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javascript - Scanline algorithm: calculate x of edges

问题描述:

I'm trying to implement the scanline algorithm in Javascript. It looks like my calculation of x is wrong as the lines are to short or to long.

These edges should be filled:

This is how my scanline code fills it:

My edge table has these fields:

et.push({

Ymin: Ymin, // minimum Y of the edge

Ymax: Ymax, // maximum Y

x: Xmin, // X value of Ymin point; updated for every scanline

dx: Xmax - Xmin, // Delta X

dy: Ymax - Ymin // Delta Y

});

The new X after every scanline is calculated this way:

// aet is active edge table

if (aet[i].dx != 0 && aet[i].dy != 0) {

// this is not a vertical edge

var incr = aet[i].dy / aet[i].dx; // =1/(dx/dy) ... dx/dy=m

if (aet[i].dy > 0) {

incr = incr;

} else {

incr = -incr;

}

aet[i].x += incr;

}

What is wrong in this calculation?

网友答案:

You don't show the scanline algorithm proper, but is looks as if you wanted to treat all scanlines from Ymin to Ymax. y starts a Ymin and increases by one for each scanline.

Therefore, the x for each scanline should increase by dx/dy.

You probably don't need to adjust the sign. Instead, the sign of dy is either positive or negative:

m = dx / dy                               # slope of the line
x[y + 1] = x[y] + m * dy = x[y] + m       # dy == (y + 1) - y == 1
x[y - 1] = x[y] + m * dy = x[y] - m       # dy == (y - 1) - y == -1

You scan in x direction and hence rule out horizontal lines for which dy == 0. That also shows in your maths: You cannot divide by dy when dy == 0.

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