feat: add setBlendedVisiblePixel to drawLineOnImage

src/draw/drawLineOnImage.ts

@@ -4,6 +4,7 @@ import { Image } from '../Image';
 import { Point } from '../utils/geometry/points';
 import { getDefaultColor } from '../utils/getDefaultColor';
 import { getOutputImage } from '../utils/getOutputImage';
+import { setBlendedVisiblePixel } from '../utils/setBlendedVisiblePixel';
 import checkProcessable from '../utils/validators/checkProcessable';
 import { validateColor } from '../utils/validators/validators';
 
@@ -56,7 +57,7 @@ export function drawLineOnImage(
     Math.round(origin.column + to.column),
     Math.round(origin.row + to.row),
     (column: number, row: number) => {
-      newImage.setVisiblePixel(column, row, color);
+      setBlendedVisiblePixel(newImage, column, row, { color });
     },
   );
   return newImage;

src/utils/setBlendedVisiblePixel.ts

@@ -0,0 +1,67 @@
+import { Image } from '../Image';
+import { Mask } from '../Mask';
+
+import { getDefaultColor } from './getDefaultColor';
+import { assert } from './validators/assert';
+
+export interface SetBlendedVisiblePixelOptions {
+  /**
+   * Color with which to blend the image pixel.
+   * @default `'Opaque black'`.
+   */
+  color?: number[];
+}
+
+/**
+ * Blend the given pixel with the pixel at the specified location in the image if the pixel is in image's bounds.
+ * @param image - The image with which to blend.
+ * @param column - Column of the target pixel.
+ * @param row - Row of the target pixel.
+ * @param options - Set blended pixel options.
+ */
+export function setBlendedVisiblePixel(
+  image: Image | Mask,
+  column: number,
+  row: number,
+  options: SetBlendedVisiblePixelOptions = {},
+) {
+  const { color = getDefaultColor(image) } = options;
+
+  if (!image.alpha) {
+    image.setVisiblePixel(column, row, color);
+  } else {
+    assert(image instanceof Image);
+
+    const sourceAlpha = color.at(-1) as number;
+
+    if (sourceAlpha === image.maxValue) {
+      image.setVisiblePixel(column, row, color);
+      return;
+    }
+
+    const targetAlpha = image.getValue(column, row, image.channels - 1);
+
+    const newAlpha =
+      sourceAlpha + targetAlpha * (1 - sourceAlpha / image.maxValue);
+    if (column >= 0 && column < image.width && row >= 0 && row < image.height) {
+      image.setValue(column, row, image.channels - 1, newAlpha);
+    }
+    for (let component = 0; component < image.components; component++) {
+      const sourceComponent = color[component];
+      const targetComponent = image.getValue(column, row, component);
+
+      const newComponent =
+        (sourceComponent * sourceAlpha +
+          targetComponent * targetAlpha * (1 - sourceAlpha / image.maxValue)) /
+        newAlpha;
+      if (
+        column >= 0 &&
+        column < image.width &&
+        row >= 0 &&
+        row < image.height
+      ) {
+        image.setValue(column, row, component, newComponent);
+      }
+    }
+  }
+}