1. Lossless rotation and related transforms

New (08-Feb-2005):
An article about the jpegtran lossless transformation functions has been published in the German c't magazine 4/2005 (released 07-Feb-2005), title "Fotoschoner", page 188 ff. The article derives and describes the lossless transformation functions in a clear and understandable form.
My draft of the article (in German) is available here:
Verlustfreie_JPEG_Drehung.doc or Verlustfreie_JPEG_Drehung.pdf .
Note that the draft is quite different from the published version, but the essence is the same.

An Exif Patch has been added to the lossless transformation code to avoid some application problems.
It is part of the release-candidate croppatch to jpegtran (see below).

A new "-perfect" switch has recently been added to the lossless transformation support in jpegtran:

    -perfect       Fail if there is non-transformable edge blocks

    jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg

    /* jtransform_perfect_transform
     *
     * Determine whether lossless transformation is perfectly
     * possible for a specified image and transformation.
     *
     * Inputs:
     *   image_width, image_height: source image dimensions.
     *   MCU_width, MCU_height: pixel dimensions of MCU.
     *   transform: transformation identifier.
     * Parameter sources from initialized jpeg_struct
     * (after reading source header):
     *   image_width = cinfo.image_width
     *   image_height = cinfo.image_height
     *   MCU_width = cinfo.max_h_samp_factor * DCTSIZE
     *   MCU_height = cinfo.max_v_samp_factor * DCTSIZE
     * Result:
     *   TRUE = perfect transformation possible
     *   FALSE = perfect transformation not possible
     *           (may use custom action then)
     */

    GLOBAL(boolean)
    jtransform_perfect_transform(JDIMENSION image_width, JDIMENSION image_height,
                                 int MCU_width, int MCU_height,
                                 JXFORM_CODE transform)
    {
      boolean result = TRUE; /* initialize TRUE */

      switch (transform) {
      case JXFORM_FLIP_H:
      case JXFORM_ROT_270:
        if (image_width % (JDIMENSION) MCU_width)
          result = FALSE;
        break;
      case JXFORM_FLIP_V:
      case JXFORM_ROT_90:
        if (image_height % (JDIMENSION) MCU_height)
          result = FALSE;
        break;
      case JXFORM_TRANSVERSE:
      case JXFORM_ROT_180:
        if (image_width % (JDIMENSION) MCU_width)
          result = FALSE;
        if (image_height % (JDIMENSION) MCU_height)
          result = FALSE;
        break;
      }

      return result;
    }

2. Lossless crop 'n' drop (cut & paste)

• Ben Jackson's lossless cropping with jpegtran
  Initial coding of the cropping feature.
• croppatch.tar.gz
  Stable jpegtran version (source code patch & Linux binary) with the crop feature:

      -crop WxH+X+Y  Crop to a rectangular subarea
  

  Windows binary compilation: jpegtran.exe or packed jpegtran.zip.
  14-Jan-2001:
  • The semantics for combining cropping and transformation has been changed!
    The crop parameters are now related to the transformed destination image, not the source!
  • Exif patch included.
  16-Jan-2001: Adjusted source code comments to reflect the changes.
  20-Jan-2001: Improved operation when specifying same input and output file.
  21-Sep-2003: Added "-perfect" switch.
  Todo: Adjust the feature description.
• jpegjoin.zip (including jpegjoin.exe, jpegjoin.txt, jpegjoin.inf, and drop logo and legend files)
  Jpegjoin Windows Application using the new crop 'n' drop feature set.

  J	P	E	G
  j	o	i	n

  Here is a sample output image and an equivalent shell script producing the same image using one temporary file. This is another sample output image.
• droppatch.tar.gz
  Stable jpegtran version (source code patch & Linux binary) with crop 'n' drop:

      -crop WxH+X+Y  Crop to a rectangular subarea
      -drop +X+Y filename          Drop another image
  

  Windows binary compilation: jpegtran.exe or packed jpegtran.zip.
  See my initial article in the IJG mailing list for important notes!
  24-Mar-2000:
  • The crop extension code has been fixed.
  • The drop memory allocation issue has been solved.
  • The drop code has been fixed.
  25-Mar-2000: The drop quantization adjustment has been finished:
  • Element-wise and largest-common-denominator adjustment in standard mode.
  • Requantizing from drop to source in "-trim" mode.
  This completes the code patch!
  01-May-2000: The semantics for combining cropping and transformation has been changed!
  The crop parameters are now related to the transformed destination image, not the source!
  14-Jan-2001: Exif patch included.
  16-Jan-2001: Adjusted source code comments to reflect the changes.
  20-Jan-2001: Improved operation when specifying same input and output file.
  21-Sep-2003: Added "-perfect" switch.
  Todo: Adjust the feature description.