HTML Canvas shadowBlur Property: Adding Blur to Shadows

The shadowBlur property of the HTML Canvas API allows you to add a blur effect to shadows of shapes, text, and images drawn on the canvas. This property controls the amount of blurring applied to the shadow, making it appear softer and more diffuse. Understanding and utilizing shadowBlur can significantly enhance the visual appeal of your canvas graphics by adding depth and dimension.

Understanding shadowBlur

The shadowBlur property specifies the blur level using a numerical value, where a higher value corresponds to a more significant blur effect. The default value for shadowBlur is 0, which means no blur is applied, resulting in a sharp shadow.

Syntax

ctx.shadowBlur = blurAmount;

• ctx: The 2D rendering context of the canvas.
• blurAmount: A number specifying the amount of blur. Non-integer values are allowed.

Important Points

• Non-negative Values: The blurAmount must be a non-negative number. Negative values are ignored by the browser.
• Applied Before Drawing: The shadowBlur property must be set before drawing the shape, text, or image that will have the shadow.
• Cumulative Effect: Like other shadow properties (offset, color), the shadowBlur property is part of the rendering context state. Once set, it will apply to all subsequent draw operations until changed.
• Performance: Extremely high blur values may impact performance, especially on less powerful devices. It is important to test your implementations for efficiency.

Practical Examples

Let's explore how to use the shadowBlur property with practical examples.

Basic Shadow Blur

In this example, we'll draw a simple rectangle and apply a basic shadow with a blur.

<canvas
  id="canvasBlurBasic"
  width="300"
  height="200"
  style="border: 1px solid #ddd;"
></canvas>

<script>
//<![CDATA[

  const canvas_blur_basic = document.getElementById("canvasBlurBasic");
  const ctx_blur_basic = canvas_blur_basic.getContext("2d");

  ctx_blur_basic.shadowColor = "rgba(0, 0, 0, 0.5)"; // Shadow color
  ctx_blur_basic.shadowBlur = 10; // Blur amount
  ctx_blur_basic.shadowOffsetX = 5; // Horizontal offset
  ctx_blur_basic.shadowOffsetY = 5; // Vertical offset

  ctx_blur_basic.fillStyle = "lightblue";
  ctx_blur_basic.fillRect(50, 50, 150, 100);

//]]]]><![CDATA[>
</script>

In this example, the rectangle has a light blue fill, and a shadow is applied with a 10px blur. The shadow is offset by 5px horizontally and vertically, and the color is set to semi-transparent black.

Shadow Blur with Different Values

Let's explore the effect of different shadowBlur values by drawing multiple circles with varying blur amounts.

<canvas
  id="canvasBlurMultiple"
  width="400"
  height="250"
  style="border: 1px solid #ddd;"
></canvas>

<script>
//<![CDATA[

  const canvas_blur_multiple = document.getElementById("canvasBlurMultiple");
  const ctx_blur_multiple = canvas_blur_multiple.getContext("2d");

  const blurValues = [0, 5, 10, 20];
  const xPositions = [50, 150, 250, 350];

  for (let i = 0; i < blurValues.length; i++) {
    ctx_blur_multiple.shadowColor = "rgba(0, 0, 0, 0.5)";
    ctx_blur_multiple.shadowBlur = blurValues[i];
    ctx_blur_multiple.shadowOffsetX = 2;
    ctx_blur_multiple.shadowOffsetY = 2;

    ctx_blur_multiple.beginPath();
    ctx_blur_multiple.arc(xPositions[i], 125, 30, 0, 2 * Math.PI);
    ctx_blur_multiple.fillStyle = "orange";
    ctx_blur_multiple.fill();
  }

//]]]]><![CDATA[>
</script>

This example draws four circles, each with a different shadowBlur value (0, 5, 10, and 20). You can observe how increasing the shadowBlur value makes the shadow softer and more spread out.

Shadow Blur with Text

The shadowBlur property also applies to text. Here's an example of how to use it with the fillText() method.

<canvas
  id="canvasBlurText"
  width="400"
  height="150"
  style="border: 1px solid #ddd;"
></canvas>

<script>
//<![CDATA[

  const canvas_blur_text = document.getElementById("canvasBlurText");
  const ctx_blur_text = canvas_blur_text.getContext("2d");

  ctx_blur_text.shadowColor = "purple";
  ctx_blur_text.shadowBlur = 10;
  ctx_blur_text.shadowOffsetX = 3;
  ctx_blur_text.shadowOffsetY = 3;

  ctx_blur_text.font = "40px Arial";
  ctx_blur_text.fillStyle = "black";
  ctx_blur_text.fillText("Blurred Text", 50, 90);

//]]]]><![CDATA[>
</script>

In this example, the text "Blurred Text" is drawn with a black fill and a purple shadow that has a 10px blur. This helps the text stand out, creating a slightly more subtle text effect.

Shadow Blur with Images

You can apply the shadowBlur to images as well. Let's see it in action.

<canvas
  id="canvasBlurImage"
  width="250"
  height="200"
  style="border: 1px solid #ddd;"
></canvas>

<script>
//<![CDATA[

  const canvas_blur_image = document.getElementById("canvasBlurImage");
  const ctx_blur_image = canvas_blur_image.getContext("2d");

  const image_blur = new Image();
    image_blur.src = 'https://dummyimage.com/70x70/000/fff';
  image_blur.onload = function () {
    ctx_blur_image.shadowColor = "rgba(0, 0, 0, 0.8)";
    ctx_blur_image.shadowBlur = 15;
    ctx_blur_image.shadowOffsetX = 5;
    ctx_blur_image.shadowOffsetY = 5;
    ctx_blur_image.drawImage(image_blur, 75, 50);
  };

//]]]]><![CDATA[>
</script>

This example loads an image and draws it onto the canvas with a shadow, which includes a 15px blur. The shadow effect creates a slight three-dimensional look.

Tips and Best Practices

• Start with Small Values: Begin experimenting with smaller shadowBlur values to understand the effect before using larger values that might result in excessive blur.
• Use Transparency: Combine shadowBlur with transparent shadow colors to create soft, subtle shadows that do not overwhelm the drawn content.
• Performance: Be mindful of performance when using shadowBlur on complex scenes. High blur values can be performance-intensive.
• Experiment with Combinations: Combine shadowBlur with other shadow properties like shadowColor, shadowOffsetX, and shadowOffsetY to achieve varied shadow effects.

Browser Support

The shadowBlur property is widely supported across all modern web browsers, making it a reliable feature for adding visual depth to your canvas graphics.

Conclusion

The shadowBlur property is a powerful tool for enhancing the visual appeal of your canvas graphics. By applying blur effects to shadows, you can create depth, dimension, and more polished designs. With a solid understanding of how to use shadowBlur alongside other shadow properties, you can produce captivating and engaging visual experiences on the web.