Appears to be texture filtering.
Using Hamoid’s example, add ((PGraphicsOpenGL)g).textureSampling(POINT);
into the setup function.
I have used that before to preserve “blockyness” of an image (in other words, the texture is not filtered).
I am not sure how to do the same in p5js.
Possibly with gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
or something like it. (I have not tried this)