Spectrum Sprites

Spectrum Sprites





Robert Newman controls sprites in Basic





This article describes a machine-code routine which adds a


number of new commands to Spectrum Basic including several


which permit the design and control of sprite graphics.


A previous article of mine on the subject of sprite


graphics appeared in the January 1984 issue of Your


Computer. This new routine has a number of new improve-


ments over the earlier version. Because the sprites can now


be controlled with Basic commands, instead of using Pokes,


programming is simpler and the program itself is easier to


read.


Sprites can now be animated. Up to four frames can now be


defined for each sprite and the routine automatically swit-


ches from one frame to the next at a chosen speed. The


method used for printing sprites has also been made faster,


and sprites can move over other subjects on the screen


without erasing them.


The routine can control up to eight sprites on the screen


at a time. An interrupt routine which is called every 1/50


second while your program is running handles all the work


needed to move the sprites, animate them, and detect


collisions.


[At this point there was a paragraph explaining how to


enter the machine code routine, but not, amusingly, how


to save it or load it back - although admittedly those are


pretty elementary procedures. Neither was any basic loader


given for the code, unlike for some similar systems by the


same author. I've provided almost the simplest possible


one; it's on the .tzx file which goes with this text under


the name of "Sprites". The machine code itself follows,


called "sprites" with a lower-case 'S'.]


The extended Basic is switched on with:


RANDOMIZE USR 60000


This command must be given before you can use any of the


new commands, and it should be the first line of any pro-


gram which uses the extended Basic. The new commands are


all preceded by an exclamation mark and can be typed in


capital or lower case letters.


Most of the commands must be followed by several numeric


expressions - a number or a variable. The !Doke and !Data


commands will also accept hex numbers as a dollar sign


followed by up to four hex digits - e.g., $FE59.


If a command cannot be executed for any reason, the pro-


gram stops with an error report as normal. Some of the


commands have new error messages, for example an !Until


which was not precedented by !Repeat gives the error


"Mismatched Until".


!GRAPHIC character number: Character number must be in


the range 1 to 32. This command tells the routine where to


store the character pattern in the !Data command.


!DATA line1, line2...: this command can be followed by up


to 16 numeric expressions, one for each line of the charac-


ter being defined. Each numeric expression can be a decimal


number - maximum 65535 - a hex number, or a Bin number.


!CHAR sprite number, character 1 (, character 2...): This


command shows what graphic character to use when printing


the sprite. If you want the sprite to be animated, you can


give a list of up to four character numbers, and the


routine will automatically switch from one frame to the


next at a speed set by the !Frames command.


!FRAMES sprite number, delay: Delay can take values from


1 to 255, and controls the speed that the routine switches


frames for an animated sprite.


!ON EDGE sprite number, edge action: This command con-


trols what happens if a sprite reaches the edge of the


screen. If edge action = 0, the sprite continues moving


with wrap-around. A value of 1 causes the sprite to stop.


A value of 2 makes it bounce.


!ON HIT sprite number, hit action: If hit action = 0, the


sprite continues moving if it hits another object on the


screen. A value of 1 makes it stop.


!SPRITE sprite number, x, y: The sprite is printed on the


screen, where x and y are the pixel co-ordinates of the top


left-hand corner of the sprite's position.


!SPRITE sprite number, x, y, xmove, ymove, steps: The


sprite is printed at position (x,y) and it then moves away


for a specified number of steps - 1 to 254. It moves by


xmove and ymove pixels at each step - possible values are


from -8 to +8. If steps = 255, the sprite moves continu-


ously.


!ERASE sprite number: The specified sprite is erased.


!ERASE 0 will erase all active sprites.


!DI and !EI: The interrupt routine which moves the


sprites is switched on automatically by the RANDOMIZE USR


60000 command, and normally continues running while the


Basic program runs. It can be switched off by !DI and re-


enabled by !EI.


To find out the position of a sprite at any time, or to


examine its collision flag, the following user-defined


functions can be used - where S=sprite number from 1 to 8:


DEF FN x(s) = PEEK(63727 + 32*s): REM X co-ordinate


DEF FN y(s) = PEEK(63728 + 32*s): REM Y co-ordinate


DEF FN c(s) = PEEK(63722 + 32*s): REM collision flag


The collision value is zero for no collision, 1 at the


edge of the screen, and 128 when colliding with background


or another sprite.


!PUT character number, x, y: The specified graphics


character is printed on the screen at position (x,y).


!REPEAT : !UNTIL condition: These two commands are found


in Pascal and structured Basics any commands between the


Repeat and the Until will be repeatedly executed until the


finishing condition becomes true.


!DOKE address, value: This command does a double POKE of


a 16-bit value to two consecutive addresses. Hex numbers


can be used for the address and for the value to be poked.


!SET INK old colour, new colour: This command changes one


ink colour on the screen to another colour. This can be


used to make a picture appear instantly on the screen by


first drawing it "invisibly" with its ink colour the same


as the background paper colour and then changing the ink


colour to make the picture appear.


!SET PAPER old colour, new colour: This command changes


paper colours.


!SCREEN paper colour, ink colour: This command changes


the colour of the screen without erasing its contents.


!BREAK OFF/!BREAK ON: These commands disable or enable


the break key. [These seem to have been left off the


internal command table; they are not accepted.]


!ZAP: This command makes a short zapping sound like a


laser.


!NOISE length: This makes white noise for a time depen-


ding upon length, which can take values from 1 to 255.


You can test the routine by typing in the short demo


program in listing 3 [on the .tzx as "Demo"]. This program


shows how to set up sprites and get them moving, either


under computer control, or in response to the keyboard.


Before you can type in any of the new commands, you must


type RANDOMIZE USR 60000. This command switches on the


extended Basic and lets the new commands be recognised and


interpreted, so it must also appear as the first line of


any program.


Listing 4 is a graphic character editor program which you


can use to design graphics instead of using the !Graphic


and !Data commands. The 32 graphics characters are stored


in memory between addresses 64280 and 65367. The editor


program lets you design graphics on a large 16 by 16 grid


and store them in the memory. The character set can be


saved and loaded into another program to be used by the


sprites routine. [This program is on the .tzx as "Editor.


Unfortunately it uses IN to read the keyboard, so it will


only work on an Issue 2 Spectrum.]