Part of the WebGL API, the WebGLRenderingContext method getUniformLocation() returns the location of a specific uniform variable which is part of a given WebGLProgram.

The uniform variable is returned as a WebGLUniformLocation object, which is an opaque identifier used to specify where in the GPU's memory that uniform variable is located.

Once you have the uniform's location, you can access the uniform itself using one of the other uniform access methods, passing in the uniform location as one of the inputs:

• getUniform()
  • : Returns the value of the uniform at the given location.
• WebGLRenderingContext.uniform[1234][fi][v]()
  • : Sets the uniform's value to the specified value, which may be a single floating point or integer number, or a 2-4 component vector specified either as a list of values or as a Float32Array or Int32Array.
• WebGLRenderingContext.uniformMatrix[234][fv]()
  • : Sets the uniform's value to the specified matrix, possibly with transposition. The value is represented as a sequence of GLfloat values or as a Float32Array.

The uniform itself is declared in the shader program using GLSL.

program

The WebGLProgram in which to locate the specified uniform variable.

name

A string specifying the name of the uniform variable whose location is to be returned. The name can't have any whitespace in it, and you can't use this function to get the location of any uniforms starting with the reserved string "gl_", since those are internal to the WebGL layer.

The possible values correspond to the uniform names returned by getActiveUniform; see that function for specifics on how declared uniforms map to uniform location names.

Additionally, for uniforms declared as arrays, the following names are also valid:

• The uniform name without the [0] suffix. E.g. the location returned for arrayUniform is equivalent to the one for arrayUniform[0].
• The uniform name indexed with an integer. E.g. the location returned for arrayUniform[2] would point directly to the third entry of the arrayUniform uniform.

A WebGLUniformLocation value indicating the location of the named variable, if it exists. If the specified variable doesn't exist, null is returned instead.

The WebGLUniformLocation is an opaque value used to uniquely identify the location in the GPU's memory at which the uniform variable is located. With this value in hand, you can call other WebGL methods to access the value of the uniform variable.

The following errors may occur; to check for errors after getUniformLocation() returns, call getError().

GL_INVALID_VALUE

The program parameter is not a value or object generated by WebGL.

GL_INVALID_OPERATION

The program parameter doesn't correspond to a GLSL program generated by WebGL, or the specified program hasn't been linked successfully.

In this example, taken from the animateScene() method in the article A basic 2D WebGL animation example, obtains the locations of three uniforms from the shading program, then sets the value of each of the three uniforms.

gl.useProgram(shaderProgram);

uScalingFactor = gl.getUniformLocation(shaderProgram, "uScalingFactor");
uGlobalColor = gl.getUniformLocation(shaderProgram, "uGlobalColor");
uRotationVector = gl.getUniformLocation(shaderProgram, "uRotationVector");

gl.uniform2fv(uScalingFactor, currentScale);
gl.uniform2fv(uRotationVector, currentRotation);
gl.uniform4fv(uGlobalColor, [0.1, 0.7, 0.2, 1.0]);

After setting the current shading program to shaderProgram, this code fetches the three uniforms "uScalingFactor", "uGlobalColor", and "uRotationVector", calling getUniformLocation() once for each uniform.

Then the three uniforms' values are set:

• The uScalingFactor uniform — a 2-component vertex — receives the horizontal and vertical scaling factors from the variable currentScale.
• The uniform uRotationVector is set to the contents of the variable currentRotation. This, too, is a 2-component vertex.
• Finally, the uniform uGlobalColor is set to the color [0.1, 0.7, 0.2, 1.0], the components in this 4-component vector represent the values of red, green, blue, and alpha, respectively.

Having done this, the next time the shading functions are called, their own variables named uScalingFactor, uGlobalColor, and uRotationVector will all have the values provided by the JavaScript code.

• WebGLRenderingContext.getAttribLocation()
• WebGLRenderingContext.getActiveUniform()
• WebGLUniformLocation