Message-ID: <001501bdacec$9b72e720$604e08c3@arthur>
From: "Arthur" <arfa AT clara DOT net>
To: "DJGPP Mailing List" <djgpp AT delorie DOT com>
Subject: Re: About ray casting and look up tables
Date: Sat, 11 Jul 1998 17:54:43 +0100
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>I'm learning how to make a ray casting engine, i'm using
>djgpp, and a friend of mine sent me some source code, but
>i have a problem with look up tables.
>
>I usually calculate them in this way
>
>#include <math.h>
>float tan_table[320];
>float rad;
>for(ang=0;ang<320;ang++)
>{
>   rad= ang *(pi/180);
>  tan_table[ang]=tan(rad);
>}
>
>but i was cheking out my friend's code and found
>
>
>#include <math.h>
>#define ANG_360 1920
>#define ANG_0  0
>#define PI 3.1415927
>#define TWOPI  ( 2 * PI)
>#define TWOPIDIV360 ( TWOPI / ANG_360)
>
>float *tan_table;
>
>tan_table = (float *) malloc( sizeof(float) * ANG_360);
>
>for(angulo=ANG_0;angulo<=ANG_360;angulo++)
>
>{
>
>    ang_rad = ang * TWOPIDIV360 ;
>
>   tan_table[ang] =  (float)tan(ang_rad);
>
>}
>
>i have no problem with malloc and c stuff
>
>I don't understand why he uses this formula
>
>    ang_rad = ang * TWOPIDIV360 ;
>insted of the one i'm using


OK, so you are using ang*(pi/180). This converts degrees to radians. It
follows that ang*(pi/180) is the same as ang*(2*pi/360) (basic mathematic
manipulation). So obviously, he has defined a macro which is equivalent to
this.

He's defined pi "manually" instead of the definition in math.h:

#define PI 3.1415927

And also 2*pi:

#define TWOPI  ( 2 * PI)

And instead of 360 degrees, he's using a higher number of increments:

#define ANG_360 1920

(this is explained in other postings).

And also 2*pi/1920:

#define TWOPIDIV360 ( TWOPI / ANG_360)

so using TWOPIDIV360 is the same as using 2*pi/1920

Advantages of doing it this way? Well, the macroes are expanded in full in
the code so TWOPIDIV360 will be compiled as: 2*3.1415927/1920 which will
give marginally smaller and quicker code, but also he obviously thinks it's
easier to read this way. I disagree, but everyone has their own opinion :^)

HTH

James Arthur
jaa AT arfa DOT clara DOT net