On 10 August 2011 12:49, Joel B <onephatcat <at> earthlink.net> wrote:
> I should know how to do this, but i don't. I only have the ability in my system to draw lines (x1,y1,x2,y2)' to a bitmap, or write to the bitmap as a byte array. Can someone tell me the stepwise procedure to draw a polygon of n sides and then fill it? Having trouble finding anything online that doesn't use pre-existing primitives or libraries.
>
> Thanks,
>
> Joel

if you already have a line drawing function, then use the code from
that to update a pair of arrays of min/max on the x axis:

int xmin[SCREEN_HEIGHT] = { SCREEN_WIDTH... }
int xmax[SCREEN_HEIGHT] = { -1... }

then for each step of your line drawing function update the min/max:

replace your: put_pixel(x,y,colour)
with: xmin[y] = min(x,xmin[y]); xmax[y] = max(x,xmax[y]);

Once you have the arrays, it's a simple case of iterating over them
and rendering all pixels between the min and max where the difference
is positive.

foreach y in SCREEN_HEIGHT:
 if( xmax[y] > xmin[y] )
   foreach( x in range( xmin[y], xmax[y] ):
     put_pixel( x,y, colour )

and you have an untextured triangle.
if you want it textured, don't forget to store only UV values when the
max/min is updated.

--
fabs();
Just because the world is full of people that think just like you,
doesn't mean the other ones can't be right.

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Some helpful Google search terms are "polygon rasterization" and "polygon scan conversion." Someone else mentioned the Foley/van Dam book/et al. book, which is a good reference that describes the classic algorithm. The book Graphics Gems I also has a discussion, though it is brief. You can also find this stuff online, for free. For example, two sets of lecture slides from University of Virginia that look good:

http://www.cs.virginia.edu/~gfx/courses/2004/Intro.Fall.04/handouts/11-polyscan.pdf
www.cs.virginia.edu/~asb/teaching/cs445-fall06/slides/09-rasterization.ppt

I even found this YouTube video:

http://www.youtube.com/watch?v=TNbkX5bYrtE

Graham

-----Original Message-----
From: Joel B [mailto:onephatcat <at> earthlink.net]

Sent: Wednesday, August 10, 2011 7:49 AM
To: Game Development Algorithms
Subject: [Algorithms] Draw & fill regular polygon?

I should know how to do this, but i don't. I only have the ability in my system to draw lines (x1,y1,x2,y2)' to a bitmap, or write to the bitmap as a byte array. Can someone tell me the stepwise procedure to draw a polygon of n sides and then fill it? Having trouble finding anything online that doesn't use pre-existing primitives or libraries.

Thanks,

Joel
Sent from my iPhone
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