[英]how to fill certain percentage area of circle in color in html canvas?
我的html頁面中有9個圓圈。每個圓圈都必須用一定的百分比填充某些顏色。我使用html5畫布元素繪制了圓圈。但我只能用顏色填充整個圓圈而不是某些優點area.How我可以實現嗎?
使用復合模式:
destination-out
主要代碼是:
var o = radius * 2, // diameter => width and height of rect
h = o - (o * percent / 100); // height based on percentage
ctx.beginPath();
ctx.arc(x, y, radius, 0, 6.28);
ctx.fill();
ctx.globalCompositeOperation = "destination-out";
ctx.fillRect(x - radius, y - radius, o, h); // this will remove a part of the top
var ctx = document.querySelector("canvas").getContext("2d"), pst = 0, dlt = 2; ctx.fillStyle = "#28f"; function drawCircle(ctx, x, y, radius, percent) { var o = radius * 2, h = o - (o * percent / 100); ctx.globalCompositeOperation = "source-over"; // make sure we have default mode ctx.beginPath(); // fill an arc ctx.arc(x, y, radius, 0, 6.28); ctx.fill(); ctx.globalCompositeOperation = "destination-out"; // mode to use for next op. ctx.fillRect(x - radius, y - radius, o, h); // "clear" part of arc ctx.globalCompositeOperation = "source-over"; // be polite, set default mode back } (function loop() { ctx.clearRect(0,0,300,150); drawCircle(ctx, 70, 70, 60, pst); pst += dlt; if (pst <= 0 || pst >= 100) dlt = -dlt; requestAnimationFrame(loop); })();
<canvas></canvas>
(tip: closePath()
實際上不需要fill()
因為fill()
會關閉隱含的路徑,但是如果你想做一個stroke()
則需要它。
關鍵部分是:
ctx.beginPath();
ctx.moveTo(x, y);
ctx.arc(x, y, radius, 0, 2 * Math.PI * percent / 100);
//ctx.closePath(); // for stroke, not needed for fill
ctx.fill();
var ctx = document.querySelector("canvas").getContext("2d"), pst = 0, dlt = 2; ctx.fillStyle = "#28f"; function drawPie(ctx, x, y, radius, percent) { ctx.beginPath(); ctx.moveTo(x, y); ctx.arc(x, y, radius, 0, 2 * Math.PI * percent /100); ctx.fill(); } (function loop() { ctx.clearRect(0,0,300,150); drawPie(ctx, 70, 70, 60, pst); pst += dlt; if (pst <= 0 || pst >= 100) dlt = -dlt; requestAnimationFrame(loop); })();
<canvas></canvas>
與派對類型幾乎相同,但有以下更改:
lineWidth
,見下面的演示) 重要組成部分:
ctx.beginPath();
ctx.moveTo(x + radius, y); // cos(0) for x = 1, so just use radius, sin(0) = 0
ctx.arc(x, y, radius, 0, 2 * Math.PI * percent /100);
ctx.stroke();
您可以使用旋轉變換調整間隙點或使用三角法計算實際點。
var ctx = document.querySelector("canvas").getContext("2d"), pst = 0, dlt = 2; ctx.strokeStyle = "#28f"; ctx.lineWidth = 8; function drawWedge(ctx, x, y, radius, percent) { ctx.beginPath(); ctx.moveTo(x + radius, y); ctx.arc(x, y, radius, 0, 2 * Math.PI * percent /100); ctx.stroke(); } (function loop() { ctx.clearRect(0,0,300,150); drawWedge(ctx, 70, 70, 60, pst); pst += dlt; if (pst <= 0 || pst >= 100) dlt = -dlt; requestAnimationFrame(loop); })();
<canvas></canvas>
您可以使用旋轉變換或使用三角函數手動計算點來更改圓弧的起點。
要手動計算這些,你可以做(弧度角):
x = radius * Math.cos(angleInRad); // end point for x
y = radius * Math.sin(angleInRad); // end point for y
只需將總角度添加到起始角度即可獲得終點。
360°弧度= 2 x PI,因此如果您想使用度數角度,請使用以下方法進行轉換:
angleInRad = angleInDeg * Math.PI / 180;
var ctx = document.querySelector("canvas").getContext("2d"), pst = 0, dlt = 2; ctx.strokeStyle = "#28f"; ctx.lineWidth = 8; function drawWedge(ctx, x, y, radius, percent) { ctx.translate(x, y); // translate to rotating pivot ctx.rotate(Math.PI * 0.5); // rotate, here 90° deg ctx.translate(-x, -y); // translate back ctx.beginPath(); ctx.moveTo(x + radius, y); ctx.arc(x, y, radius, 0, 2 * Math.PI * percent /100); ctx.stroke(); ctx.setTransform(1,0,0,1,0,0); // reset transform } (function loop() { ctx.clearRect(0,0,300,150); drawWedge(ctx, 70, 70, 60, pst); pst += dlt; if (pst <= 0 || pst >= 100) dlt = -dlt; requestAnimationFrame(loop); })();
<canvas></canvas>
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