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.