Re: [E-devel] drawing circles.

[Moritz A. <mor...@gm...>]

if you talk about evas(2|1) i think you'r right, about evas(1|0.66) i
don'T know
but i think the Berenham algorithm would be better since it's faster
( when i tryed it )
here is the 'php' source wich should be easily understandable.

function circle($center_x,$center_y,$r,$color){
    global $img;
    $d = 3-(2*$r);
    $y = $r;
    for($x=0;$x<=$y;$x++){
        imagesetpixel($img['src'],$center_x+$x, $center_y+$y, $color);
        imagesetpixel($img['src'],$center_x+$x, $center_y-$y, $color);
        imagesetpixel($img['src'],$center_x-$x, $center_y+$y, $color);
        imagesetpixel($img['src'],$center_x-$x, $center_y-$y, $color);
        
        imagesetpixel($img['src'],$center_x+$y, $center_y+$x, $color);
        imagesetpixel($img['src'],$center_x+$y, $center_y-$x, $color);
        imagesetpixel($img['src'],$center_x-$y, $center_y+$x, $color);
        imagesetpixel($img['src'],$center_x-$y, $center_y-$x, $color);

        if($d < 0) $d = $d + ( 4 * $x ) + 6;
        else{
            $d = $d + 4 * ($x - $y) + 10;
            $y = $y -1;
        }
    }
}

i got a same running 
here: http://mindbox.mine.nu/~ss/imagery.php
and there source here: http://mindbox.mine.nu/~ss/imagery_src.php

regards

On Mon, 2002-08-26 at 00:51, ronny abraham wrote:
> 
> 
> Hi, I've only browsed through the Evas code, and that was a while ago,
> so I don't know if you have this in the new code.  In any case, I think
> people will get a kick out of this.
> 
> I found the code in "mathematical elements for computer graphics" by
> rogers & adams. pg 215-16
> 
> Anyway, I'm going to go through the math of it, so just bear with me.
> I'll try to paraphrase as much as I can, but their style is very
> concise, so there isn't much I can do to improve on it.
> 
> 
> given a circle of radious r
> 
> x = rcos(theta)
> y = rsin(theta)
> 
> 0 <= theta <= pi
> 
> 
> assume a fixed number of points equally spaced apart are used to
> represent the circle
> 
> delta_theta = 2 * pi/num_points
> 
> the intial points are:
> 
> x_0 = rcos(0)
> y_0 = rsin(0)  
> 
> from that you can do this:
> 
> x_i+1 = rcos( theta_i + delta_theta )
> y_i+1 = rsin( theta_i + delta_theta )
> 
> where theta_i is the angle we used for the last points ( x_i and y_i )we
> got.
> 
> x_i+1 and y_i+1 are the current points we're trying to find.
> 
> 
> sum of angles formula gives us:
> 
> x_i+1 = r( cos(theta_i) cos(delta_theta) 
> 		- sin(theta_i) sin(delta_theta) )
> 
> y_i+1 = r( cos(theta_i) sin(delta_theta) 
> 		- cose(delta_theta) sin(theta_i) )
> 
> 
> if we take the original equations, with theta as theta_i
> 
> x_i = rcos(theta_i)
> y_i = rsin(theta_i)
> 
> we then get:
> 
> x_i+1 = x_i * cos( delta_theta ) - y_i * sin(delta_theta)
> y_i+1 = y_i * sin( delta_theta ) - y_i * cos(delta_theta)
> 
> the nifty thing about all this, is that you only need to compute:
> 
> delta_theta
> cos(delta_theta)
> sin(delta_theta)
> 
> ONCE.
> 
> Which allows us to kick out the time-intensive trig stuff.
> 
> I think that this makes having a circle function in evas very possible
> since it's fast.
> 
> Of course, if it already exists, then at least this email was
> (hopefully) entertaining. :D
> 
> 
> 
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-- 
- Moritz Angermann / mind -

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