From 2c805b25f112035e8e47674bf22300a2b60b6e7d Mon Sep 17 00:00:00 2001
From: Igor Zinken <730069+igorski@users.noreply.github.com>
Date: Sat, 8 Feb 2025 15:20:37 +0100
Subject: [PATCH] Add method for pixel-accurate content copying

---
 src/math/point-math.ts   | 25 ++++++++++++++++++++++++-
 src/utils/canvas-util.ts | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/src/math/point-math.ts b/src/math/point-math.ts
index 9650a00..d152022 100644
--- a/src/math/point-math.ts
+++ b/src/math/point-math.ts
@@ -1,7 +1,7 @@
 /**
  * The MIT License (MIT)
  *
- * Igor Zinken 2020-2022 - https://www.igorski.nl
+ * Igor Zinken 2020-2025 - https://www.igorski.nl
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy of
  * this software and associated documentation files (the "Software"), to deal in
@@ -51,6 +51,29 @@ export const isPointInRange = ( point1x: number, point1y: number, point2x: numbe
            isCoordinateInVerticalRange( point1y, point2y, margin );
 };
 
+export const isPointInsidePolygon = ( point: Point, polygon: Point[] ): boolean => {
+    const { x, y } = point;
+    let isInside = false;
+
+    for ( let i = 0, l = polygon.length, j = polygon.length - 1; i < l; j = i++ ) {
+        const compare1 = polygon[ i ];
+        const compare2 = polygon[ j ];
+
+        const x1 = compare1.x;
+        const y1 = compare1.y;
+        const x2 = compare2.x;
+        const y2 = compare2.y;
+
+        const intersect = (( y1 > y ) !== ( y2 > y )) && ( x < ( x2 - x1 ) * ( y - y1 ) / ( y2 - y1 ) + x1 );
+        if ( intersect ) {
+            // we are ray-casting, an odd number of line crossing determines whether a point is inside the polygon.
+            // when even, the point is outside the polygon. We cannot directly return true here!
+            isInside = !isInside;
+        }
+    }
+    return isInside;
+};
+
 export const isCoordinateInHorizontalRange = ( point1x: number, point2x: number, margin = 5 ): boolean => {
     const left   = point2x - margin;
     const right  = point2x + margin;
diff --git a/src/utils/canvas-util.ts b/src/utils/canvas-util.ts
index 4b878de..105d581 100644
--- a/src/utils/canvas-util.ts
+++ b/src/utils/canvas-util.ts
@@ -24,6 +24,8 @@ import { loader } from "zcanvas";
 import type { Point } from "zcanvas";
 import type { CanvasContextPairing, CanvasDrawable } from "@/definitions/editor";
 import { JPEG, PNG } from "@/definitions/image-types";
+import { isPointInsidePolygon } from "@/math/point-math";
+import { fastRound } from "@/math/unit-math";
 import type ZoomableCanvas from "@/rendering/canvas-elements/zoomable-canvas";
 import { blobToResource, disposeResource } from "@/utils/resource-manager";
 
@@ -235,3 +237,40 @@ export const blobToCanvas = ( blob: Blob ): Promise<HTMLCanvasElement> => {
 };
 
 export const getPixelRatio = (): number => window.devicePixelRatio ?? 1;
+
+
+/**
+ * Pixel-accurate copying of selection content. This is quite slow though, depending
+ * on the source content quality, using drawImage() (provides aliasing) is more than satisfactory.
+ */
+export const copySelectionCrisp = ( sourceCtx: CanvasRenderingContext2D, destCtx: CanvasRenderingContext2D, selection: Point[] ): void => {
+    // get bounding box of the selection
+    const minX = fastRound( Math.min( ...selection.map( point => point.x )));
+    const minY = fastRound( Math.min( ...selection.map( point => point.y )));
+    const maxX = fastRound( Math.max( ...selection.map( point => point.x )));
+    const maxY = fastRound( Math.max( ...selection.map( point => point.y )));
+
+    const width  = maxX - minX;
+    const height = maxY - minY;
+
+    const sourceData = sourceCtx.getImageData( minX, minY, width, height ).data;
+    const outputImageData = destCtx.createImageData( width, height );
+    const outputData = outputImageData.data;
+
+    // Loop through each pixel in the bounding box
+    for ( let y = 0; y < height; y++ ) {
+        for ( let x = 0; x < width; x++ ) {
+            const pixelIndex = ( y * width + x ) * 4;
+
+            if ( isPointInsidePolygon({ x: minX + x, y: minY + y }, selection )) {
+                outputData[ pixelIndex ]     = sourceData[ pixelIndex ];
+                outputData[ pixelIndex + 1 ] = sourceData[ pixelIndex + 1 ];
+                outputData[ pixelIndex + 2 ] = sourceData[ pixelIndex + 2 ];
+                outputData[ pixelIndex + 3 ] = sourceData[ pixelIndex + 3 ];
+            } else {
+                outputData[ pixelIndex + 3 ] = 0; // make transparent
+            }
+        }
+    }
+    destCtx.putImageData( outputImageData, minX, minY );
+};