Need help with code snippet using trig.

BlitzMax Forums/BlitzMax Programming/Need help with code snippet using trig.

JoJo	(Posted 2008) [#1]

I downloaded this code from the net that draws a circle using the sine and cosine functions, but I can't figure out what I'm doing wrong in Max.

Code I converted to Max:

Global SCREEN_W:Int = 800
Global SCREEN_H:Int = 600
Graphics SCREEN_W, SCREEN_H, 0


Local x:Int, y:Int
Local length:Int = 50
Local angle:Float = 0.0
Local angle_stepsize:Float = 0.1;


SetColor 255, 255, 255
Cls
'go through all angles from 0 To 2 * Pi radians
While (angle < 2 * Pi)

    ' calculate x, y from a vector with known length and angle
    x = length * Cos (angle)
    y = length * Sin (angle)
	Plot((x + (SCREEN_W / 2)), (y + (SCREEN_H / 2)))
	angle:+angle_stepsize
	
Wend
Flip

WaitKey

And here is the code written in C using Allegro:

/*
    CIRCLE 1
    Written by Amarillion (amarillion@...)

    This program demonstrates the use of sin and cos with floats.
    All it does is draw a circle on the screen.
*/

#include <allegro.h>
#include <math.h>

// Make sure PI is defined.
// MinGW has some problems with this.
#ifndef PI
#define PI 3.1415927
#endif

void draw_circle ()
{
    int x, y;
    int length = 50;
    float angle = 0.0;
    float angle_stepsize = 0.1;

    // go through all angles from 0 to 2 * PI radians
    while (angle < 2 * PI)
    {
        // calculate x, y from a vector with known length and angle
        x = length * cos (angle);
        y = length * sin (angle);

        putpixel (screen,
            x + SCREEN_W / 2, y + SCREEN_H / 2,
            makecol (255, 255, 255));
        angle += angle_stepsize;
    }
}

int main ()
{
    // initialize Allegro
    if (allegro_init () < 0)
    {
        allegro_message ("Error: Could not initialize Allegro");
        return -1;
    }
    // initialize gfx mode
    if (set_gfx_mode (GFX_AUTODETECT, 320, 200, 0, 0) < 0)
    {
        allegro_message ("Error: Could not set graphics mode");
        return -1;
    }
    // initialize keyboard
    install_keyboard ();
    clear_keybuf ();

    // call the example function
    draw_circle ();

    // wait for a user key-press
    readkey ();

    // exit Allegro
    allegro_exit ();

    return 0;

} END_OF_MAIN ();

Retimer

(Posted 2008) [#2]

Different example, although I believe you're looking for the same thing:


Const Radius:Int = 100
Const Step_Angle:Float = 2.0
Const ScreenWidth:Int = 800
Const ScreenHeight:Int = 600

Global Angle:Int = 0

Graphics ScreenWidth,ScreenHeight

SetColor 255, 255, 255

Cls

While angle < 360
	
	Local X:Int = (ScreenWidth * 0.5) + (Radius * Sin(angle))
	Local Y:Int = (ScreenHeight * 0.5) + (Radius * Cos(angle))
	Plot(x,y)
	Angle:+Step_Angle
Wend

	Flip
WaitKey

Edit: If this is your first time getting into small trig, you're probobly just about to hit a time in coding that starts to really get fun, especially when messing around with fractal stuff. And even understanding this small bit will get you through a LOT of 3d formulas. Good luck.

Brucey

(Posted 2008) [#3]

Remember that BlitzMax works in Degrees, and your example expects Radians... :-)

JoJo	(Posted 2008) [#4]

@Retimer
Yeah, I'm really looking forward to using math routines instead of writing hacks.
I figure its about time. I wish I would have started 10-15 years ago though.

@Brucey
Ah...thanks! It's working now!

Rob Farley

(Posted 2008) [#5]

And just as an aside, what you're doing there has nothing to do with trigonometry.

JoJo	(Posted 2008) [#6]

@Rob Farley
I thought the sine and cosine functions were trigonometry functions.

plash

(Posted 2008) [#7]

I thought the sine and cosine functions were trigonometry functions.

http://en.wikipedia.org/wiki/Trigonometry
win.

Rob Farley

(Posted 2008) [#8]

I'm not sure who's winning what but Sine and Cosine functions are used in Trigonometry, however, Sine and Cosine functions do not equal trigonometry.

pmc	(Posted 2008) [#9]

sine and cosine are two of the three primary trigonometric functions. This thread (and the code) are entirely about trigonometry.

That said, I've been coding a bunch of vector object transformations this way and I really enjoyed stick-building the math code and re-aquainting myself with basic high-school and undergrad math.

-pmc

sswift

(Posted 2008) [#10]

And trigonometry deals with triangles. And how do you draw a circle on a computer screen? By calculating lots and lots of triangles. One for each pixel in fact.

Each pixel in a circle is one point of a right triangle, where the point's radius is the length of the hypoteneuse, the point's X coordinate is the length of the opposite side, and the point's Y coordinate is the length of the adjacent side.

Trigonometry! Ta da!