Comparing Images With idiff


The idiff program compares two images, printing a report about how different they are and optionally producing a third image that records the pixel-by-pixel differences between them. There are a variety of options and ways to compare (absolute pixel difference, various thresholds for warnings and errors, and also an optional perceptual difference metric).

Because idiff is built on top on OpenImageIO, it can compare two images of any formats readable by ImageInput plugins on hand. They may have any (or different) file formats, data formats, etc.

Using idiff

The idiff utility is invoked as follows:

idiff [options] image1 image2

Where input1 and input2 are the names of two image files that should be compared. They may be of any format recognized by OpenImageIO (i.e., for which image-reading plugins are available).

If the two input images are not the same resolutions, or do not have the same number of channels, the comparison will return FAILURE immediately and will not attempt to compare the pixels of the two images. If they are the same dimensions, the pixels of the two images will be compared, and a report will be printed including the mean and maximum error, how many pixels were above the warning and failure thresholds, and whether the result is PASS, WARNING, or FAILURE. For example:

$ idiff a.jpg b.jpg

Comparing "a.jpg" and "b.jpg"
  Mean error = 0.00450079
  RMS error = 0.00764215
  Peak SNR = 42.3357
  Max error  = 0.254902 @ (700, 222, B)
  574062 pixels (82.1%) over 1e-06
  574062 pixels (82.1%) over 1e-06

The “mean error” is the average difference (per channel, per pixel). The “max error” is the largest difference in any pixel channel, and will point out on which pixel and channel it was found. It will also give a count of how many pixels were above the warning and failure thresholds.

The metadata of the two images (e.g., the comments) are not currently compared; only differences in pixel values are taken into consideration.

Raising the thresholds

By default, if any pixels differ between the images, the comparison will fail. You can allow some differences to still pass by raising the failure thresholds. The following example will allow images to pass the comparison test, as long as no more than 10% of the pixels differ by 0.004 (just above a 1/255 threshold):

idiff -fail 0.004 -failpercent 10 a.jpg b.jpg

But what happens if a just a few pixels are very different? Maybe you want that to fail, also. The following adjustment will fail if at least 10% of pixels differ by 0.004, or if any pixel differs by more than 0.25:

idiff -fail 0.004 -failpercent 10 -hardfail 0.25 a.jpg b.jpg

If none of the failure criteria are met, and yet some pixels are still different, it will still give a WARNING. But you can also raise the warning threshold in a similar way:

idiff -fail 0.004 -failpercent 10 -hardfail 0.25 \
         -warn 0.004 -warnpercent 3 a.jpg b.jpg

The above example will PASS as long as fewer than 3% of pixels differ by more than 0.004. If it does, it will be a WARNING as long as no more than 10% of pixels differ by 0.004 and no pixel differs by more than 0.25, otherwise it is a FAILURE.

Output a difference image

Ordinary text output will tell you how many pixels failed or were warnings, and which pixel had the biggest difference. But sometimes you need to see visually where the images differ. You can get idiff to save an image of the differences between the two input images:

idiff -o diff.tif -abs a.jpg b.jpg

The -abs flag saves the absolute value of the differences (i.e., all positive values or zero). If you omit the -abs, pixels in which a.jpg have smaller values than b.jpg will be negative in the difference image (be careful in this case of using a file format that doesn’t support negative values).

You can also scale the difference image with the -scale, making them easier to see. And the -od flag can be used to output a difference image only if the comparison fails, but not if the images pass within the designated threshold (thus saving you the trouble and space of saving a black image).

idiff Command Line Argument Reference

The various command-line options are discussed below:

General options


Prints usage information to the terminal.


Verbose output — more detail about what it finds when comparing images, even when the comparison does not fail.


Quiet mode – output nothing for successful match), output only minimal error messages to stderr for failure / no match. The shell return code also indicates success or failure (successful match returns 0, failure returns nonzero).


Compare all subimages. Without this flag, only the first subimage of each file will be compared.

Thresholds and comparison options

-fail A
-failpercent B
-hardfail C

Sets the threshold for FAILURE: if more than B % of pixels (on a 0-100 floating point scale) are greater than A different, or if any pixels are more than C different. The defaults are to fail if more than 0% (any) pixels differ by more than 0.00001 (1e-6), and C is infinite.

-warn A
-warnpercent B
-hardwarn C

Sets the threshold for WARNING: if more than B % of pixels (on a 0-100 floating point scale) are greater than A different, or if any pixels are more than C different. The defaults are to warn if more than 0% (any) pixels differ by more than 0.00001 (1e-6), and C is infinite.


Does an additional test on the images to attempt to see if they are perceptually different (whether you are likely to discern a difference visually), using Hector Yee’s metric. If this option is enabled, the statistics will additionally show a report on how many pixels failed the perceptual test, and the test overall will fail if more than the “fail percentage” failed the perceptual test.

Difference image output

-o outputfile

Outputs a difference image to the designated file. This difference image pixels consist are each of the value of the corresponding pixel from image1 minus the value of the pixel image2.

The file extension of the output file is used to determine the file format to write (e.g., out.tif will write a TIFF file, out.jpg will write a JPEG, etc.). The data format of the output file will be format of whichever of the two input images has higher precision (or the maximum precision that the designated output format is capable of, if that is less than either of the input imges).

Note that pixels whose value is lower in image1 than in image2, this will result in negative pixels (which may be clamped to zero if the image format does not support negative values)), unless the -abs option is also used.


Will cause the output image to consist of the absolute value of the difference between the two input images (so all values in the difference image \(\ge 0\).

-scale factor

Scales the values in the difference image by the given (floating point) factor. The main use for this is to make small actual differences more visible in the resulting difference image by giving a large scale factor.


Causes a difference image to be produce only if the image comparison fails. That is, even if the -o option is used, images that are within the comparison threshold will not write out a useless black (or nearly black) difference image.

Process return codes

The idiff program will return a code that can be used by scripts to indicate the results:


OK: the images match within the warning and error thresholds.


Warning: the errors differ a little, but within error thresholds.


Failure: the errors differ a lot, outside error thresholds.


The images weren’t the same size and couldn’t be compared.


File error: could not find or open input files, etc.