Red filter in Processing.py

process_py_image_filter.pyde

WIDTH = 360
HEIGHT = 240

def setup():
    global img
    img = None
    noLoop()
    size(WIDTH, HEIGHT)
    frame.setResizable(True)
    background(200)

def draw():    
    print("called draw")
    global img
    if img != None:
        print(img)
        applyRedFilter(img)
        image(img, 0, 0)
    
def chooseFile():
    selectInput("Select a file to process:", "fileSelected")
    
def fileSelected(file):
    if file == None:
        return

    global img
    img = loadImage(file.getAbsolutePath())
    frame.setSize(img.width, img.height)
    redraw()

def keyPressed():
    print("key pressed: " + key)
    if key == 'c':
        chooseFile()
    elif key == 'p':
        pass
    elif key == ' ':
        pass

def applyRedFilter(img):
    img.loadPixels()
    numberOfPixels = len(img.pixels)
    for i in range(numberOfPixels):
        pixel = img.pixels[i]

        a = alpha(pixel)
        r = 0
        g = green(pixel)
        b = blue(pixel)

        img.pixels[i] = color(r, g, b, a)

I made it by referring to 500lines - Make your own image filters.