问题
I'm trying to draw a filled polygon using individual pixels in a scanline loop (so no lineTo
or fill
Canvas methods).
I was able to achieve a triangle in this method (example below), but I'm not sure where to begin with a more complex polygon (such as a star shape ★). Can anyone give any advice on how to approach this or existing shape algorithm examples for Javascript & Canvas?
I have researched Bresenham’s Algorithm but was unsuccessful implementing it for polygons because of my limited understanding of it. Please let me know if anything I've explained is unclear.
Thank you!
var canvas = document.querySelector('#canvas')
var ctx = canvas.getContext('2d');
var widthRange = document.querySelector('#widthRange')
var heightRange = document.querySelector('#heightRange')
ctx.fillStyle = 'blue';
var DrawPixel = function (x, y) {
ctx.fillRect(x, y, 1, 1);
}
var x = 100;
var y = 100;
var width = widthRange.value;
var height = heightRange.value;
const draw = () =>
{
ctx.clearRect(0,0,canvas.width,canvas.height);
wHRatio = width/height;
for (var j=0; j<height; j++)
{
w = width-j*wHRatio;
for (var i=0; i<w; i++)
{
DrawPixel(Math.floor(i+(j*(wHRatio/2))),height-j);
}
}
}
draw();
widthRange.addEventListener("input", function(e){
width = e.currentTarget.value;
draw();
})
heightRange.addEventListener("input", function(e){
height = e.currentTarget.value;
draw();
})
#canvas {
outline: 1px solid grey;
}
.slidecontainer
{
display: inline-block;
}
<div class="slidecontainer">
<label for="widthRange">Width</label>
<input type="range" min="1" max="300" value="100" class="slider" id="widthRange">
</div>
<div class="slidecontainer">
<label for="heightRange">Height</label>
<input type="range" min="1" max="150" value="100" class="slider" id="heightRange">
</div>
<canvas width=300 height=150 id="canvas"></canvas>
回答1:
Scanline for non self intersecting polygons.
Draw any concave or convex polygon with 3 or more sides using scanline method.
- Is the easiest, also slowest.
- Polygon is a set of lines with start and end points.
- Polygon lines can be unordered
- Polygon must be closed
- None of polygon lines can cross any of the other polygon lines.
Steps
Find bounding box of lines. top, left, right, bottom.
Set x, y to top left of bounding box.
while y is less than bottom.
Find all lines that will cross the line from left, y to right, y
Sort lines in distance from x to point where above line crossed
while there are sorted lines
shift two lines from sorted lines and scan the pixels between
add 1 to y
An implementation.
Function scanlinePoly(lines, col)
draw the pixels. lines
is create with createLines
which is an array of lines with helper functions to add lines, find lines, sort lines, and get bounds.
Helper functions
createStar(x, y, r1, r2, points)
will create alines
array for a star,x
,y
center of star,r1
radius,r2
second radius,points
number of pointsP2(x, y)
returns 2D pointL2(p1, p2)
returns 2D line that includes the slope of the line.p1
,p2
are points as created byP2
atLineLevelY(y)
returns true if line crosses scan line aty
const scanlinePoly = (lines, col) => {
const b = lines.getBounds();
var x, y, xx;
ctx.fillStyle = col;
b.left = Math.floor(b.left);
b.top = Math.floor(b.top);
for (y = b.top; y <= b.bottom; y ++) {
// update
// old line was const ly = lines.getLinesAtY(y).sortLeftToRightAtY(y);
// changed to
const ly = lines.getLinesAtY(y + 0.5).sortLeftToRightAtY(y + 0.5);
x = b.left - 1;
while(x <= b.right) {
const nx1 = ly.nextLineFromX(x);
if (nx1 !== undefined) {
const nx2 = ly.nextLineFromX(nx1);
if (nx2 !== undefined) {
const xS = Math.floor(nx1);
const xE = Math.floor(nx2);
for (xx = xS; xx < xE; xx++) {
ctx.fillRect(xx, y, 1, 1);
}
x = nx2;
} else { break }
} else { break }
}
}
}
function createLines(linesArray = []) {
return Object.assign(linesArray, {
addLine(l) { this.push(l) },
getLinesAtY(y) { return createLines(this.filter(l => atLineLevelY(y, l))) },
sortLeftToRightAtY(y) {
for (const l of this) { l.dist = l.p1.x + l.slope * (y - l.p1.y) }
this.sort((a,b) => a.dist - b.dist);
return this;
},
nextLineFromX(x) { // only when sorted
const line = this.find(l => l.dist > x);
return line ? line.dist : undefined;
},
getBounds() {
var top = Infinity, left = Infinity;
var right = -Infinity, bottom = -Infinity;
for (const l of this) {
top = Math.min(top, l.p1.y, l.p2.y);
left = Math.min(left, l.p1.x, l.p2.x);
right = Math.max(right, l.p1.x, l.p2.x);
bottom = Math.max(bottom, l.p1.y, l.p2.y);
}
return {top, left, right, bottom};
},
});
}
const createStar = (x, y, r1, r2, points) => {
var i = 0, pFirst, p1, p2;
const lines = createLines()
while (i < points * 2) {
const r = i % 2 ? r1 : r2;
const ang = (i / (points * 2)) * Math.PI * 2;
p2 = P2(Math.cos(ang) * r + x, Math.sin(ang) * r + y);
if (pFirst === undefined) { pFirst = p2 };
if (p1 !== undefined) { lines.addLine(L2(p1, p2)) }
p1 = p2;
i++;
}
lines.addLine(L2(p2, pFirst));
return lines;
}
const ctx = canvas.getContext("2d");
const P2 = (x = 0,y = 0) => ({x, y});
const L2 = (p1 = P2(), p2 = P2()) => ({p1, p2, slope: (p2.x - p1.x) / (p2.y - p1.y)});
const atLineLevelY = (y, l) => l.p1.y < l.p2.y && (y >= l.p1.y && y <= l.p2.y) || (y >= l.p2.y && y <= l.p1.y);
canvas.addEventListener("click", () => {
ctx.clearRect(0,0,200,200);
const star = createStar(
100, 90,
Math.random() * 80 + 10,
Math.random() * 80 + 10,
Math.random() * 20 + 2 | 0
);
scanlinePoly(star, "#F00")
})
const star = createStar(100, 90, 90, 40, 10);
scanlinePoly(star, "#F00")
canvas {border: 1px solid black;}
<canvas id="canvas" width="200" height="180"></canvas>Click for rand star
Note that inner loop
for (xx = xS; xx < xE; xx++) {
ctx.fillRect(xx, y, 1, 1);
}
Can be replaced with ctx.fillRect(xS, y, xE - xS, 1)
to greatly improve performance.
UPDATE
Looking at my answer again to see if it could be improved I noticed a problem that resulted in lines incorrectly rendered.
To fix the the first line inside the outer loop of function scanlinePoly
needs to be changed from.
const ly = lines.getLinesAtY(y).sortLeftToRightAtY(y);
To
const ly = lines.getLinesAtY(y + 0.5).sortLeftToRightAtY(y + 0.5);
来源:https://stackoverflow.com/questions/65573101/draw-a-filled-polygon-using-scanline-loop