Painting, Layout, and Compositing in Browser Rendering

When a web page is loaded in a browser, the browser goes through several steps to render the content on the screen. These steps include

1. parsing the HTML and CSS,
2. building the DOM and CSSOM, and
3. finally rendering the page.

During the rendering process, the browser performs several operations, including layout, painting, and compositing, to display the content correctly. In the context of browser rendering, painting, layout, and compositing refer to different stages of the process the browser uses to render web pages onto the screen. These stages occur after the browser parses the HTML and CSS to build the DOM (Document Object Model) and CSSOM (CSS Object Model). Here’s a breakdown of what each term means and how they differ:

1. Layout (Reflow):

• What it is: The layout stage involves calculating the geometry and positioning of elements on the page. The browser determines where each element should appear, what size it should be, and how it interacts with other elements (e.g., margins, padding, borders, etc.).
• When it happens: Layout occurs when changes are made to the document that affect element positions, sizes, or visibility, such as modifying the DOM, changing styles (e.g., width, height, display properties), or resizing the window.
• Purpose: It answers questions like:
  • How wide and tall is each element?
  • Where should elements be placed relative to each other on the screen?

Example: Changing the width or height of an element, or adding a new element to the DOM, triggers layout to recalculate the positions of affected elements.

2. Painting:

• What it is: Painting is the process where the browser fills in the pixels of the elements. After determining the layout, the browser starts to "paint" each visual part of the page — applying colors, borders, shadows, text, and images — based on the computed styles.
• When it happens: Painting occurs when visual properties that don’t affect layout change, such as background color, text color, or box-shadow. These changes don’t require recalculating positions or dimensions, but they do require re-drawing the pixels on the screen.
• Purpose: It answers the question:
  • How should each element look on the screen in terms of colors, text, and visuals?

Example: Changing the color, background-color, or box-shadow of an element triggers a repaint (painting) of the affected region.

3. Compositing:

• What it is: Compositing is the process of taking separate painted layers (created in the previous steps) and combining them into a single image to be displayed on the screen. The browser breaks the page into multiple layers (e.g., for different z-index values, 3D transforms, or opacity changes), and compositing ensures that these layers are combined in the correct order.
• When it happens: Compositing happens when elements are transformed (using CSS properties like transform, opacity, or z-index), or if the page uses features like hardware acceleration. These elements might be placed into their own layers to improve performance.
• Purpose: It answers the question:
  • How should the layers of elements be stacked and blended together before being rendered?

Example: Applying transform: translateX(100px) or changing opacity causes compositing because these properties are often rendered on separate layers to avoid triggering a full layout or paint.

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Summary of the Differences:

• Layout: Calculates positions and dimensions of elements on the page (how elements are laid out on the screen).
• Painting: Fills in the pixels of each element according to their visual properties (colors, borders, backgrounds, etc.).
• Compositing: Combines painted layers (which might be drawn separately for performance reasons) into a single visual image, respecting their stacking order and other effects.

Workflow Overview:

1. Layout is triggered first if any structural changes are made to the page (e.g., DOM or size changes).
2. Painting is done next, where the browser paints the pixels for each element.
3. Compositing then takes place if multiple layers need to be combined or layered for effects like transformations, opacity, or z-index.

Example in Practice:

• Layout: You add a new <div> to the page or change its size. The browser recalculates where everything should be positioned and how much space each element takes.
• Paint: You change the background color of the <div>. The browser repaints the background with the new color, but doesn't need to recalculate positions.
• Compositing: You apply a transform: rotate(45deg) to the <div>. The browser moves that element to a separate layer and combines it with the other layers during compositing, without needing to repaint everything else.

By understanding the distinction between these stages, you can better optimize your web application for performance, as each stage has a different cost in terms of computation and rendering time.