Python: How to Script Image Adjustments

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I’ll level with you. I rarely need to manipulate specific pixels in an image, but when those occasions come along, it’s a massive pain in the neck. One that’s preventable with Python. So here it is. A quick starter guide to coding pixel by pixel adjustments.

The Set Up

Ok, let’s start off by installing the Pillow module. I hate installing modules through the command prompt, so I use the Python shell. Here’s what I typed in to make that happen:

Importing the Pillow Module

A Simple Shell Script

To make sure Pillow installed properly, try popping this into the Python shell. Remember, when you’re specifying a file path in Python, you’ll need to double up on your back slashes. Use .open to stash your tester image into a variable, and then append .size to get the width and height out.

The Pillow Module

Print RGBA Values for Every Pixel

The width and height will be returned as a tuple. To make the script adaptable, you’ll want to assign width and height variables. Accessing values in a tuple is just a matter of referencing the appropriate position within brackets. The x value is at 0, and the y is at 1.

*Tuples, because they are the devil, start counting at 0*

Once you’ve called .load, you can access the red, green, blue, and alpha values using a nested loop like so:

from PIL import Image
im = Image.open('C:\\Users\\Li\\Desktop\\HexTest.png')
pix = im.load()
width = im.size[0]
height = im.size[1]
for y in range(height):
for x in range(width):
print(pix[x,y])

Keep in mind, if your image is 700 by 800, this is going to loop through 560,000 pixels. It might take a while.

Print Coordinates

Find the Black Pixels

What’s fun about this is when you combine that last script with an if statement, you can get a list of coordinates with a specific color.

Coordinate Extractor

Accessing Red, Green, and Blue Values Separately

Alright, what if you don’t want to find pixels at a specific RGB value, but more or less than a value?

See that pix[x,y] line above? That returns a tuple made up of four values. Meaning you can access each of those values separately in the normal access-a-tuple fashion. It’s going to look a little hokey with brackets on top of brackets, but all you really need to remember is red is in the [0] slot of the tuple, green is at [1], blue is at [2], and alpha is at [3].

So, say, if you wanted to have your if statement look specifically for pixels that have a red value less than 50, you could use a this expression:

if pix[x,y][0] < 50:

The full Python script is over at gist.

Here’s a look at what those results look like with different values plugged in:

Variations on a Script

 

*Scripts tested in Python 3.6

 

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