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Full Name	JPEG File Interchange Format
Format ID	ImGearFormats.JPG
File Extension(s)	*.ipg, *jpeg
Data Type	Raster Image
Data Encoding	Binary
IG Multipage Support	No
IG Alpha Channel Support	Read/write
IG Platforms Support	WIN32, WIN64, Linux32, Linux64, MAC32, MAC64

ImageGear Supported Versions

• Version 1.01 - 1991
• Version 1.02 - Added ability for thumbnails to be color-mapped and JPEG compressed.

ImageGear Supported Features

• DETECTSUPPORT - autodetection
• PAGEREADSUPPORT - single page file reading
• PAGEINSERTSUPPORT - single-page file writing

ImageGear Read Support

• ImGearCompressions.JPEG (Lossy):
• • Grayscale: 8, 12 bpp;
  • RGB: 24, 36 bpp;
  • CMYK: 32 bpp;
  • RGB + Alpha: 32 bpp
• ImGearCompressions.JPEG (Lossless):
• • Grayscale: 8, 16 bpp;
  • RGB: 24, 48 bpp;
  • CMYK: 32 bpp;
  • RGB + Alpha: 32 bpp
• ImGearCompressions.JPEG (Progressive JPEG):
• • Grayscale: 8, 12 bpp;
  • RGB: 24 bpp;
  • CMYK: 32 bpp;
  • RGB + Alpha: 32 bpp

ImageGear Write Support

• ImGearCompressions.JPEG (Lossy):
• • Grayscale: 8, 12 bpp;
  • RGB: 24, 36 bpp;
  • RGB + Alpha: 32 bpp (RGBA saving is disabled by default. Set AllowRGBASaving control parameter to true to enable it.)
  • CMYK: 32 bpp2 (CMYK saving is enabled by default. You can disable it by setting AllowCMYKSaving control parameter to false.)
• ImGearCompressions.JPEG (Lossless):
• • Grayscale: 8, 16 bpp;
  • RGB: 24 bpp;

ImageGear Filter Control Parameters

Filter Control Parameter	Type	Default Value	Available Values	Description
AllowLossy12bpcSaving	boolean	true	true, false	Controls the saving to Lossy JPEG format. If true, saving of 12 bits per channel Lossy JPEG is allowed. This is the maximum channel depth supported by Lossy JPEG format. If source image's depth is more than 8 bits per channel, it will be saved to a 12 bpc JPEG. For example, 48-bit RGB image will be saved to 36-bit JPEG. Note that many viewers do not support 12 bpc JPEG. If False, all images are saved to 8 bpc JPEG, regardless of their channel depth.
AllowRGBASaving	boolean	true	true, false	Set this parameter to true to enable RGBA JPEG saving. Note, that there is no official standard for RGBA JPEG. This format is recognized as CMYK rather than RGBA by most third-party software.
Quality	int	70	Possible value in range [1,100]	Gets/Sets quality value for Lossy and Progressive JPEG compression. See See JPEG Quality. Section for details.
DecimationType	ImGearDecimationTypes Enumeration	Decim_2x2_1x1_1x1	ImGearDecimationTypes Enumeration values	Get/Set decimation value. For all available values see the ImGearDecimationTypes Enumeration.
Predictor	int	1		Gets/Sets predictor for lossless JPEG. See Lossless JPEG Predictor Settings section for details.
SaveType	int	0 (Lossy compression)	0 (Lossy compression), 1 (Lossless compression), 2 (Progressive compression)	Get/Set type for output JPEG format. See Loading and Saving JPEG-Compressed Images. Section for details.
AllowCMYKSaving	boolean	true	true, false	Set to true (default) to allow saving JPEG CMYK images. Set to false to save CMYK images to JPEG as RGB.
SaveEXIFMetadata	boolean	false	true, false	If true, EXIF metadata is saved.
SaveExistingEXIFMetadata	boolean	true	true, false	This parameter controls the saving of existing EXIF metadata. By default (SaveEXIFMetadata = False, SaveExistingEXIFMetadata = True), ImageGear saves EXIF metadata if it exists in the source page, and does not save EXIF metadata if it does not exist in the source page.

SaveExistingEXIFMetadata works in combination with SaveEXIFMetadata parameter, as shown in the table below:

SaveEXIFMetadata	SaveExistingEXIFMetadata	Source Page has EXIF Metadata	ImageGear Action
False	False	No	Does not save EXIF metadata
False	False	Yes	Does not save EXIF metadata
False	True	No	Does not save EXIF metadata
False	True	Yes	Saves existing EXIF metadata
True	False	No	Saves default EXIF metadata
True	False	Yes	Ignores existing and saves default EXIF metadata
True	True	No	Saves default EXIF metadata
True	True	Yes	Saves existing EXIF metadata

Comments

JPEG is normally associated with the JPEG compression scheme, but it is also implemented into the JFIF file format. This format was developed to store JPEG-encoded data, and to exchange it between applications or operating systems that are normally incompatible.

The header of the JFIF contains the version number, the image dots per inch (DPI), or dots per centimeter, and an optional thumbnail (miniature) RGB representation of the main image. Version 1.02 handles thumbnails differently by storing them separately, rather than in the identification marker of the header.

The raw JPEG data is surrounded by two markers, an "SOI" (start of image) marker, and an "EOI" (end of image) marker. See JPEG Compression (ImageGear Supported Compressions Reference) for more about JPEG compression.

JFIF is considered a non-proprietary file format. Many proprietary file formats contain JPEG data, incorporating their own application-specific data structures. Another non-proprietary format that uses JPEG-encoded data is TIFF file format, version 6.0.

Loading and Saving JPEG-Compressed Images

ImageGear supports the reading and writing of three types of JPEG compression: baseline JPEG (Lossy), Progressive JPEG, and Lossless JPEG. When you load a JPEG-compressed file, ImageGear detects the type of JPEG compression and decompress the image automatically. But if you want to save an image with a JPEG compression scheme other than baseline JPEG, you must use the "SaveType" parameter to specify the type of JPEG compression.

JPEG Thumbnails

The JPEG format can store thumbnails, which are small representations of the original image. These images are stored in uncompressed form and can significantly decrease your overall compression ratio. (Uncompressed thumbnails sometimes occupy more space than the original JPEG image when compressed). Use this option carefully.

Lossless JPEG Predictor Settings

The Lossless JPEG scheme is "predictive" in nature--it uses the values of surrounding pixels in addition to the value of the original pixel to calculate a predictor value, which it then subtracts from the value of the original pixel. The resulting pixel value will be reduced such that it can be compressed more than the original value. The higher the number of neighboring pixels used, the higher the compression will be.

Regardless of the predictor value setting, the quality of the image will remain the same. The difference is that if you choose to optimize for space by setting a high predictor value, you will have to give up some speed, as the decompression will take longer to perform.

ImageGear lets you set the predictor value using Predictor control parameter. The ImageGear default for this setting is 1. The allowed range is 1--7. The graphic below shows a predictor (x) and three reconstructed samples (a,b,c) immediately to the left, immediately above, and diagonally to the left of the current sample.

 

Lossless JPEG does not apply DCT for an image as per the Lossy JPEG compression. Instead, it uses a DPCM difference coding, which can be carried out with any one of seven different prediction modes. Correspondingly, the Predictor control parameter can be set to a value between 0 and 7. In Table 4, you will see what algorithm your setting of 1--7 will use, where Pr Px is the predictor, and Ra, Rb, and Rc are the reconstructed samples:

Value	JPEG DIS Prediction
0	no prediction
1	Px = Ra
2	Px = Rb
3	Px = Rc
4	Px = Ra + Rb - Rc
5	Px = Ra + ((Rb-Rc)/2)
6	Px = Rb + ((Ra-Rc)/2)
7	Px = (Ra + Rb)/2

JPEG Quality

The baseline JPEG specification calls for a quality setting. The lower the setting, the greater the number of original pixels lost, and therefore the smaller the resulting compressed file will be. ImageGear lets you set the quality of compression with values of type INT between 1-100, where 100 provides the highest retention of original pixel values. A setting of 100 does not mean that the image includes 100% of all original pixel values. With Lossy JPEG, there is no such thing as "no loss". Control parameter for setting Lossy JPEG quality is Quality.

References Used

Brown, C. Wayne, and Barry J. Shepherd. Graphics File Formats: Reference and Guide. Greenwich, CT.: Manning Publications, 1992.

Kay, David C. and John R. Levine. Graphics File Formats. Windcrest Books, 1992.

Murray, James D. and William vanRyper. Encyclopedia of Graphics File Formats. Sebastopol, CA: O'Reilly & Associates, Inc., 1994.

 

 

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