How to Use the 'pnmcrop' Command (with Examples)

pnmcrop is a versatile command-line utility from the Netpbm toolkit used for cropping PNM (Portable Any Map) images, which include formats like PBM, PGM, and PPM. This tool is beneficial when you need to trim unnecessary borders from images, often common in scanner outputs and various image processing tasks. With pnmcrop, you can automatically detect and remove borders of specific colors, making it a valuable asset in batch image preprocessing.

Use case 1: Remove white borders on a PNM image

Code:

pnmcrop -white path/to/image.pnm > path/to/output.pnm

Motivation:

In digital image processing, particularly when scanning documents or converting images, unwanted white borders may be introduced. These borders can interfere with further processing tasks such as image segmentation or optical character recognition (OCR). Removing them ensures that subsequent operations are applied only to the actual content of the image, increasing efficiency and accuracy.

Explanation:

• -white: This option instructs pnmcrop to remove contiguous white areas from the edges of the image. The tool will continue cropping until a non-white pixel is encountered.
• path/to/image.pnm: This is the path to the input image file from which white borders are to be removed.
• >: This redirects the output from pnmcrop to a new file.
• path/to/output.pnm: Specifies the path where the cropped image will be saved.

Example output:

After running the command, the output file path/to/output.pnm will contain the original image with all white borders removed, presenting a cleaner and more focused representation of the subject matter.

Use case 2: Remove borders of the specified color on the top and left side of the image

Code:

pnmcrop -bg-color color -top -left path/to/image.pnm > path/to/output.pnm

Motivation:

Sometimes, images have undesired colored borders rather than white ones—this might occur in artwork or graphical designs with frames. You often only need to target specific borders for removal, such as those on the top or left, to maintain certain image constraints or avoid cutting into vital parts of the image.

Explanation:

• -bg-color color: Specifies the border color to be removed. Replace color with the desired color value or name.
• -top: Indicates that only the border at the top should be removed.
• -left: Indicates that only the border on the left should be removed.
• path/to/image.pnm: Path to the source PNM image.
• path/to/output.pnm: Destination path for the cropped image.

Example output:

The resulting path/to/output.pnm will show the original image now devoid of its top and left colored borders, while other sides remain untouched, preserving your framing choices.

Use case 3: Determine the color of the borders to be removed by the color of the pixel in the specified corner

Code:

pnmcrop -bg-corner topleft|topright|bottomleft|bottomright path/to/image.pnm > path/to/output.pnm

Motivation:

When dealing with images of varying border colors, manually specifying the border color each time can be inefficient. By allowing the corner pixel’s color to determine the border to remove, pnmcrop streamlines the cropping process, particularly in batch processing or scripts.

Explanation:

• -bg-corner: This option directs pnmcrop to use the color of a corner pixel as the border color for cropping. Choose from topleft, topright, bottomleft, or bottomright.
• path/to/image.pnm: File path of the input image.
• path/to/output.pnm: Defines the output path for the cropped image.

Example output:

Using this command, path/to/output.pnm will show the image free of borders that matched the color of the selected corner pixel, enhancing consistency across a series of differently bordered images.

Use case 4: Leave a border with a width of n pixels and specify behavior if the image is entirely made out of the background

Code:

pnmcrop -margins n -blank-image pass|minimize|maxcrop path/to/image.pnm > path/to/output.pnm

Motivation:

There are scenarios where entirely removing all boundaries might be undesirable, like when a slight margin is necessary for visual framing or to meet print specifications. Moreover, when processing images that might not contain any distinguishable content, having options for handling such cases becomes crucial to avoid errors.

Explanation:

• -margins n: Leaves a specified number of n pixels as a margin around the cropped image. Adjust n for the desired border thickness.
• -blank-image pass|minimize|maxcrop: Defines the treatment of images that are completely border-colored. pass outputs the original image, minimize retains one pixel of image data, and maxcrop removes all content, producing a one-pixel image.
• path/to/image.pnm: Path to the input file you wish to crop.
• path/to/output.pnm: Path for saving the modified image.

Example output:

Once executed, the resulting file will maintain a border of n pixels and adhere to the specified behavior for an entirely border-colored image. This ensures an image ready for various display or printing requirements.

Conclusion:

The pnmcrop command is an essential tool for anyone dealing with automated image processing, offering numerous options for managing and customizing the boundaries of PNM images effectively. By utilizing different arguments and switches, users can tailor their cropping needs to meet specific requirements, whether it involves removing unwanted borders, retaining necessary margins, or optimizing batch processing tasks.