What Does setOutput Do in GPU.js?

The setOutput method in GPU.js is a fundamental configuration function that defines the dimensions and shape of the array returned by a GPU-accelerated kernel. This article explains how setOutput works, its role in allocating GPU execution threads, how it maps internal thread coordinates, and how it determines the structure of your output data.

Defining Output Dimensions

When executing a kernel, the GPU needs to know the exact size and dimensionality of the calculation it must perform. The setOutput method accepts an array of up to three integers—[width], [width, height], or [width, height, depth]—corresponding to 1D, 2D, or 3D data structures:

• 1D Output: kernel.setOutput([1000]) produces a single array with 1,000 elements.
• 2D Output: kernel.setOutput([512, 512]) generates a nested matrix containing 512 rows and 512 columns.
• 3D Output: kernel.setOutput([10, 10, 10]) produces a three-dimensional array.

Determining the GPU Thread Grid

Beyond shaping the returned JavaScript array, setOutput dictates the number of parallel threads dispatched across GPU compute units. Every unique position in the declared output dimensions corresponds to an individual execution thread. For example, declaring setOutput([1920, 1080]) tells GPU.js to instantiate over two million parallel threads, each running the kernel function simultaneously to calculate the value of its designated position.

Mapping Internal Thread Coordinates

Inside the kernel function body, setOutput directly drives the built-in coordinate system:

• this.thread.x indexes the first dimension (horizontal coordinate).
• this.thread.y indexes the second dimension (vertical coordinate).
• this.thread.z indexes the third dimension (depth coordinate).

Each thread reads these coordinate properties to identify its place within the overall output grid, allowing the kernel to perform spatially distinct calculations—such as reading matching pixel locations from an input texture or computing specific cells in matrix multiplication.

Summary

In GPU.js, calling setOutput is mandatory prior to executing a kernel. It specifies the physical shape of the returned JavaScript array, controls the thread allocation across the graphics hardware, and establishes the index bounds used by this.thread inside your shader logic.