How can I draw an arrow using PIL?

PIL doesn't provide an easy way to draw an arrow, so I would suggest converting your PIL Image to a NumPy array and using OpenCV, or Matplotlib, or Cairo, or Wand, to draw an arrow on it then converting back to a PIL Image:

#!/usr/bin/env python3

import cv2
import numpy as np
from PIL import Image

# Create an empty solid blue image
w, h = 640, 480
im = Image.new('RGB', (w,h), (0,0,255))

# Make into Numpy array so we can use OpenCV drawing functions
na = np.array(im)

# Draw arrowed line, from 10,20 to w-40,h-60 in black with thickness 8 pixels
na = cv2.arrowedLine(na, (10,20), (w-40, h-60), (0,0,0), 8)

# Revert back to PIL Image and save
Image.fromarray(na).save('result.png')

Note that OpenCV uses BGR rather than RGB ordering, so if you want a red line in PIL, you need to use (0, 0, 255) in OpenCV.

If you really, really want to draw lines with arrows using PIL, you can draw a line and then add a triangle over the end of it with draw.polygon() like this:

#!/usr/bin/env python3

import math
import random
from PIL import Image, ImageDraw

def arrowedLine(im, ptA, ptB, width=1, color=(0,255,0)):
    """Draw line from ptA to ptB with arrowhead at ptB"""
    # Get drawing context
    draw = ImageDraw.Draw(im)
    # Draw the line without arrows
    draw.line((ptA,ptB), width=width, fill=color)

    # Now work out the arrowhead
    # = it will be a triangle with one vertex at ptB
    # - it will start at 95% of the length of the line
    # - it will extend 8 pixels either side of the line
    x0, y0 = ptA
    x1, y1 = ptB
    # Now we can work out the x,y coordinates of the bottom of the arrowhead triangle
    xb = 0.95*(x1-x0)+x0
    yb = 0.95*(y1-y0)+y0

    # Work out the other two vertices of the triangle
    # Check if line is vertical
    if x0==x1:
       vtx0 = (xb-5, yb)
       vtx1 = (xb+5, yb)
    # Check if line is horizontal
    elif y0==y1:
       vtx0 = (xb, yb+5)
       vtx1 = (xb, yb-5)
    else:
       alpha = math.atan2(y1-y0,x1-x0)-90*math.pi/180
       a = 8*math.cos(alpha)
       b = 8*math.sin(alpha)
       vtx0 = (xb+a, yb+b)
       vtx1 = (xb-a, yb-b)

    #draw.point((xb,yb), fill=(255,0,0))    # DEBUG: draw point of base in red - comment out draw.polygon() below if using this line
    #im.save('DEBUG-base.png')              # DEBUG: save

    # Now draw the arrowhead triangle
    draw.polygon([vtx0, vtx1, ptB], fill=color)
    return im

# Create an empty solid blue image
w, h = 640, 480
im = Image.new('RGB', (w,h), (0,0,255))

# Get some controlled randomness
random.seed(58)

# Draw some random arrows
for _ in range(10):
    ptA = (random.randint(0,w), random.randint(0,h))
    ptB = (random.randint(0,w), random.randint(0,h))
    im = arrowedLine(im, ptA, ptB)

# Save
im.save('result.png')

Keywords: Python, Image Processing, PIL, Pillow, arrow, arrows, arrowed line, OpenCV.