Encoding format used by GraphicsData?

Question

I am trying to extract some graphics stored in PICT format from a Mathematica notebook, using a platform that doesn't support PICT. If I look at the .nb file in a plain text editor, or if I use FullForm, then I can see that the picture is stored as a GraphicsData head with a text string encoding the picture. It doesn't appear that the GraphicsData head uses Base64 or any other binary-to-text encoding that I know of off the top of my head, but I would like to be able to copy-and-paste the GraphicsData string and decode it manually. Is this possible? Does GraphicsData use a well-known encoding scheme?

Answer 1

I found an example on the web. Here is code that will convert the PICT data from the format stored in the notebook file into a .pict file that can be opened by an image viewer (e.g. Photoshop).

DecodePICT[data_String] := Module[
    {slash, backslash, zero, LF, CR, decode, codes, len, i},
    {slash, backslash, zero, LF, CR} = ToCharacterCode["/\\0\n\r"];
    decode[char_] := If[char == slash, backslash-zero, char-zero];
    len = Length[codes = ToCharacterCode[data]];
    i = 1;
    Join[Table[0,{512}], Last@Last@Reap@While[i <= len-1,
        Which[
            codes[[i]] == LF || codes[[i]] == CR,
            i++,
            codes[[i]] == backslash,
            i += 4,
            True,
            Sow@BitAnd[BitOr[
                BitShiftLeft[decode[codes[[i]]], 2],
                BitShiftRight[decode[codes[[i+1]]], 4]
            ], 255];
            i++;
            If[i <= len-1,
                Sow@BitAnd[BitOr[
                    BitShiftLeft[decode[codes[[i]]], 4],
                    BitShiftRight[decode[codes[[i+1]]], 2]
                ], 255]
            ];
            i++;
            If[i <= len-1,
                Sow@BitAnd[BitOr[
                    BitShiftLeft[decode[codes[[i]]], 6],
                    BitShiftRight[decode[codes[[i+1]]], 0]
                ], 255]
            ];
            i += 2;
        ]
    ]]
];

str = "0N801`0]05815@0A0_l<0?ooool0;@0000L0004E0000DP000000002Q0O@0hd=U
  K6aK8U`lG0eSKgEbHVEKM5mM83Xm80eLM51QLV5]IGAbJF=@K6md<dAKN`eLM2Pb
  :b1d84=_Le]fGBU3Kg=K<WIM;0eLM2Pb:b1d84=_Le]fGBUCJFiK<WIM;0eLM7@P
  DfU^FgIMOBakMR`P<2`P<R1@JGeM83]L3E`n8R`P8TU^L7Ed8R`=8219KF5WIE=Y
  NVD]?W/b<c8/83LeOB`=8219KF5WIDeQLVMYKW<]?W]k<2`P<7d/87/`;20`OGd/
  3B0PBFeQIfEBIFMYKfh]?W]k<2`P<Gd/87/`;20aOGeM000N01[oooooool00@0:
  00L0;@1B0AD0<@0602d0DP9203401`920582@P0J0000000002`01@R^0Te30003
  2:h0104000d0300^00@0o`0002/]4PiSKgEbHVEKM5mM83Xm8000:b0?4U1QLV5]
  IGAbJF=@K6md<dAKN`00:PlE:38[87@P@fmcFgIM:D=_Le/bMUd/000Z3aDX<R/P
  M213Kg=KMUdYDfU^Fc9fGB`002X?6G@PDfU^FgIMOBakMR`P<2`P<R1@JGeM83/0
  0?l";

Export["~/Desktop/foo.pict", DecodePICT[str], "Binary"]

Answer 2

This is data compressed using Compress. You can uncompress it using Uncompress.

Demonstration:

Rasterize["x", ImageSize -> 25]

Open this cell with Cell>Show Expression (ctrlshift-E):

Cell[BoxData[
 GraphicsBox[RasterBox[CompressedData["
1:eJxTTMoPSmJmYGDQA2IpIAax/4+CUTAKhgbYlsaAHaRtI0MhdU1DVWo1gRT/
YJpDsoFEGEWsgROsiDQKVT02A/FIkWogeUZh1UuJUWgmUG4UioHUMArFNKIj
EQdApBZSUwVOo2AGUGAg9sROdBYgVhOJBhJWTnImJaQQET238crjL7aIU0Vt
00bBKBgFo2DgAQD6B2Cc
   "], {{0, 0}, {7.284046692607004, 12.88715953307393}}, {0, 255},
   ColorFunction->RGBColor],
  ImageSize->25,
  PlotRange->{{0, 7.284046692607004}, {0, 12.88715953307393}}]], "Output",
 CellChangeTimes->{{3.546020312347924*^9, 3.546020338348411*^9}}]

Copy the compressed string and uncompress:

Uncompress["
  1:eJxTTMoPSmJmYGDQA2IpIAax/4+CUTAKhgbYlsaAHaRtI0MhdU1DVWo1gRT/
  YJpDsoFEGEWsgROsiDQKVT02A/FIkWogeUZh1UuJUWgmUG4UioHUMArFNKIj
  EQdApBZSUwVOo2AGUGAg9sROdBYgVhOJBhJWTnImJaQQET238crjL7aIU0Vt
  00bBKBgFo2DgAQD6B2Cc
     "] // Image