter of the User Manual for details). Even using these characters, however, there are serious limitations. They can only use one colour per character, and so even if you make different characters different colours, your screen will still appear rather dull. Also, they are difficult to use when dealing with large areas of the screen.

This is where MOVE, DRAW and PLOT are useful. Using these commands, lines can be drawn across the screen, and, using PLOT, triangles can be drawn. These commands are covered in the Graphics chapter of the User Manual. The other major advantage of these graphics commands is that they can be used not just to set the colour of an area, but also to invert it. The GCOL command, which selects the colour in which graphics are displayed, takes two parameters:

GCOL x,c

Lines may now be drawn across the screen by typing:

MOVE x,y:DRAW xb,yb

The range of the screen is always 1280 pixels across, and 1024 pixels down. The bottom left corner is point (0,0), and the top right corner is (1280,1024). Larger graphics can be drawn using the PLOT command. The most common usage is as follows:

      MOVE x,y:MOVE xb,yb:PLOT 85,xc,yc

This plots a filled triangle with corners at (x,y), (xb,yb) and (xc,yc). PLOT also has many other functions - see the User Guide for details.

At first sight, it may seem that user defined characters serve no purpose once a programmer has learnt about the more flexible graphics commands, but this is not the case. If you try to produce detailed graphics using MOVE and DRAW, you will find that your program crawls along at an unacceptable rate. User defined characters, on the other hand, can produce detail quickly, especially if the pattern repeats, so that one character definition can be used repeatedly in several areas of the screen. This is demonstrated by the program which accompanies this tutorial.

So, it seems that programmers are faced with a difficult problem. They can use graphics commands and get colour, or they can use user defined characters and get detail, but what if they want both at the same time? There must be a solution - how else would games like Citadel manage to produce such detailed colourful graphics, and at such a speed?

There are, in fact, two possible solutions, both of which involve dealing with an area of memory known as screen memory. This is, however, very complicated, and will be discussed further in the next article.