How to use the command pnmcolormap (with examples)

The pnmcolormap command is used to create a quantization color map for a PNM (Portable aNyMap) image. It allows you to generate an output image using a specified number of colors or less, which closely resembles the input image. The command provides additional options such as using the splitspread strategy for determining output colors, as well as sorting the resulting colormap.

Use case 1: Generate an image using only n_colors or less colors

Code:

pnmcolormap n_colors path/to/input.pnm > path/to/output.ppm

MOTIVATION: When working with images, reducing the number of colors can help reduce the file size and improve processing time. This can be useful when displaying images on low-bandwidth networks or on devices with limited capabilities. The n_colors argument allows you to specify the maximum number of colors to be used in the output image.

EXPLANATION:

• n_colors: The number of colors to be used in the output image. The command will generate a color map with n_colors or fewer colors closest to the input image.
• path/to/input.pnm: The path to the input PNM image file.
• > path/to/output.ppm: Redirects the output of the command to the specified path, creating a new PPM (Portable Pixel Map) image file.

Example output: A PPM image file (output.ppm) will be generated with n_colors or fewer colors, resembling the input image.

Use case 2: Use the splitspread strategy for determining the output colors

Code:

pnmcolormap -splitspread n_colors path/to/input.pnm > path/to/output.ppm

MOTIVATION: The splitspread strategy can be used when dealing with images that have small details, such as textures or fine patterns. By using this strategy, the pnmcolormap command can produce a better result by preserving more details in the output image, compared to the default strategy.

EXPLANATION:

• -splitspread: An optional flag that enables the use of the splitspread strategy for determining the output colors.
• n_colors: The number of colors to be used in the output image.
• path/to/input.pnm: The path to the input PNM image file.
• > path/to/output.ppm: Redirects the output of the command to the specified path, creating a new PPM image file.

Example output: A PPM image file (output.ppm) will be generated using the splitspread strategy to determine n_colors or fewer colors that closely resemble the input image, preserving small details.

Use case 3: Sort the resulting colormap

Code:

pnmcolormap -sort path/to/input.pnm > path/to/output.ppm

MOTIVATION: Sorting the resulting colormap is useful when you want to compare different colormaps. By sorting the colormap, you can easily identify any changes between different versions or variations of an image. This can be helpful for tasks such as color analysis or evaluation of different image processing techniques.

EXPLANATION:

• -sort: An optional flag that enables the sorting of the resulting colormap.
• path/to/input.pnm: The path to the input PNM image file.
• > path/to/output.ppm: Redirects the output of the command to the specified path, creating a new PPM image file.

Example output: A PPM image file (output.ppm) will be generated using the default strategy to determine the colors that closely resemble the input image, with the colormap sorted for easier comparison with other colormaps.

Conclusion:

The pnmcolormap command is a powerful tool for creating quantization color maps for PNM images. By specifying the number of colors and using different strategies, you can generate output images that closely resemble the input while reducing file size and preserving fine details. The ability to sort the resulting colormap adds an additional level of functionality, allowing for easy comparison and analysis of different colormaps.