How to Measure Image Similarity Using 'pnmpsnr' (with examples)

The pnmpsnr command is a tool from the Netpbm library that calculates the peak signal-to-noise ratio (PSNR) between two images. The PSNR is a popular metric used to measure the quality of reconstructed or compressed images relative to the original by quantifying the difference. It is widely used in image processing fields, such as compression, restoration, and enhancement, to assess how much two images differ in quality.

Use case 1: Compute the difference, i.e., the peak signal-to-noise ratio (PSNR) between two images

Code:

pnmpsnr path/to/file1.pnm path/to/file2.pnm

Motivation:

Understanding the difference in quality between two images is crucial for professionals dealing with image processing, such as photographers, graphic designers, or video editors. They require tools to assess variations in image quality when they apply changes or compress images. Calculating the PSNR provides them with a quantifiable metric to evaluate how much a processed image deviates from its original form.

Explanation:

• pnmpsnr: The command for calculating the PSNR.
• path/to/file1.pnm: The path to the first image in PNM format; the original or reference image.
• path/to/file2.pnm: The path to the second image in PNM format; the altered or compressed image.

Example output:

PSNR = 30.27 dB

Use case 2: Compare the color components rather than the luminance and chrominance components of the images

Code:

pnmpsnr path/to/file1.pnm path/to/file2.pnm -rgb

Motivation:

In some cases, professionals might be more interested in assessing how changes to specific color components (red, green, and blue) affect overall image quality rather than looking at the luminance (brightness-related) components. This is particularly useful in fields like computer graphics and digital art, where color accuracy and consistency are paramount.

Explanation:

• -rgb: This option instructs the command to compare individual color components (red, green, blue) instead of the usual luminance and chrominance values.

Example output:

PSNR (R channel) = 32.45 dB
PSNR (G channel) = 31.89 dB
PSNR (B channel) = 30.67 dB

Use case 3: Run in comparison mode, i.e., only output nomatch or match depending on whether the computing PSNR exceeds n or not

Code:

pnmpsnr path/to/file1.pnm path/to/file2.pnm -target n

Motivation:

For automated processing systems, such as batch processing or quality check systems in digital media companies, having a simple match or nomatch response helps in rapidly filtering images that need attention. This use case allows streamlined operations where quick decisions are necessary based on specific quality thresholds.

Explanation:

• -target n: Sets a PSNR threshold n. If the computed PSNR is greater than this threshold, the output is match, otherwise nomatch.

Example output:

match

Use case 4: Run in comparison mode and compare the individual image components, i.e., Y, Cb, and Cr, to the corresponding thresholds

Code:

pnmpsnr path/to/file1.pnm path/to/file2.pnm -target1 threshold_Y -target2 threshold_Cb -target3 threshold_Cr

Motivation:

For applications where luminance (Y) and chrominance (Cb, Cr) components are critical, such as in video compression where you want to preserve specific attributes more than others, this option allows targeted analysis that is component-specific. This is particularly beneficial in situations where preserving color accuracy or brightness is more important than a general similarity.

Explanation:

• -target1 threshold_Y: Sets a PSNR threshold for the Y (luminance) component.
• -target2 threshold_Cb: Sets a PSNR threshold for the Cb (chrominance blue) component.
• -target3 threshold_Cr: Sets a PSNR threshold for the Cr (chrominance red) component.

Example output:

nomatch Y
match Cb
match Cr

Use case 5: Run in comparison mode and compare the individual image components, i.e., red, green, and blue to the corresponding thresholds

Code:

pnmpsnr path/to/file1.pnm path/to/file2.pnm -rgb -target1 threshold_red -target2 threshold_green -target3 threshold_blue

Motivation:

In scenarios where specific color channels need to meet certain quality criteria, such as in digital printing or screen displays, this use case helps by ensuring each color component exceeds its respective quality threshold. This way, color integrity is maintained which is crucial for high-quality outputs in print media and visual displays.

Explanation:

• -rgb: Indicates to compare RGB color components.
• -target1 threshold_red: Sets a PSNR threshold for the red component.
• -target2 threshold_green: Sets a PSNR threshold for the green component.
• -target3 threshold_blue: Sets a PSNR threshold for the blue component.

Example output:

match R
nomatch G
match B

Use case 6: Produce machine-readable output

Code:

pnmpsnr path/to/file1.pnm path/to/file2.pnm -machine

Motivation:

Large-scale data analysis and automated pipelines often require data to be in a more accessible, programmatically interpretable format. Machine-readable output is ideal for logging data, feeding quality metrics into other tools or applications, or integrating into software that makes decisions based on PSNR values.

Explanation:

• -machine: Outputs data in a format that can be easily parsed by software tools, which is essential for machine-to-machine communication and processing.

Example output:

{ "PSNR": "30.27" }

Conclusion

The pnmpsnr command is a versatile tool for measuring image similarity that provides several options for analyzing image quality across various dimensions. From comparing color components to automating comparisons, it offers multiple use cases tailored to specific industry needs. Whether for research, quality control, or automated processing, pnmpsnr facilitates precise evaluation of image quality using PSNR as a reliable metric.