The ImageMath Module

(New in 1.1.6) The ImageMath module can be used to evaluate “image expressions”. The module provides a single eval function, which takes an expression string and one or more images.


Using the ImageMath module:

import Image, ImageMath

im1 ="image1.jpg")
im2 ="image2.jpg")

out = ImageMath.eval("convert(min(a, b), 'L')", a=im1, b=im2)"result.png")


ImageMath.eval(expression, environment)

Evaluate expression in the given environment.

The expression argument is a string which uses the standard Python expression syntax. In addition to the standard operators, you can also use the functions described below.

The environment argument is a dictionary that maps image names to Image instances. You can use one or more keyword arguments instead of a dictionary, as shown in the above example. Note that the names must be valid Python identifiers.

In the current version, ImageMath only supports single-layer images. To process multi-band images, use the split and merge functions.

This function returns an image, an integer value, a floating point value, or a pixel tuple, depending on the expression.

Expression Syntax

Expressions are standard Python expressions, but they’re evaluated in a non-standard environment. You can use PIL methods as usual, plus the following set of operators and functions:

Standard Operators

You can use standard arithmetical operators for addition (+), subtraction (-), multiplication (*), and division (/).

The module also supports unary minus (-), modulo (%), and power (**) operators.

Note that all operations are done with 32-bit integers or 32-bit floating point values, as necessary. For example, if you add two 8-bit images, the result will be a 32-bit integer image. If you add a floating point constant to an 8-bit image, the result will be a 32-bit floating point image.

You can force conversion using the convert, float, and int functions described below.

Bitwise Operators

The module also provides operations that operate on individual bits. This includes and (&), or (|), and exclusive or (^). You can also invert (~) all pixel bits.

Note that the operands are converted to 32-bit signed integers before the bitwise operation is applied. This means that you’ll get negative values if you invert an ordinary greyscale image. You can use the and (&) operator to mask off unwanted bits.

Bitwise operators don’t work on floating point images.

Logical Operators

Logical operators like and, or, and not work on entire images, rather than individual pixels.

An empty image (all pixels zero) is treated as false. All other images are treated as true.

Note that and and or return the last evaluated operand, while not always returns a boolean value.

Built-in Functions

These functions are applied to each individual pixel.

abs(image): Absolute value.

convert(image, mode): Convert image to the given mode. The mode must be given as a string constant.

float(image): Convert image to 32-bit floating point. This is equivalent to convert(image, 'F').

int(image): Convert image to 32-bit integer. This is equivalent to convert(image, 'I').

Note that 1-bit and 8-bit images are automatically converted to 32-bit integers if necessary to get a correct result.

max(image1, image2): Maximum value

min(image1, image2): Minimum value