Merge pull request #35 from FarhadGhE/fixPinholeImageLengthDetection

[e57] fixed corrupted images when binary blob section header crossed CRC

Author: stefanmaierhofer

src/Aardvark.Data.E57/ASTM_E57.cs

@@ -2444,7 +2444,6 @@ internal static byte[] ReadLogicalBytes(Stream stream, E57PhysicalOffset start,
                 return buffer;
             }
         }
-
         /// <summary>
         /// Read given number of logical bytes (excluding 32-bit CRC at the end of each 1024 byte page) from stream,
         /// starting at logical stream position 'start'.
@@ -2544,15 +2543,18 @@ private static byte[] GetImageBytes(XElement root, string elementName, Stream st
             var element = GetElement(root, elementName);
             if (element == null) return null;
             var blob = E57Blob.Parse(GetElement(root, elementName));
-            // Mysterious!  Why is +16 required? 
-            var start = blob.FileOffset.Value + 16;
-            // If start falls in checksum region move it forward the the next page
-            if(start % 1024 >= 1020)
-            {
-                start += 1024 - (start % 1024);
-            }
-            var offest = new E57PhysicalOffset(start);
-            return ReadLogicalBytes(stream, offest, (int)blob.Length);            
+
+            // The blob.FileOffset is the physical offset to the start of a Blob
+            // binary section and not directly to the binary blob data.
+            // And such a section has a header of exactly 16 bytes.
+            // Because the header could cross a CRC (in which case we would have
+            // to skip 20 bytes instead of 16), we immediately convert the physical offset
+            // to a logical offset, and skip the 16 bytes header in "logical address space",
+            // which will automatically handle all CRC-crossing corner cases for us.
+            // Finally, ReadLogicalBytes already has an overload for E57LogicalOffset,
+            // so we don't even have to convert back to a physical offset.
+            var start = (E57LogicalOffset)blob.FileOffset + 16;
+            return ReadLogicalBytes(stream, start, (int)blob.Length);            
         }
         private static double? GetFloatOrInteger(XElement root, string elementName, bool required)
         {