consistent checksums...

john_P37

I sometimes look at checksums with datafiles to track if I have the right files.  When I use Onshape to output two PDF files, one right after the other, the checksums are different.  Without being an expert on checksums, this doesn't seem correct.  Shouldn't those files represent exactly the same information?
thanks, John

NeilCooke

The PDFs are created on-the-fly so they are not the same file. I’m not an expert on checksums so ignore me if I’m off base. 

jakeramsley

Hi john_P37,

A PDF is a binary file that Adobe has written their own encoding for.  I exported the same drawing twice, converted to hex and made a diff:

>>>
  0004d510: 3020 6f62 6a0d 0a3c 3c0d 0a2f 5072 6f64&nbsp; 0 obj..<<../Prod
  0004d520: 7563 6572 2028 4444 5420 5044 4620 4578&nbsp; ucer (DDT PDF Ex
  0004d530: 706f 7274 2076 332e 382e 312e 3020 2876&nbsp; port v3.8.1.0 (v
  0004d540: 332e 382e 312e 3029 290d 0a2f 4372 6561&nbsp; 3.8.1.0))../Crea
  0004d550: 7469 6f6e 4461 7465 2028 443a 3230 3139&nbsp; tionDate (D:2019
+ 0004d560: 3038 3234 3033 3435 3336 290d 0a3e 3e0d&nbsp; 0824034536)..>>.
<<<
  0004d510: 3020 6f62 6a0d 0a3c 3c0d 0a2f 5072 6f64  0 obj..<<../Prod
  0004d520: 7563 6572 2028 4444 5420 5044 4620 4578  ucer (DDT PDF Ex
  0004d530: 706f 7274 2076 332e 382e 312e 3020 2876  port v3.8.1.0 (v
  0004d540: 332e 382e 312e 3029 290d 0a2f 4372 6561  3.8.1.0))../Crea
  0004d550: 7469 6f6e 4461 7465 2028 443a 3230 3139  tionDate (D:2019
- 0004d560: 3038 3234 3033 3436 3133 290d 0a3e 3e0d  0824034613)..>>.

There is a single line that is different, comparing 0824034536 to 0824034613 (which are time stamps when combined with the `2019` on the line before it)

I included a bit more to make it more obvious why it is different.

This is because each time you export, you are generating a new file.  If you export once, store in a tab, and then download it then you are referencing the same file.