إعداد Canvas وسياق 2D

عنصر canvas

<canvas id="myCanvas" width="800" height="400">
  <!-- Fallback for browsers without canvas support -->
  <p>Your browser does not support HTML5 Canvas.</p>
</canvas>

سمتا العرض والارتفاع

<!-- CORRECT: set intrinsic size via attributes -->
<canvas width="800" height="400"></canvas>

<!-- WRONG: CSS changes display size but not canvas resolution -->
<canvas style="width: 800px; height: 400px;"></canvas>
<!-- This stretches a default 300x150 canvas to 800x400
     Result: blurry graphics -->

الحصول على سياق 2D

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');
// ctx is a CanvasRenderingContext2D object
// All drawing operations are methods on ctx

// Check browser support:
if (!canvas.getContext) {
  console.error('Canvas not supported');
}

نظام الإحداثيات

// (0, 0) = top-left corner
// (width, 0) = top-right
// (0, height) = bottom-left
// (width, height) = bottom-right

// For a 800x400 canvas:
// x increases left → right (0 to 800)
// y increases top → bottom (0 to 400)
//
// Example: draw at center:
ctx.fillRect(400, 200, 10, 10);  // centered dot

حالة canvas

ctx.save();       // push current state onto a stack

// Change state:
ctx.fillStyle = 'red';
ctx.globalAlpha = 0.5;

// Draw with modified state...
ctx.fillRect(100, 100, 200, 200);

ctx.restore();    // pop state: back to pre-save settings

مسح canvas

// Clear entire canvas:
ctx.clearRect(0, 0, canvas.width, canvas.height);

// Or with a background color:
ctx.fillStyle = 'white';
ctx.fillRect(0, 0, canvas.width, canvas.height);

fillStyle وstrokeStyle

ctx.fillStyle = 'blue';                    // CSS color name
ctx.fillStyle = '#ff6600';                 // hex color
ctx.fillStyle = 'rgb(255, 100, 0)';       // rgb
ctx.fillStyle = 'rgba(255, 100, 0, 0.5)'; // rgba with alpha
ctx.fillStyle = gradient;                  // gradient object
ctx.fillStyle = pattern;                   // pattern object

ctx.strokeStyle = '#333333';               // border color

lineWidth وخصائص الأنماط الأخرى

ctx.lineWidth = 4;           // stroke width in pixels
ctx.lineCap = 'round';       // butt | round | square
ctx.lineJoin = 'round';      // miter | round | bevel
ctx.setLineDash([5, 3]);     // dashed line: 5px dash, 3px gap
ctx.globalAlpha = 0.8;       // transparency for all drawing
ctx.globalCompositeOperation = 'multiply'; // blend modes

canvas متجاوب مع HiDPI

function setupHiDPI(canvas, width, height) {
  const dpr = window.devicePixelRatio || 1;
  canvas.width = width * dpr;
  canvas.height = height * dpr;
  canvas.style.width = width + 'px';
  canvas.style.height = height + 'px';
  const ctx = canvas.getContext('2d');
  ctx.scale(dpr, dpr);  // scale all drawing
  return ctx;
}

canvas مقابل SVG

إمكانية الوصول في canvas

<canvas
  id="chart"
  width="800"
  height="400"
  aria-label="Bar chart showing Q1-Q4 revenue growth"
  role="img"
>
  <!-- Fallback for non-canvas browsers and screen readers -->
  <table><!-- accessible data table --></table>
</canvas>

تحقق سريع

مراجعة: إعداد Canvas