pmouseX()
A Number system variable that tracks the mouse's previous horizontal position.
pmouseX keeps track of the mouse's position relative to the top-left corner of the canvas. Its value is mouseX from the previous frame. For example, if the mouse was 50 pixels from the left edge of the canvas during the last frame, then pmouseX will be 50.
Examples

function setup()
size(100, 100)
-- Slow the frame rate.
frameRate(10)
describe('A line follows the mouse as it moves. The line grows longer with faster movements.')
end
function draw()
background(200)
line(pmouseX, pmouseY, mouseX, mouseY)
end
Syntax
pmouseX
Returns
Number: x-coordinate of mouse from previous frame.