Add development instructions and scripts to README.

README.md

@@ -6,3 +6,34 @@ early in development and only works on a single dance, Isaac Banner's
 ["Two Hearts in Time"](https://contradb.com/dances/2014), chosen as a
 simple dance (i.e. single progression improper, no shadow or
 next/previous neighbor interactions).
+
+
+## Development
+
+To develop locally, you need
+[TypeScript](https://www.typescriptlang.org/download/) installed. You
+can check if you have the **`tsc`** (TypeScript Compiler) command available.
+
+While developing, leave the [watch-tsc.sh](./watch-tsc.sh) script running
+or, equivalently, run the following the command from the root of the
+repository:
+```sh
+tsc --watch
+```
+
+Also, serve the `www/` directory from a local webserver.
+The [serve.sh](./serve.sh) script will do this or you can run the
+following command from the `www/` directory of the repository:
+```sh
+python -m http.server --bind localhost 8085
+```
+(Nothing special about [Python](https://www.python.org/downloads/) here,
+just the easiest web server to set up.)
+
+Then open http://localhost:8085/ in a web browser. The site should work
+in any modern browser (tested in Firefox and Chromium).
+
+Any text editor/IDE works, but I find
+[VS Code](https://code.visualstudio.com/Download)'s
+[TypeScript support](https://code.visualstudio.com/Docs/languages/typescript)
+works well.

serve.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+# Change directory to www/ directory.
+cd "$(dirname "$0")/www/" || exit
+echo "Serving $(pwd)"
+# From www/
+python -m http.server --bind localhost 8085

watch-tsc.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+# Change directory to repo root.
+cd "$(dirname "$0")" || exit
+echo "Watching $(pwd)"
+# From /
+tsc --watch

www/js/main.ts

@@ -421,6 +421,46 @@ displaySettingsDiv.appendChild(document.createElement('br'));
 displaySettingsDiv.appendChild(extraLinesLabel);
 displaySettingsDiv.appendChild(extraLinesSelector);
 
+const trailIncrementsSelector = document.createElement('input');
+trailIncrementsSelector.type = 'number';
+trailIncrementsSelector.min = '0';
+trailIncrementsSelector.step = '1';
+trailIncrementsSelector.value = r.trailIncrements!.toString();
+trailIncrementsSelector.id = 'trailIncrements';
+trailIncrementsSelector.style.width = '3em';
+trailIncrementsSelector.addEventListener('input', (ev) => {
+  r.trailIncrements = trailIncrementsSelector.valueAsNumber;
+  drawAtCurrentBeat();
+  restartAnimation(false);
+})
+const trailIncrementsLabel = document.createElement('label');
+trailIncrementsLabel.innerText = '# trails (faded previous positions): ';
+trailIncrementsLabel.htmlFor = 'trailIncrements';
+
+displaySettingsDiv.appendChild(document.createElement('br'));
+displaySettingsDiv.appendChild(trailIncrementsLabel);
+displaySettingsDiv.appendChild(trailIncrementsSelector);
+
+const trailBeatsSelector = document.createElement('input');
+trailBeatsSelector.type = 'number';
+trailBeatsSelector.min = '0';
+trailBeatsSelector.step = '0.1';
+trailBeatsSelector.value = r.trailLengthInBeats!.toPrecision(1);
+trailBeatsSelector.id = 'trailBeats';
+trailBeatsSelector.style.width = '3em';
+trailBeatsSelector.addEventListener('input', (ev) => {
+  r.trailLengthInBeats = trailBeatsSelector.valueAsNumber;
+  drawAtCurrentBeat();
+  restartAnimation(false);
+})
+const trailBeatsLabel = document.createElement('label');
+trailBeatsLabel.innerText = '# max age of trails (faded previous positions) in beats: ';
+trailBeatsLabel.htmlFor = 'trailBeats';
+
+displaySettingsDiv.appendChild(document.createElement('br'));
+displaySettingsDiv.appendChild(trailBeatsLabel);
+displaySettingsDiv.appendChild(trailBeatsSelector);
+
 displaySettingsDiv.appendChild(document.createElement('br'));
 displaySettingsDiv.appendChild(debugRender);
 displaySettingsDiv.appendChild(debugRenderLabel);

www/js/renderer.ts

@@ -52,6 +52,8 @@ export class Renderer {
   animation?: Animation;
   extraSets?: number;
   extraLines?: number;
+  trailIncrements: number = 6;
+  trailLengthInBeats: number = 1;
   drawDebug: boolean = false;
 
   constructor(canvas: HTMLCanvasElement, ctx: CanvasRenderingContext2D) {
@@ -170,16 +172,15 @@ export class Renderer {
     if (!this.animation) throw new Error("Attempted to render before setting animation.");
 
     this.clear();
-    const increments = 7;
+    const increments = this.trailLengthInBeats > 0 && this.trailIncrements > 0 ? this.trailIncrements : 0;
-    const trailLengthInBeats = 1;
+    const incrementLength = this.trailLengthInBeats / (increments + 1);
-    const incrementLength = trailLengthInBeats / increments;
     progression ??= 0;
     const offsetSets = this.animation.progression.y === 0
       ? 0
       : -((progression - (progression % 2)) / 2) / ((this.animation.progression.y * 2) / setDistance);
     for (var i = increments; i >= 0; i--) {
-      const beatToDraw = beat - i*incrementLength;
+      const beatToDraw = i == 0 ? beat : beat - i*incrementLength;
-      this.ctx.globalAlpha = i == 0 ? 1 : (1 - i/increments)*0.3;
+      this.ctx.globalAlpha = i == 0 ? 1 : (1 - i / (increments + 1)) * 0.3;
       const positions = this.animation.positionsAtBeat(beatToDraw, progression % 2);
       if (this.drawDebug) this.drawSets(positions, offsetSets, true, true);
       this.drawSets(positions, offsetSets, i === 0, false);