Grafcode

Machine code colour graphics





Robert Erskine shows you how to speed up


your colour graphics on the ZX Spectrum





Although Spectrum BASIC is fairly fast, there are many


occasions when it is not nearly fast enough; particularly


when you need to print large or multiple images to the


screen.


The most efficient way of solving this problem is to use


machine code routines to do the job, for the speed of the


Spectrum's microprocessor is such that several graphics, or


even the entire screen, can be printed in a small fraction


of a second. Although there is a routine in the ROM which


enables you to do this (using an RST 10 instruction) the


process can be rather long winded unless each character in


the graphic follows the previous one on the screen. Fur-


thermore, since this routine draws its graphics symbols


from existing memory, you will always be confined to using


the standard character set and user defined graphics.


What would be ideal would be a program which enables you


to 'read' every byte of your most complex graphic master-


pieces and store them away in RAM for instant recall to any


position on the screen. The two programs in this article,


Grafcode and Grafprint, are designed to do exactly that.





Dynamic duo





Grafprint is a machine code program which builds up a


graphic image on the screen, including attributes, from a


data file held in memory. It can either be called from


within a BASIC program or incorporated in a larger machine


code program where it can be used to greater effect in


generating laser fire or swooping aliens.


The Demo program illustrates Grafprint at work, printing


a large green moon lander at the top left of the screen. By


altering lines 20 and 30 which hold the low and high byte


values of the first attribute position on the screen, you


will see that the image can be printed anywhere, in spite


of the peculiar design of the Sinclair display file.


Line 90 of Demo holds the machine code data of Grafprint


and line 100 holds the data for the graphic.


The Grafprint program is designed to be loaded from ad-


dress 32300 to address 32411 and therefore RAMtop should be


at 32299 or less. The first eight bytes are used as tempo-


rary pigeon holes by the main program, which starts at


32308. Although Grafprint can be moved elsewhere in memory,


it contains references to these first eight bytes and these


would therefore have to be changed.


Grafcode is a BASIC program which generates data files


of graphics for Grafprint. To use it, first create your


graphic image (or text) anywhere on the screen, preferably


from the top left-hand corner where you can find the start


address easily. The image may be created in the normal way,


using BASIC colour commands, user defined graphics and so


on. Then, when it is ready, activate Grafcode by entering


the command GO TO 9600. On no account RUN the program or


your splendid new handiwork will disappear without trace.





What's in store?





You will then be prompted to input the address from which


you want the graphics data to be stored, the /first/ attri-


bute address currently occupied by the graphic and the


number of character squares in the graphic. You will then


be asked to input each of the displacements between the


characters. These refer to attribute file displacements so


that one square to the right = 1, one square below = 32,


and so on. You may then sit back and hum a little tune for


a few seconds whilst Grafcode translates your handiwork


into a sequential data file which can be accessed by


Grafprint.


Having loaded Grafprint and prepared your data file, the


next step is to load the start address if the data file


into 32302/3 and the attribute address of the chosen screen


location into 32300/1. Obviously in a moving graphics


program, this would be done in machine code. To print the


graphic, simply enter the command RANDOMIZE USR 32308.


Grafprint works by moving a pointer around the attri-


butes file of the Spectrum and loading the attributes of


the current character into an address, followed by the


corresponding eight display file bytes for each character.


The data file which it reads is formatted so that each


character is represented by 10 bytes. The first byte holds


the displacement between the current character and the one


before, the second holds the attributes code and the


remaining eight hold the graphic image. The data for the


first character in a graphic also uses 10 bytes but instead


of starting with a displacement value, it holds the total


number of characters in the graphic.


The program incorporates a routine which calculates


which of the three screen zones the current character will


occupy and selects the corresponding display addresses


accordingly. This ensures that if the graphic crosses a


border between zones then the relative positions of the


characters are maintained.





The sky's the limit





There is no limit to the size of the data file which can be


used by Grafprint, other than the size of your machine's


memory, and if files are built up section by section, there


is no limit to the different shapes you can store. Having


used 21 user defined graphics in a picture you can convert


it to data using Grafcode, store it temporarily on tape and


continue with a new set of graphics, bringing the whole lot


together in one file when you have finished. Graphics in-


volving circles and other shapes can equally be stored.


If you intend to move the graphics rapidly around the


screen using Grafprint, it will be necessary to erase each


preceding image before printing the next. One way of doing


this is to use a machine code equivalent of PRINT OVER, say


by holding a blank graphic of the same size and shape as


the original and using Grafprint to print it over the top.


In some cases, it will be sufficient to clear the screen


between each printing because the speed of machine code is


such that the illusion of continuous action will be main-


tained. Screen clearing can be achieved by filling all the


display file addresses with zeros and all the attribute


addresses with an appropriate code. If the background is


complex and you don't wish to clear it between moves, you


can store a complete copy of the screen above RAMtop using


a block move routine in machine code and reprint it to the


screen between moves by means of a similar routine. Since


this technique uses up nearly seven thousand bytes of RAM


it is hardly worth contemplating on a 16K Spectrum, unless


the rest of your program is entirely in machine code.