DDA Algorithm of a Star

Neo — Mon, 18 Feb 2008 20:13:02 -0500

Hi, I'm new to Matlab and am in need of a little help. I
currently study a University module which involves a lot of
matlab.

My task is to draw a rasterisation of a star using a DDA
algorithm. I'm told this is really simple, I am almost there
I have managed to program half of the star but cant work
out how to program the other half.

Below is my 2 M-files so far...I'm told the first one is
100% correct but I have slight errors in the second.

1) Star.m

screen=ones(110,110);

coord=[40 70; 5 55; 40 40; 55 5; 70 40; 105 55; 70 70; 55 105];
for i=1:7
    first=coord(i,:);
    last=coord(i+1,:);

    screen=dda(screen,first,last,0);
end

clf
imagesc(flipud(screen'));
title('..');
xlabel('..');
ylabel('..');
axis square;
colormap(gray);

2) DDA.m

function raster=dda(raster,first,last,color)

%check whether line is drawn from left to right
if(last(1)<first(1))
temp=last;
last=first;
first=temp;
end

%calculate the gradient
grad=(last(2)-first(2))/(last(1)-first(1));

x=first(1);
y=first(2);

if (abs(grad)<1)
for x=first(1):last(1)
raster(x,round(y))=color;
y=y+grad;
end


elseif (abs(grad)>1)
for y=first(2):last(2)
raster(y,round(x))=color;
x=x+1/grad;
end

end

Any help is much appreciated,

Neo.

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DDA Algorithm of a Star