Aren’t the blend modes supported just the Porter-Duff compositing modes? You might think that’s overkill, but it’s a really good mapping of what other rendering pipelines offer and it can really help reduce the work to produce a PDF.
The Porter-Duff operators are appealingly rigorous and easy to implement because they’re simply the possible combinations of a simple formula. But many of these operators are not very useful either.
The Photoshop blending modes are practically the opposite: they are not derived from anything mathematically appealing, it’s really just a collection of algorithms that Photoshop’s designers originally found useful. They reflect the limitations of their early 1990s desktop computer implementations (for example, no attempt is made to account for gamma correction when combining the layers, which makes many of these operations behave very differently from actual light that they mean to emulate).
