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https://github.com/mozilla/mozjpeg

Improved JPEG encoder.
https://github.com/mozilla/mozjpeg

image-optimization jpeg-encoder libjpeg mozjpeg web-performance

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Improved JPEG encoder.

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README 

        
Mozilla JPEG Encoder Project

============================



mozjpeg is a fork of libjpeg-turbo that aims to speed up load times of web

pages by reducing the size (and, by extension, the transmission time) of JPEG

files.  It accomplishes this by enabling optimized Huffman trees and

progressive entropy coding by default in the JPEG compressor, as well as

splitting the spectrum of DCT coefficients into separate scans and using

Trellis quantisation.



Although it is based on libjpeg-turbo, mozjpeg is not intended to be a

general-purpose or high-performance JPEG library.  Its performance is highly

"asymmetric".  That is, the JPEG files it generates require much more time to

compress than to decompress.  When the default settings are used, mozjpeg is

considerably slower than libjpeg-turbo or even libjpeg at compressing images.

Thus, it is not generally suitable for real-time compression.  It is best used

as part of a web encoding workflow.



libjpeg API Extensibility Framework

===================================



mozjpeg's implementation of the libjpeg API includes an extensibility framework

that allows new features to be added without modifying the transparent libjpeg

compress/decompress structures (which would break backward ABI compatibility.)

Extension parameters are placed into the opaque jpeg_comp_master structure, and

a set of accessor functions and globally unique tokens allows for

getting/setting those parameters without directly accessing the structure.



Currently, only the accessor functions necessary to support the mozjpeg

extensions are implemented, but the framework can be easily extended in the

future to accommodate additional simple parameter types, complex or

multi-valued parameters, or decompressor extensions.



The currently-implemented accessor functions are as follows:



boolean jpeg_c_bool_param_supported (j_compress_ptr cinfo,

                                     J_BOOLEAN_PARAM param)

        Returns TRUE if the given boolean extension parameter is supported by

        this implementation of the libjpeg API, or FALSE otherwise.



void jpeg_c_set_bool_param (j_compress_ptr cinfo,

                            J_BOOLEAN_PARAM param, boolean value);

        Set the given boolean extension parameter to the given value (TRUE or

        FALSE.)



boolean jpeg_c_get_bool_param (j_compress_ptr cinfo, J_BOOLEAN_PARAM param)

        Get the value of the given boolean extension parameter (TRUE or FALSE.)



boolean jpeg_c_float_param_supported (j_compress_ptr cinfo,

                                      J_FLOAT_PARAM param)

        Returns TRUE if the given floating point extension parameter is

        supported by this implementation of the libjpeg API, or FALSE

        otherwise.



void jpeg_c_set_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param,

                             float value)

        Set the given floating point extension parameter to the given value.



float jpeg_c_get_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param);

        Get the value of the given floating point extension parameter.



boolean jpeg_c_int_param_supported (j_compress_ptr cinfo,

                                    J_INT_PARAM param)

        Returns TRUE if the given integer extension parameter is supported by

        this implementation of the libjpeg API, or FALSE otherwise.



void jpeg_c_set_int_param (j_compress_ptr cinfo, J_INT_PARAM param,

                          int value)

        Set the given integer extension parameter to the given value.



int jpeg_c_get_int_param (j_compress_ptr cinfo, J_INT_PARAM param)

        Get the value of the given integer extension parameter.



Boolean Extension Parameters Supported by mozjpeg

-------------------------------------------------



* JBOOLEAN_OPTIMIZE_SCANS (default: TRUE)

  Specifies whether scan parameters should be optimized.  Parameter

  optimization is done as in jpgcrush. jpeg_simple_progression() should be called

  after setting JBOOLEAN_OPTIMIZE_SCANS.

  When disabling JBOOLEAN_OPTIMIZE_SCANS, cinfo.scan_info should additionally be

  set to NULL to disable use of the progressive coding mode, if so desired.



* JBOOLEAN_TRELLIS_QUANT (default: TRUE)

  Specifies whether to apply trellis quantization.  For each 8x8 block, trellis

  quantization determines the best tradeoff between rate and distortion.



* JBOOLEAN_TRELLIS_QUANT_DC (default: TRUE)

  Specifies whether to apply trellis quantization to DC coefficients.



* JBOOLEAN_TRELLIS_EOB_OPT (default: FALSE)

  Specifies whether to optimize runs of zero blocks in trellis quantization.

  This is applicable only when JBOOLEAN_USE_SCANS_IN_TRELLIS is enabled.



* JBOOLEAN_USE_LAMBDA_WEIGHT_TBL currently has no effect.



* JBOOLEAN_USE_SCANS_IN_TRELLIS (default: FALSE)

  Specifies whether multiple scans should be considered during trellis

  quantization.



* JBOOLEAN_TRELLIS_Q_OPT (default: FALSE)

  Specifies whether to optimize the quantization table after trellis

  quantization.  If enabled, then a revised quantization table is derived so

  as to minimize the reconstruction error of the quantized coefficients.



* JBOOLEAN_OVERSHOOT_DERINGING (default: TRUE)

  Specifies whether overshooting is applied to samples with extreme values

  (for example, 0 and 255 for 8-bit samples).  Overshooting may reduce ringing

  artifacts from compression, in particular in areas where black text appears

  on a white background.



Floating Point Extension Parameters Supported by mozjpeg

--------------------------------------------------------



* JFLOAT_LAMBDA_LOG_SCALE1 (default: 14.75)

  JFLOAT_LAMBDA_LOG_SCALE2 (default: 16.5)

  These parameters specify the lambda value used in trellis quantization.  The

  lambda value (Lagrange multiplier) in the

    R + lambda * D

  equation is derived from

    lambda = 2^s1 / ((2^s2 + n) * q^2),

  where s1 and s2 are the values of JFLOAT_LAMBDA_LOG_SCALE1 and

  JFLOAT_LAMBDA_LOG_SCALE2, n is the average of the squared unquantized AC

  coefficients within the current 8x8 block, and q is the quantization table

  entry associated with the current coefficient frequency.  If

  JFLOAT_LAMBDA_LOG_SCALE2 is 0, then an alternate form is used that does not

  rely on n:

    lambda = 2^(s1-12) / q^2.



* JFLOAT_TRELLIS_DELTA_DC_WEIGHT (default: 0.0)

  This parameter controls how distortion is calculated in DC trellis quantization

  (enabled with JBOOLEAN_TRELLIS_QUANT_DC). It defines weighting between distortion

  of the DC coefficient and distortion of the vertical gradient of DC coefficients.

  The value of the parameter corresponds to the weight applied to the distortion

  of the vertical gradient.



Integer Extension Parameters Supported by mozjpeg

-------------------------------------------------



* JINT_COMPRESS_PROFILE (default: JCP_MAX_COMPRESSION)

  Select a compression profile, which is a set of default parameters that will

  achieve a desired compression goal.  This parameter controls the behavior of

  the jpeg_set_defaults() function.  Thus, setting JINT_COMPRESS_PROFILE does

  not cause any other parameters to be modified until jpeg_set_defaults() is

  called.  The following compression profiles are supported:



  - JCP_MAX_COMPRESSION (default)

    Increase the compression ratio as much as possible, at the expense of

    increased encoding time.  This enables progressive entropy coding and all

    mozjpeg extensions.



  - JCP_FASTEST

    Use the libjpeg[-turbo] defaults (baseline entropy coding, no mozjpeg

    extensions enabled.)



* JINT_TRELLIS_FREQ_SPLIT (default: 8)

  Specifies the position within the zigzag scan at which the split between

  scans is positioned in the context of trellis quantization.

  JBOOLEAN_USE_SCANS_IN_TRELLIS must be enabled for this parameter to have any

  effect.



* JINT_TRELLIS_NUM_LOOPS (default: 1)

  Specifies the number of trellis quantization passes.  Huffman tables are

  updated between passes.



* JINT_BASE_QUANT_TBL_IDX (default: 3)

  Specifies which quantization table set to use.  The following options are

  available:

  0 = Tables from JPEG Annex K

  1 = Flat table

  2 = Table tuned for MSSIM on Kodak image set

  3 = Table from http://www.imagemagick.org/discourse-server/viewtopic.php?f=22&t=20333&p=98008#p98008

  4 = Table tuned for PSNR-HVS-M on Kodak image set

  5 = Table from:  Relevance of Human Vision to JPEG-DCT Compression

      (1992) Klein, Silverstein and Carney

  6 = Table from:  DCTune Perceptual Optimization of Compressed Dental X-Rays

      (1997) Watson, Taylor, Borthwick

  7 = Table from:  A Visual Detection Model for DCT Coefficient Quantization

      (12/9/93) Ahumada, Watson, Peterson

  8 = Table from:  An Improved Detection Model for DCT Coefficient Quantization

      (1993) Peterson, Ahumada and Watson



* JINT_DC_SCAN_OPT_MODE (default: 0)

  Specifies the DC scan optimization mode.  The following options are

  available:

  0 = One scan for all components

  1 = One scan per component

  2 = Optimize between one scan for all components and one scan for the first

      component plus one scan for the remaining components