iResolution

iResolution is a uniform variable used in shader programming, most notably in the Shadertoy web platform, to convey the rendering canvas resolution to a fragment shader. Implemented as a three-component vector (vec3), its components are typically used as follows: iResolution.x is the width in pixels, iResolution.y is the height, and iResolution.z is either set to 1.0 or used by some environments to encode additional data such as aspect or a device-dependent parameter.

Purpose and usage: By exposing the current output size, iResolution allows shader code to compute normalized coordinates and operate in a resolution-independent manner. Common patterns include computing uv = gl_FragCoord.xy / iResolution.xy or computing an aspect ratio as iResolution.x / iResolution.y. This enables effects to scale appropriately across different window sizes and aspect ratios.

Origins and scope: The variable originated with Shadertoy, where it is provided by the host environment and updated on resize. It has since been adopted by other GLSL-based tools and editors that emulate Shadertoy-style uniforms. iResolution is not part of the core GLSL specification, but it is a widely used convention in tutorials, examples, and shader templates.

Variations and related terms: Some engines provide analogous uniforms, such as Unity’s _ScreenParams, which exposes width, height, and a third value. Other environments may implement iResolution-like data under different names or may leave the third component unused. In many shaders, the z component is not relied upon and remains at 1.0.