TICT TUTORIAL SERIES 1 - Part VII (c) TI-Chess Team 2001
Fast Pixel Access Macros and Line Drawing

Focus Of This Tutorial

This tutorial isn't a real tutorial, but a code demonstration. It shows how to implement the Bresenham line drawing algorithm in C using the TIGCC environment. My english is not good enough to explain the details of the algorithm, but looking on the code should be explanatory enough. If you need such a routine in one of your projects, feel free to copy-and-paste in your source.

Fast Pixel Access Macros

Line drawing, cycle drawing and similar routines have at least one point in common: each of them manipulate single pixels and these manipulations have to be done as fast as possible.
Manipulating single pixels is a little bit cumbersome on the TIs (as well as slow), because 8 pixels are packed into each single byte of the graphics buffer. The following macros are the fasted way I know to access and manipulate single pixels.
Note that using "(y<<5)-(y<<1)" as replacement of "y*30" is even much faster than using a lookup table.
#define PIXOFFSET(x,y)  ((y<<5)-(y<<1)+(x>>3))
#define PIXADDR(p,x,y)  (((unsigned char*)(p))+PIXOFFSET(x,y))
#define PIXMASK(x)      ((unsigned char)(0x80 >> ((x)&7)))
#define GETPIX(p,x,y)   (!(!(*PIXADDR(p,x,y) & PIXMASK(x))))
#define SETPIX(p,x,y)   (*PIXADDR(p,x,y) |=  PIXMASK(x))
#define CLRPIX(p,x,y)   (*PIXADDR(p,x,y) &= ~PIXMASK(x))
#define XORPIX(p,x,y)   (*PIXADDR(p,x,y) ^=  PIXMASK(x))

Using the Macros on Buffers with Different Sizes

If you want to use a backbuffer which is not 240 pixels (30 bytes) wide, just modify the first macro which calculates the pixel offset. The rest of macros can be used as they are.

For example: suppose you have a backbuffer which is only 160 pixels (20 bytes) wide. The corresponding PIXOFFSET macro looks like this:
#define PIXOFFSET(x,y) ((y<<4)+(y<<2)+(x>>3))

Mixing Buffers with Different Sizes

If you want to mix backbuffers of different sizes you can to this by writting two complete separate sets of macros or by using #undef and #define to redefine just macro PIXOFFSET for a specific routine or block of code. Suppose you have defined the macros as above somewhere at the beginning of your sourcecode. By default the macros will now handle buffers which are 240 pixels wide. The following sourcecode shows how to use #undef and #define to redefine the PIXOFFSET macro just for one routine (or a couple of routines), so that the macros will handle "temporary" another buffer width:
#define PIXOFFSET(x,y) ((y<<4)+(y<<2)+(x>>3))

// now pixel access macros handles 160 pixel wide buffers ...

void RoutineWhichOperatesOn160PixelWideBuffer() {
   // some code ...

#define PIXOFFSET(x,y)  ((y<<5)-(y<<1)+(x>>3))

// now pixel access macros handles again 240 pixel wide buffers ...
Of course this way of re-defining macros is not the best style of programming, but sometimes it very handy.

... And The Credits go to:

Contact TI-Chess Team Members

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Suggestions, bug reports and similar are VERY welcome (use our Messageboard for this!!).

How to Thanx the Author?

Like Xavier from the Doors Team I like it to get postcards from all over the world. If you want to thank me, just send me a postcard with greetings on it. Thats enough.

My snail mail address is:

Thomas Nussbaumer
Heinrichstrasse 112a
A-8010 Graz

... and please: no mail bombs if one of my programs had crashed your calculator!


This program may be distributed by any other website for non-commercial use only.

DISTRIBUTIONS ON ANY OTHER MEDIUM (Disc,CD-ROM,DVD etc.) are PROHIBITED without separate allowance of the author.

The author makes no representations or warranties about the suitability of the software and/or the data files, either express or implied. The author shall not be liable for any damages suffered as a result of using or distributing this software and/or data files.

You are free to re-use any part of the sourcecode in your products as long as you give credits including a reference to the TICT-HQ (http://tict.ticalc.org/).

Thomas Nussbaumer Graz,Austria 05/04/2001