internal static void FindNextSegment(Stream jpegStream, byte[] headerBuffer)
{
/*
* Well I'll be.
* Really this is no longer a valid jpeg at all.
* The last segment size was incorrect as we should have found a 0xff for the next segment but we did not.
* We could have jumped anywhere in the file. Could still be in the same segment or jumped into the middle of another segment.
* We really have no idea where we are!
* If it was up to me we would just throw invalid jpeg here.
*
* However, libjpeg and most other jpeg parsing libraries do not throw.
* So, in order to keep compatibly with them we do what they do. We go byte by byte looking for 0xff.
*
* This results in a silent failure. We ignore and throw anyway the data we are reading and/or missed.
*/
try
{
/*
* We read 4 bytes at a time skiping over any non-FF bytes.
* This code is only used we the jpeg stream is invalid so not performance critical.
*/
int i = 0;
while (headerBuffer[i] != Jpeg.SectionStartMarker)
{
++i;
if (i == headerBuffer.Length)
{
Jpeg.CheckedRead(jpegStream, headerBuffer, 0);
i = 0;
}
}
if (i != 0)
{
// found the marker in the data we have move the data down the buffer
Buffer.BlockCopy(headerBuffer, i, headerBuffer, 0, headerBuffer.Length - i);
// fill the rest of the buffer with data from the stream
Jpeg.CheckedRead(jpegStream, headerBuffer, headerBuffer.Length - i);
}
}
catch (StreamTooSmallException ex)
{
// we shit we ran into the end of the stream, we could not recover from the bad jump
throw new BadSegmentSizeException(ex);
}
}
private static void ThrowIfNotJpeg(Stream jpegStream)
{
if (jpegStream == null)
{
throw new ArgumentNullException(nameof(jpegStream));
}
byte[] buffer = new byte[2];
Jpeg.CheckedRead(jpegStream, buffer);
if (buffer[0] != Jpeg.SectionStartMarker || buffer[1] != Markers.FileMarker)
{
// jpegs have to start with 0xff, 0xd8 because them the rules!
throw new InvalidJpegException("Invalided JPEG/JFIF. Does not start with SOI marker.");
}
}
/// <summary>
/// Reads the passed stream and returns the jpeg image dimensions in pixels.
/// It stops reading the stream when the dimensions are found.
/// </summary>
/// <param name="jpegStream"></param>
/// <exception cref="InvalidJpegException">This exception is thrown when the passed stream does not conform to the jpeg standard.</exception>
/// <exception cref="StreamTooSmallException">This exception is thrown when the passed stream does not contain the required jpeg headers before the end of the stream.</exception>
/// <exception cref="BadSegmentSizeException">This exception is thrown when an attempt to recover from a bad segment size fails due to not finding a section marker before the end of the stream.</exception>
/// <returns>the image dimensions in pixels</returns>
public static Dimensions GetDimensions(Stream jpegStream)
{
Jpeg.ThrowIfNotJpeg(jpegStream);
Dimensions imageDetails = new Dimensions();
byte[] headerBuffer = new byte[4];
do
{
Jpeg.CheckedRead(jpegStream, headerBuffer);
if (headerBuffer[0] != Jpeg.SectionStartMarker)
{
Jpeg.FindNextSegment(jpegStream, headerBuffer);
}
ushort length = Jpeg.ReadLength(headerBuffer, 2);
//TODO make this a seek if we do not need to read the data
byte[] headerData = new byte[length - 2];
Jpeg.CheckedRead(jpegStream, headerData);
if (headerBuffer[1] == Markers.SOF0 || headerBuffer[1] == Markers.SOF2)
{
imageDetails = new Dimensions(headerData);
}
}while (headerBuffer[1] != Markers.SOS && imageDetails.Height == 0);
if (imageDetails.Height == 0)
{
// because it has no Start Of Frame header
throw new InvalidJpegException("Failed to find Start Of Frame header.");
}
return(imageDetails);
}
private static void CheckedRead(Stream jpegStream, byte[] buffer)
{
Jpeg.CheckedRead(jpegStream, buffer, 0);
}