Exploring the CSS Paint API: Rounding Shapes

Mastering rounded corners on complex shapes with CSS can be challenging. The CSS Paint API offers an elegant solution! This article, part of our "Exploring the CSS Paint API" series, demonstrates how to achieve this.

Exploring the CSS Paint API series:

• Part 1: Image Fragmentation Effect
• Part 2: Blob Animation
• Part 3: Polygon Border
• Part 4: Rounding Shapes (this article)

This technique, currently supported by Chrome and Edge, provides a clean CSS-only approach, simplifying maintenance and code understanding.

Alternatives to the Paint API

While other methods exist for rounding complex shapes, they often involve SVG and present challenges for adjustment and maintainability. These include:

clip-path: path()

This SVG-based approach is straightforward if the shape is pre-defined, but modifying the curvature or size requires complex path edits.

SVG Filters

Utilizing SVG filters, as seen in Lucas Bebber's gooey effect tutorial, allows for corner rounding. However, it still necessitates significant SVG expertise for adjustments.

Ana Tudor's CSS-only Method

Ana Tudor's innovative CSS-only technique offers an alternative, but it can be complex to implement and adjust for varying scenarios, especially with transparency or images.

The CSS Paint API Solution

The CSS Paint API provides a more manageable solution. This approach leverages a similar structure to the polygon border example in a previous article.

CSS Setup

We start with a simple rectangle and define the shape using the --path variable, similar to clip-path: polygon(). Clippy can assist in generating these paths.

.box {
  display: inline-block;
  height: 200px;
  width: 200px;
  --path: 50% 0,100% 100%,0 100%;
  --radius: 20px;
  -webkit-mask: paint(rounded-shape);
}

The --path variable defines the shape, and --radius controls the corner curvature.

JavaScript Implementation

The JavaScript code adds midpoints to the path segments and utilizes the arcTo() function to create smooth rounded corners. Understanding arcTo()'s use of control points is crucial. The code iterates through the points, calculating midpoints and applying arcTo() to generate the rounded shape. A radius limit is implemented to prevent overflow with large radius values.

(JavaScript code omitted for brevity, but described in detail in the original article)

The final step involves filling or stroking the shape to achieve the desired visual effect.

Drawbacks and Solutions

The primary drawbacks are the hoverable area (using a mask affects interaction) and potential overflow with large radius values. The hoverable area issue is addressed by using a second mask on a pseudo-element for the border, and the overflow issue is handled by dynamically limiting the radius based on the shape's geometry.

Extending Functionality

The --path variable is enhanced to accept individual radius values for each corner, providing more granular control.

Examples

Several examples showcase the versatility of this technique: CSS shapes, speech bubbles, frames, section dividers, navigation menus, gooey effects, and shape morphing. These examples demonstrate the power and flexibility of the CSS Paint API for creating visually appealing and easily maintainable designs.

Conclusion

The CSS Paint API offers a powerful and efficient method for creating rounded corners on complex shapes, overcoming the limitations of traditional CSS and SVG approaches. Its straightforward implementation and flexibility make it a valuable tool for modern web design.

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