Understanding the PDF file Format – PDF image color depth

This article is all about color in PDF images, and exactly what the depth of an image means in practice.

All PDF images will specify a width, a height, a ColorSpace and the number of bits per color component (a color depth). This is the number of bits used to store each color component for each pixel. It is usually 8 bits (ie a byte), 1 bit or sometimes 2 or 4 bits. I have not seen an odd value or a number greater that 8. If the number is less that 8, the bits are packed together into pixels (so a single byte would contain 2 values).

To get a real indication of the total number amount of memory used, you also need to look at the type of color (the PDF ColorSpace). An RGB image will have 3 ‘channels’ (Red, Green and Blue) so an 8 bit RGB pixel needs 24 bits (3 bytes) in total. A CMYK 8 bit image would need 4 bytes.

Some Colorspaces only define one channel so an 8 bit Grayscale image needs 1 byte per pixel.

Indexed, Separation and DeviceN colorspaces work slightly differently. They still have a set number of pixels but this specifies the number of ‘palete colors’ you can choose from. So a 1 bit indexed or Separation colorspace PDF image can only contain TWO colors, but they can be any TWO colors defined by the user.  They are very useful if you want to use a limited number of colors and reduce the size of the image data.

Now that you understand exactly how PDF image color depth works you can see how to interpret it and also how you can choose different ColorSpaces to optimise your images in different scenarios.

This post is part of our “Understanding the PDF File Format” series. In each article, we aim to take a specific PDF feature and explain it in simple terms. If you wish to learn more about PDF, we have 13 years worth of PDF knowledge and tips, so click here to visit our series index!

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Mark Stephens

System Architect and Lead Developer at IDRSolutions
Mark Stephens has been working with Java and PDF since 1999 and has diversified into HTML5, SVG and JavaFX. He also enjoys speaking at conferences and has been a Speaker at user groups, Business of Software, Seybold and JavaOne conferences. He has a very dry sense of humor and an MA in Medieval History for which he has not yet found a practical use.
Markee174

About Mark Stephens

Mark Stephens has been working with Java and PDF since 1999 and has diversified into HTML5, SVG and JavaFX.

He also enjoys speaking at conferences and has been a Speaker at user groups, Business of Software, Seybold and JavaOne conferences. He has a very dry sense of humor and an MA in Medieval History for which he has not yet found a practical use.

2 thoughts on “Understanding the PDF file Format – PDF image color depth

  1. Jacob

    Hi,
    I want to detect the images in a pdf file and get the pixel color of the images. How it can be done??

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