I need to programmatically categorize which shape a pdf form field radiobutton has. Therefore I created a test pdf using *crobat. I added a radiobutton group where each widget is using a different style.
One way could be to check the CA key of the appearance characteristics dictionary (MK) which is mapped to the ZapfDingbats font:
/MK<</BC[0.0]>> //CIRCLE (normally l)
/MK<</BC[0.0]/CA(4)>> //CHECK
/MK<</BC[0.0]/CA(8)>> //CROSS
/MK<</BC[0.0]/CA(u)>> //DIAMOND
/MK<</BC[0.0]/CA(n)>> //SQUARE
/MK<</BC[0.0]/CA(H)>> //STAR
However in the example PDF for the circle the CA key does not exist (it should have been /CA(l)). To implicitly assume a round shape does not seem correct.
Another idea would be to look at the appearance dictionary. For the example given in the pdf spec it seems possible:
stream
q
0 0 1 rg
BT
/ZaDb 12 Tf
0 0 Td
(l) Tj
ET
Q
endstream
However the normal appearance generated by *crobat looks like that:
stream
q
1 0 0 1 9 9 cm
8.5 0 m
8.5 4.6946 4.6946 8.5 0 8.5 c
-4.6946 8.5 -8.5 4.6946 -8.5 0 c
-8.5 -4.6946 -4.6946 -8.5 0 -8.5 c
4.6946 -8.5 8.5 -4.6946 8.5 0 c
s
Q
0.501953 G
q
0.7071 0.7071 -0.7071 0.7071 9 9 cm
7.5 0 m
7.5 4.1423 4.1423 7.5 0 7.5 c
-4.1423 7.5 -7.5 4.1423 -7.5 0 c
S
Q
0.75293 G
q
0.7071 0.7071 -0.7071 0.7071 9 9 cm
-7.5 0 m
-7.5 -4.1423 -4.1423 -7.5 0 -7.5 c
4.1423 -7.5 7.5 -4.1423 7.5 0 c
S
Q
q
1 0 0 1 9 9 cm
3.5 0 m
3.5 1.9331 1.9331 3.5 0 3.5 c
-1.9331 3.5 -3.5 1.9331 -3.5 0 c
-3.5 -1.9331 -1.9331 -3.5 0 -3.5 c
1.9331 -3.5 3.5 -1.9331 3.5 0 c
f
Q
endstream
My question: Is there a way to detect that a widget annotation has a round shape / circular style? I know that any arbitrary shape can be defined as an appearance however for the use case at hand the differentiation of those 6 styles is more than enough.
If the answer somehow depends on the pdf lib (due to certain functionality): currently openPDF is used and other libs like pdfbox or iText are in use, too.
First you can check if the /CA entry is 'l'. If /CA does not exist you can check if the appearance stream contains 'c', 'v', or 'y' operators (curve operators). If they are present you can assume a circular style.
This is a empiric approach but it might work for you situation.