Tool to display DICOM overlays

When enabled this tool will disable the DICOM overlays stored in `image.overlays`

The overlays data is created by `cornerstoneWADOImageLoader` - see cornerstonejs/cornerstoneWADOImageLoader#239

Implements DICOM standard C.9 - http://dicom.nema.org/dicom/2013/output/chtml/part03/sect_C.9.html

src/imageTools/overlays.js

@@ -0,0 +1,77 @@
+import external from '../externalModules.js';
+
+// const toolType = 'overlays';
+
+function onImageRendered (e) {
+  const eventData = e.detail;
+
+  if (!eventData || !eventData.enabledElement || !eventData.image || !eventData.image.overlays || !eventData.image.overlays.length === 0) {
+    return;
+  }
+
+  const enabledElement = eventData.enabledElement;
+  const context = enabledElement.canvas.getContext('2d');
+  const overlays = eventData.image.overlays;
+
+  const imageWidth = Math.abs(enabledElement.viewport.displayedArea.brhc.x - enabledElement.viewport.displayedArea.tlhc.x) * enabledElement.viewport.displayedArea.columnPixelSpacing;
+  const imageHeight = Math.abs(enabledElement.viewport.displayedArea.brhc.y - enabledElement.viewport.displayedArea.tlhc.y) * enabledElement.viewport.displayedArea.rowPixelSpacing;
+
+  context.save();
+
+  overlays.forEach((overlay) => {
+    if (!overlay.visible) {
+      return;
+    }
+    const layerCanvas = document.createElement('canvas');
+
+    layerCanvas.width = imageWidth;
+    layerCanvas.height = imageHeight;
+    const layerContext = layerCanvas.getContext('2d');
+    const transform = external.cornerstone.internal.getTransform(enabledElement);
+
+    layerContext.setTransform(transform.m[0], transform.m[1], transform.m[2], transform.m[3], transform.m[4], transform.m[5]);
+    layerContext.save();
+    layerContext.setTransform(1, 0, 0, 1, 0, 0);
+    layerContext.fillStyle = overlay.fillStyle;
+
+    if (overlay.type === 'R') {
+      layerContext.fillRect(0, 0, layerCanvas.width, layerCanvas.height);
+      layerContext.globalCompositeOperation = 'xor';
+    }
+
+    let i = 0;
+
+    for (let y = 0; y < overlay.rows; y++) {
+      for (let x = 0; x < overlay.columns; x++) {
+        if (overlay.pixelData[i++] > 0) {
+          layerContext.fillRect(x, y, 1, 1);
+        }
+      }
+    }
+    layerContext.restore();
+    context.drawImage(layerCanvas, 0, 0);
+  });
+
+  context.restore();
+}
+
+// Enables the overlays tool for a given element
+function enable (element) {
+  element.removeEventListener(external.cornerstone.EVENTS.IMAGE_RENDERED, onImageRendered);
+  element.addEventListener(external.cornerstone.EVENTS.IMAGE_RENDERED, onImageRendered);
+  external.cornerstone.updateImage(element);
+}
+
+// Disables the overlays tool for the given element
+function disable (element) {
+  element.removeEventListener(external.cornerstone.EVENTS.IMAGE_RENDERED, onImageRendered);
+  external.cornerstone.updateImage(element);
+}
+
+// Module/private exports
+const tool = {
+  enable,
+  disable
+};
+
+export default tool;