title: what does "imageData[ (width + height * stageHeight) * 4 -1 ]" mean?
body: <pre><code>dotPos(density, stageWidth, stageHeight) {
      const imageData = this.ctx.getImageData(0, 0, stageWidth, stageHeight).data;
  
      const imageData = this.ctx.getImageData(0, 0, stageWidth, stageHeight).data;
  
      const particles = [];
      let i = 0;
      let width = 0;
      let pixel;
  
      for (let height = 0; height &lt; stageHeight; height++) {
        ++i;
        const slide = i % 2 === 0;
        width = 0;
        if (slide === 1) {
          width += 6;
        }
  
        for (width; width &lt; stageWidth; width += density) {
          pixel = imageData[(width + height * stageWidth) * 4 - 1];
          if (pixel !== 0 &amp;&amp; width &gt; 0 &amp;&amp; width &lt; stageWidth &amp;&amp; height &gt; 0 &amp;&amp; height &lt; stageHeight) {
            particles.push({
              x: width,
              h: height,
            });
          }
        }
      }
  </code></pre>
  <p>I'm studying the code above but I don't know what the below line means:</p>
  <pre><code>pixel = imageData[(width + height * stageWidth) * 4 - 1]; 
  </code></pre>
  <p>please help me.</p>