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Adding photos and videos into the canvas is cool, but the real fun comes when we add effects to them through tools like filters.

Filters in p5.js work in two ways: across the entire canvas and applied to individual images.

To apply a filter across the canvas, use the filter() function. It requires the type of filter, which can be one of eight different types as defined by p5.js. For a complete list, visit the p5.js reference on filters.

For example, to add a GRAY filter across the canvas, use:

filter(GRAY);

A diagram showing what the GRAY filter looks like when applied to a canvas with a pink background and an image of a puppy. The entire canvas becomes grayscale.

Certain filter types require an additional numerical argument. For example, the POSTERIZE filter, which reduces the number of colors in the image, requires a value between 2 and 255.

filter(POSTERIZE, 3);

A diagram showing what the POSTERIZE filter looks like when applied to a canvas with a pink background and an image of a puppy. The canvas becomes reduced to fewer colors.

The filter() function applies the filter to everything drawn on the canvas before it’s called—this lets us layer filters together, combining them with ones that were called previously.

A diagram showing what the GRAY and POSTERIZE filter look like when used one after another. The canvas turns grayscale and is reduced to a few shades of gray.

It also lets us apply filters to certain parts of the canvas—for example, we can apply a filter to the entire canvas, then draw new elements that’ll be unaffected.

A diagram showing the grayscale filter applied to a canvas, but another colored image of a flower is drawn on top, unaffected by the filter.

To apply filters across an individual p5 image element, use the .filter() method. You’ll need to call this before drawing the image.

//Invert colors in an image
img.filter(INVERT); 
image(img, 0, 0);

A diagram showing the invert filter only being applied to one image on the canvas. The rest of the elements are unaffected.

We can’t, however, apply filters to individual video elements (though you could instead draw a video to the canvas, then add a filter to the entire canvas).

A weakness of the filter() function is its slow performance—especially when called frequently. Later, we’ll learn how to achieve similar effects (and more!) with pixel manipulation.

Instructions

In this exercise, you’ll see we’ve already drawn four identical images of a cute puppy to the screen. Let’s turn this sketch into pop art—or rather, pup art!

In the setup() function, add a GRAY filter to img1 before it is drawn to the canvas.

Before img2 is drawn, apply an INVERT filter to it.

Before img3 is drawn, apply a POSTERIZE filter to it. For the additional parameter, use the value 4.

Before img4 is drawn, apply a THRESHOLD function to it. Look up details on how to apply it in the p5.js reference.

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