Drawing diagonal lines in SDL

[Vladwow91]

Good Day/Night,

I am currently trying to draw a bunch of lines in a empty SDL window.
I am having diffeculties constructing my DrawLine function so that it will place the pixel in the correct approximate place where the line is going. For horisontal & vertical lines its not a herculean task, but i am having problems in defining the content of my loops within DrawLine.

I am aware of the Bresenham's line algorithm, my problem thereafter is understanding the consept.

Here goes my thoughts so far;

void DrawLine(SDL_Surface *screen, int x1, int y1, int x2, int y2, unsigned int color)
{
    int i,j;

    for (i=x1; i<x2; i++)
    	//
    	for(j=y1; j<y2; j++)
    	{
    		if // some statement that determines j & i are infact on line
    			SetPixel(screen, i, m, color);
    	}

}

Now, what i am asking here is wheter or not you can describe the thought prossess behind how to determin where to SetPixel. I am aware of the more complex versions of this function to allow more versetile coordinates for drawing the acctual line.

I am hoping you could shed some light on this issue for me, if you have the time and patience. I am a fast learner, but i just gotta get it right first.

Sincerely, Vladwow

[Disch]

This probably isn't the ideal solution, but....


You can use pythagorean theorum to find the length of a line:

a*a + b*b = c*c

or... to rephrase:

x*x + y*y = length*length

If you divide a vector (line) by its length, you have a 'unit' vector (a line with a length of 1)

You can then step through each point on the line and draw it at the appropriate coords onscreen.

double x = x2 - x1;
double y = y2 - y1;
double length = sqrt( x*x + y*y );

double addx = x / length;
double addy = y / length;

x = x1;
y = y1;

for(double i = 0; i < length; i += 1)
{
  SetPixel( (int)x, (int)y, your_color );
  x += addx;
  y += addy;
}

[Vladwow91]

Thats freaking brilliant mate! Absolutly a valid way to draw a line with c in SDL, since this has seemingly no restrictions to in the code to wheter its horrisontal, vertical or diagonal.

This is something i'm absolutly going to remember. Thank you.


Sincerly, Vladwow