Question

This thing has been bothering me for a while now. Althought the code is really simple, something is wrong.

Frame
n=w;
drawmode=1;
s=0.5;


Pixel
s=-s;
x=0;
y=s;

(I take you mean x=i-.5; instead of x=0 for your example?)

It has to do with how the lines are drawn since there's only a one-pixel shift for 2 points.

I always use n=w+2 (or n=w*2+2 for fullscreen). This makes it one pixel wider, but it fixes the problem and noone will notice

Yeah I ment x=i-0.5;.

It never hit me that n=w+2; would solve it. Thanks.

I don't know why it works either, but it does

you could always just use n=2, x=i-0.5 and mega fat linesize.

The reason this happens is quite obvious if you step through the code. x = i - .5 means that for each point it will be a little further across in the x-axis. with your s = -s you are alternating successive points between s and -s in the y axis. putting the two together creates a zig-zag, sawtooth wave... whatever you want to call it. by using the (pretty wasteful i might add) n = w you are creating one which is so dense that it appears to be a solid rectangle.

by convenience n = w is the exact amount needed for it to break down like this since you are covering half the width of the screen with w/2 points per side of rectangle. e.g. each subsequent point is so close on the x axis that the point moves one pixel to the left for every two points evaluated. the only reason for it to not look perfect is rounding errors, since the n = w/2 means each point is half a pixel infront of the other. n = w+2 pushes it past the magic point of w/2 and so there is no chance of a visible artefact from a rounding error. such artefacts are still there but the zig-zag is so dense that they always get drawn over.

a better way to do this is to work out a single pixel width per frame then, still using s = -s for y, incrementing x manually with this width only when s is +ve or -ve (which ever you choose will probably not matter) you can then skip redundant lines too.

per frame:
ix = .5/w
xi = -.5

per point:
s=-s;
xi = if(above(s,0),xi + ix,xi);
x = xi;
y = s;

(untested since i am at work)

an even better way to draw a solid rectangle is with triangle.ape :P