Documentation
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Overview ¶
Package offscreen renders a widget.Widget tree into an *image.RGBA without a GPU, window, or running application.
This enables headless widget rendering for screenshot testing, multi-process compositors (e.g. github.com/gogpu/compose), PDF/image export, and CI pipelines.
The renderer uses CPU-only rasterization via github.com/gogpu/gg. A Material 3 light theme is applied by default; override with WithTheme.
Basic usage ¶
r := offscreen.NewRenderer(400, 120)
r.Render(primitives.Text("Hello, World!").FontSize(24))
img := r.Image() // *image.RGBA — ready for png.Encode, testing, compositing
HiDPI rendering ¶
r := offscreen.NewRenderer(800, 240, offscreen.WithScale(2.0))
Custom theme and background ¶
dark := material3.NewDark(widget.Hex(0x00BFA5))
r := offscreen.NewRenderer(400, 120,
offscreen.WithTheme(dark),
offscreen.WithBackground(widget.ColorWhite),
)
Index ¶
Constants ¶
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Variables ¶
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Functions ¶
This section is empty.
Types ¶
type Option ¶
type Option func(*Renderer)
Option configures a Renderer.
func WithBackground ¶
WithBackground sets the canvas clear color before drawing. The default is transparent (zero alpha).
func WithFitSize ¶ added in v0.1.17
func WithFitSize() Option
WithFitSize enables measure-then-render mode. The renderer measures the widget's preferred size before allocating the canvas, so the caller does not need to know dimensions upfront. Pass width=0, height=0 to NewRenderer.
This follows the universal enterprise pattern (Flutter IntrinsicWidth, Qt6 sizeHint, SwiftUI sizeThatFits, Compose SubcomposeLayout).
Use WithMaxSize to constrain expansion for widgets that grow unbounded.
func WithMaxSize ¶ added in v0.1.17
WithMaxSize sets maximum dimensions for fit-to-content mode. A value of 0 means unlimited in that axis. Only effective when WithFitSize is also set.
func WithScale ¶
WithScale sets the display scale factor for HiDPI rendering. The default is 1.0. A value of 2.0 renders at Retina density.
func WithTheme ¶
func WithTheme(tp widget.ThemeProvider) Option
WithTheme overrides the default Material 3 light theme.
type Renderer ¶
type Renderer struct {
// contains filtered or unexported fields
}
Renderer renders a widget tree into an offscreen image.
Create with NewRenderer. Configure with Option functions. Call Renderer.Render to draw a widget, then Renderer.Image to retrieve the result.
func NewRenderer ¶
NewRenderer creates an offscreen renderer with the given pixel dimensions.
Width and height must be positive; they are clamped to a minimum of 1. By default, a Material 3 light theme is applied and the background is transparent. Override with WithTheme, WithBackground, and WithScale.
func (*Renderer) Render ¶
Render lays out and draws the widget into the offscreen buffer.
In default mode, the widget receives the renderer dimensions as loose constraints. In fit-to-content mode (WithFitSize), the widget is measured first to determine its preferred size, then the canvas is allocated at that size. Calling Render again replaces the previous image.
Source Files