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The Pixels Array

Before diving into more complex image manipulation techniques, let’s look at what’s happening under the hood with the get() and set() functions.

The pixels array is a special data structure that enables us to access and manipulate pixel data—whether in the canvas or in individual images or videos. The get() and set() functions are actually functions that make working with pixels array easier, but slower.

We normally think of an image as a two-dimensional matrix—an array of arrays—with each pixel having a red, green, blue, and alpha (RGBA) value.

The pixels array is a representation of all those pixels, flattened down to a single list. Starting from the first pixel and moving left-to-right by rows to the last pixel, pixels stores the RGBA values for each pixel into one, single array. The one-dimensional array format, while harder to work with, makes accessing the data fast.

The pixels array is also more like a snapshot of a canvas or image, rather than a source of truth for what’s on the screen. This is where the updatePixels() function comes back in—after we modify the pixels array, it’s how we indicate that we’re done making changes, and we’re ready to see it updated on the sketch.

In some cases, the pixels array will be affected by pixel density. The pixel density in a p5.js sketch depends on your computer monitor—many high-resolution monitors treat a single pixel on the screen as multiple smaller pixels, creating a sharper image—accordingly, the number of elements in the pixels array factors in that density.

Instructions

View the diagram to the right to see how the pixels array consists of the RGBA values of each pixel in an image.

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