Draw a triangle in the rasterization

ElHacker · Nov 28, 2019 · 2c6ee89 · 2c6ee89
1 parent 825d8ca
commit 2c6ee89
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diff --git a/raster-03.html b/raster-03.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<html lang="en" dir="ltr">
+  <head>
+    <meta charset="utf-8">
+    <title>Raster 03</title>
+  </head>
+  <body>
+    <canvas id="canvas" width=600 height=600 style="border: 1px grey solid">
+    <script src="./raster-03.js" charset="utf-8"></script>
+  </body>
+</html>
diff --git a/raster-03.js b/raster-03.js
@@ -0,0 +1,140 @@
+
+// ======================================================================
+//  Low-level canvas access.
+// ======================================================================
+let canvas = document.getElementById('canvas');
+let canvas_context = canvas.getContext('2d');
+let canvas_buffer = canvas_context.getImageData(0, 0, canvas.width, canvas.height);
+let canvas_pitch = canvas_buffer.width * 4;
+
+
+let PutPixel = (x, y, color) => {
+  x = canvas.width / 2 + Math.floor(x);
+  y = canvas.height / 2 - Math.floor(y) - 1;
+
+  if (x < 0 || x >= canvas.width || y < 0 || y >= canvas.height) {
+    return;
+  }
+
+  let offset = 4 * x + canvas_pitch * y;
+  canvas_buffer.data[offset++] = color[0];
+  canvas_buffer.data[offset++] = color[1];
+  canvas_buffer.data[offset++] = color[2];
+  canvas_buffer.data[offset++] = 255; // Alpha = 255 (full opacity)
+}
+
+
+// Displays the contents of the offscreen buffer into the canvas.
+let UpdateCanvas = () => {
+  canvas_context.putImageData(canvas_buffer, 0, 0);
+}
+
+
+// ======================================================================
+//  Rasterization code.
+// ======================================================================
+
+// A Point representation.
+let Pt = function(x, y) {
+  if (!(this instanceof Pt)) {
+    return new Pt(x, y);
+  }
+
+  this.x = x;
+  this.y = y;
+}
+
+let Interpolate = (i0, d0, i1, d1) => {
+  if (i0 == i1) {
+    return [d0];
+  }
+
+  let values = [];
+  const a = (d1 - d0) / (i1 - i0);
+  let d = d0;
+  for (let i = i0; i <= i1; i++) {
+    values.push(d);
+    d += a;
+  }
+
+  return values;
+}
+
+let DrawLine = (p0, p1, color) => {
+  let dx = p1.x - p0.x;
+  let dy = p1.y - p0.y;
+
+  if (Math.abs(dx) > Math.abs(dy)) {
+    // The line is horizontal-ish. Make sure it's left to right.
+    if (dx < 0) {
+      [p0, p1] = [p1, p0];
+    }
+
+    // Compute the Y values and draw.
+    let ys = Interpolate(p0.x, p0.y, p1.x, p1.y);
+    for (let x = p0.x; x <= p1.x; x++) {
+      // Using bitwise "or" | operator with 0 here is to remove the fractional part of the index.
+      PutPixel(x, ys[(x - p0.x) | 0], color);
+    }
+  } else {
+    // The line is vertical-ish. Make sure it's bottom to top.
+    if (dy < 0) {
+      [p0, p1] = [p1, p0];
+    }
+
+    // Compute the X values and draw.
+    let xs = Interpolate(p0.y, p0.x, p1.y, p1.x);
+    for (let y = p0.y; y <= p1.y; y++) {
+      // Using bitwise "or" | operator with 0 here is to remove the fractional part of the index.
+      PutPixel(xs[(y - p0.y) | 0], y, color);
+    }
+  }
+}
+
+let DrawWireframeTriangle = (p0, p1, p2, color) => {
+  DrawLine(p0, p1, color);
+  DrawLine(p1, p2, color);
+  DrawLine(p2, p0, color);
+}
+
+let DrawFilledTriangle = (p0, p1, p2, color) => {
+  // Sort the points from bottom to top;
+  if (p1.y < p0.y) { [p0, p1] = [p1, p0]; }
+  if (p2.y < p0.y) { [p0, p2] = [p2, p0]; }
+  if (p2.y < p1.y) { [p1, p2] = [p2, p1]; }
+
+  // Compute the x coordinates of the triangle edges
+  const x01 = Interpolate(p0.y, p0.x, p1.y, p1.x);
+  const x12 = Interpolate(p1.y, p1.x, p2.y, p2.x);
+  const x02 = Interpolate(p0.y, p0.x, p2.y, p2.x);
+
+  // Concatenate the short sides
+  x01.pop()
+  const x012 = x01.concat(x12);
+
+  // Determine which side is left and which is right.
+  let x_left;
+  let x_right;
+  const m = (x012.length / 2) | 0;
+  if (x02[m] < x012[m]) {
+    x_left = x02;
+    x_right = x012;
+  } else {
+    x_left = x012;
+    x_right = x02;
+  }
+
+  // Draw the horizontal segments.
+  for (let y = p0.y; y <= p2.y; y++) {
+    for (let x = x_left[y - p0.y]; x <= x_right[y - p0.y]; x++) {
+      PutPixel(x, y, color);
+    }
+  }
+}
+var p0 = Pt(-200, -250);
+var p1 = Pt(200, 50);
+var p2 = Pt(20, 250);
+DrawFilledTriangle(p0, p1, p2, [0, 255, 0]);
+DrawWireframeTriangle(p0, p1, p2, [0, 0, 0]);
+
+UpdateCanvas();