img2brush: allow customising the channel to pull from

.docs/img2brush/img2brush.js

@@ -13,6 +13,23 @@ window.addEventListener("load", () => {
 	})
 });
 
+function get_source_channel_offset() {
+	const select = document.querySelector("#img2brush-channel");
+	console.info(`get_source_channel_offset: channel is ${select.value}`)
+	switch(select.value) {
+		case "alpha":
+			return 3;
+		case "red":
+			return 0;
+		case "green":
+			return 1;
+		case "blue":
+			return 2;
+		default:
+			throw new Error(`Error : Unknown channel name ${select.value}.`);
+	}
+}
+
 function select_output() {
 	let output = document.querySelector("#brushimg-tsv");
 	
@@ -26,7 +43,6 @@ function select_output() {
 	selection.addRange(range);
 }
 
-
 function handle_drag_enter(event) {
 	event.target.classList.add("dropzone-active");
 }
@@ -80,12 +96,15 @@ function handle_new_image(image) {
 }
 
 function pixels2tsv(pixels) {
+	const offset = get_source_channel_offset();
+	console.info(`pixels2tsv: offset is ${offset}`);
 	let result = "";
 	for(let y = 0; y < pixels.height; y++) {
 		let row = [];
 		for(let x = 0; x < pixels.width; x++) {
 			// No need to rescale here - this is done automagically by WorldEditAdditions.
-			row.push(pixels.data[((y*pixels.width + x) * 4) + 3] / 255);
+			// r/b/g/alpha
+			row.push(pixels.data[((y*pixels.width + x) * 4) + offset] / 255);
 		}
 		result += row.join(`\t`) + `\n`;
 	}

.docs/img2brush/index.html

@@ -6,7 +6,17 @@ title: Image to brush converter
 <section class="panel-generic">
 	<h1>Image to sculpting brush converter</h1>
 	
-	<p>Convert any image to a sculpting brush here! The <strong>alpha (opacity) channel</strong> is used to determine the weight of the brush - <strong>all colour is ignored</strong>.</p>
+	<p>Convert any image to a sculpting brush here!</p>
+	<p>
+		<strong>Use this channel to convert:</strong>
+		<select name="img2brush-channel" id="img2brush-channel">
+			<option value="alpha" selected>Alpha (opacity)</option>
+			<option value="red">Red</option>
+			<option value="green">Green</option>
+			<option value="blue">Blue</option>
+		</select> 
+	</p>
+	<p>Only the selected channel is used to determine the weight of the brush - <strong>all other channels are ignored</strong>! Change the channel option <strong>before</strong> you drag + drop the image onto this page.</p>
 </section>
 
 <section id="dropzone" class="bigbox panel-generic">