When you create an image in a PDF file it is possible to specify that it is inverted. You do this by using the decodeArray. This consists of 2 values for each colour element – a minimum and a maximum. So for a gray colorspace you might see
decodeArray [0.0 1.0]
while CMYK would have
decodeArray [0.0 1.0 0.0 1.0 0.0 1.0 0.0 1.0]
It can also be defined as an 8 bit value ( decodeArray [0 254])
This allows control over the allowable values and out of range values will be set to the minimum or maximum. If you reverse these the image can be inverted (ie decodeArray [1.0 0.0]) which is a neat little trick. You can also do some simple image processing with it.
Where it can get slightly tricky is that you can also apply a decoder Array to a Mask on an image – I’ll leave you to figure this one out…
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!