fresnel_shader #

The shader-based Liquid Glass fallback for fresnel.

Apps should not depend on this package directly. Depend on fresnel, which uses this automatically on every platform without native glass: Android, web, Windows, Linux, and iOS below 26.

What it does #

Pure Dart, no native code, so it runs anywhere Flutter does.

ShaderGlassSurface renders fresnel_glass.frag as an ImageFilter.shader over the backdrop, composed on top of a hardware gaussian:

ui.ImageFilter.compose(
  outer: ui.ImageFilter.shader(shader),
  inner: ui.ImageFilter.blur(sigmaX: sigma, sigmaY: sigma),
)

Blurring in the filter chain rather than inside the shader matters — Flutter's blur is a tuned separable pass, and a hand-written multi-tap loop is both slower and worse. The shader is left to do only what the built-in filters cannot:

• Edge refraction. A signed-distance field gives a surface normal; samples bend along it, strongest at the rim. This lensing is what separates glass from a frosted pane.
• Specular rim. Directional highlight with an ambient floor so the rim stays legible on dark backgrounds.
• Thickness falloff, so the surface reads as a slab rather than a decal.
• System intensity. The whole effect scales toward "off" with the user's glass-intensity preference, so hand-rendered glass tracks the OS instead of drifting away from it.

FallbackGlassSurface is the last resort — a plain BackdropFilter with a tint and hairline rim, used on Skia and web where backdrop shaders are unavailable, and whenever the shader fails to compile.

Precision #

The shader declares precision highp float. mediump's 10-bit mantissa quantises the displacement into roughly 1.5px of visible banding at typical thickness values on mobile GPUs.

Notes for anyone writing their own #

Flutter's .frag dialect is GLSL, not SkSL: uniform sampler2D and texture(sampler, uv) with normalised coordinates. uniform shader and .eval() are SkSL and fail impellerc with a bare syntax error.

Size a glass surface from inside a Positioned.fill, not from an outer LayoutBuilder. LayoutBuilder reports the constraints handed down, so any unbounded parent — a Row, Column, or scrollable — yields infinity and silently disables the effect.

License #

MIT