[poppler] Problem understanding Optional Content

[Brad Hards]

Bug 12661 (https://bugs.freedesktop.org/show_bug.cgi?id=12661) is about 
rendering layers that should be turned off.

OK, I've since read section 4.10 of the PDF specification, but I still don't 
understand exactly how the file is working.

I do see how we get from the document Catalog to the OCProperties, which 
layers are on or off by default, the absence of any radio buttons, the names 
of the layers, the order and structure of the layers. In fact, I think I 
understand almost everything except the for how an object is actually decided 
to be rendered or not.

 In particular, I don't see how any content (except for a few in the group for 
Object 25) actually gets turned on or off.

I do get this example:
33 0 obj<</Rect[51.0236 368.632 290.763 379.333]/Subtype/Link/BS<</S/S/W 
0/Type/Border>>/OC 25 0 R/A 76 0 R/H/N/Border[0 0 0]/Type/Annot>>endobj

I just don't see any other groups work - I only see the /OC property for obj 
25.


I see this part (which I take it says MCx is somehow a name for obj (19+x) ), 
but I can't see how the labels match up to anything:
31 0 obj<</CropBox[0.0 0.0 595.276 841.89]/Annots 32 0 R/Parent 13 0 
R/Contents[37 0 R 38 0 R 40 0 R 41 0 R 50 0 R 51 0 R 52 0 R 53 0 R]/Rotate 
0/BleedBox[0.0 0.0 595.276 841.89]/ArtBox[0.0 0.0 595.276 
841.89]/MediaBox[0.0 0.0 595.276 841.89]/TrimBox[0.0 0.0 595.276 
841.89]/Resources<</XObject<</Im0 56 0 R/Im1 56 0 R/Im2 56 0 R/Im3 56 0 
R>>/Shading<</Sh0 59 0 R/Sh1 63 0 R/Sh2 66 0 R>>/ColorSpace<</CS0 35 0 R/CS1 
35 0 R>>/Font<</T1_0 42 0 R/T1_1 43 0 R/T1_2 44 0 R/T1_3 45 0 
R>>/ProcSet[/PDF/Text/ImageB]/Properties<</MC0 19 0 R/MC1 20 0 R/MC2 21 0 
R/MC3<<>>/MC4 22 0 R/MC5 23 0 R/MC6 24 0 R/MC7<<>>/MC8 25 0 R/MC9 26 0 
R>>/ExtGState<</GS0 36 0 R/GS1 39 0 R/GS2 73 0 R/GS3 74 0 R/GS4 75 0 
R>>>>/Type/Page>>
endobj

Any suggestions?

Brad
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