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page_name: rendertext
title: RenderText and Chrome UI text drawing
---

Most text in Chrome’s UI is rendered either through `gfx::Canvas` text drawing
facilities or by using `gfx::RenderText` directly. Indeed, `gfx::Canvas` itself
uses `gfx::RenderText` for drawing and measuring text, so that nearly all UI
text drawn in Chrome Windows and ChromeOS ends up going through
`gfx::RenderText`.

`gfx::RenderText` has platform-specific subclasses for shaping text runs using
platform APIs: Uniscribe on Windows, Pango on Linux + ChromeOS and CoreText on
the Mac. Although the shaping/layout code is platform-specific, drawing the text
uses a common path, using Skia. The `gfx::RenderText` subclasses also share
common implementations of the various effects that can be applied to the text
such as fading and shadows, as well as the more rudimentary settings such as
text alignment.

In addition to text layout, each `gfx::RenderText` subclass also performs any
necessary selection of fallback fonts needed to render particular runs of text.
While this is mostly delegated to Pango on Linux and CoreText on the Mac, the
Windows implementation has to do this work itself through the font linking
information from the registry.

`gfx::RenderText` is a stateful API - an instance of a `gfx::RenderText`
subclass will cache its layout information between draw calls. Because of this,
it is often more efficient to use the `gfx::RenderText` API directly instead of
using a state-less abstraction such as the `gfx::Canvas` drawing calls. In
particular, for text that changes rarely but that may be drawn multiple times,
it is more efficient to keep an instance of `gfx::RenderText` around, so that
the text layout would be performed only when the text is updated and not on
every draw operation. Prior to the introduction of `gfx::RenderText`, this
pattern was not possible, so you may see existing code still doing its text
drawing through `gfx::Canvas` text drawing APIs (which in the past were not
based on `gfx::RenderText`).

`gfx::RenderText` is also used to render footers and headers in printed page.
This works because Skia has a PDF backend so that the glyph drawing calls done
by `gfx::RenderText` are appropriately transformed to PDF output. This is
currently the case on Mac OS X and Windows, but not yet Linux or ChromeOS
because of sandboxing issues.