Add documentation for mozjpeg
A brief explanation is provided for the parameters introduced in mozjpeg
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Frank Bossen
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Mozilla JPEG Encoder Project
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============================
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This project's goal is to reduce the size of JPEG files without reducing
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quality or compatibility with the vast majority of the world's deployed decoders.
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The idea is to reduce transfer times for JPEGs on the Web, thus reducing page load times.
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'mozjpeg' is not intended to be a general JPEG library replacement. It makes tradeoffs that
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are intended to benefit Web use cases and focuses solely on improving encoding. It is best
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used as part of a Web encoding workflow. For a general JPEG library (e.g. your system libjpeg),
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especially if you care about decoding, we recommend libjpeg-turbo.
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Data structures
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===============
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New parameters introduced by the Mozilla JPEG encoder are placed into the jpeg_comp_master data
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structure which his not directly accessible. Several functions are introduced to get and set these
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parameters:
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EXTERN(boolean) jpeg_c_bool_param_supported (j_compress_ptr cinfo, J_BOOLEAN_PARAM param);
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EXTERN(void) jpeg_c_set_bool_param (j_compress_ptr cinfo, J_BOOLEAN_PARAM param, boolean value);
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EXTERN(boolean) jpeg_c_get_bool_param (j_compress_ptr cinfo, J_BOOLEAN_PARAM param);
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EXTERN(boolean) jpeg_c_float_param_supported (j_compress_ptr cinfo, J_FLOAT_PARAM param);
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EXTERN(void) jpeg_c_set_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param, float value);
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EXTERN(float) jpeg_c_get_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param);
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EXTERN(boolean) jpeg_c_int_param_supported (j_compress_ptr cinfo, J_INT_PARAM param);
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EXTERN(void) jpeg_c_set_int_param (j_compress_ptr cinfo, J_INT_PARAM param, int value);
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EXTERN(int) jpeg_c_get_int_param (j_compress_ptr cinfo, J_INT_PARAM param);
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Boolean parameters
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------------------
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* JBOOLEAN_USE_MOZ_DEFAULTS indicates whether the Mozilla default settings should be used. Otherwise
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the behavior reverts to the default from libjpeg-turbo. Note that this parameter should be set
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before calling jpeg_set_defaults(). By default this parameter is enabled.
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* JBOOLEAN_OPTIMIZE_SCANS indicates whether to optimize scan parameters. Parameter optimization is
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done as in jpgcrush. By default this parameter is enabled.
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* JBOOLEAN_TRELLIS_QUANT indicates whether to apply trellis quantization. For each 8x8 block trellis
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quantization determines the best trade-off between rate and distortion. By default this parameter
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is enabled.
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* JBOOLEAN_TRELLIS_QUANT_DC indicates whether to apply trellis quantization to DC coefficients. By
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default this parameter is enabled.
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* JBOOLEAN_TRELLIS_EOB_OPT indicates whether to optimize runs of zero blocks in trellis quantization.
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This is applicable only when JBOOLEAN_USE_SCANS_IN_TRELLIS is enabled. By default this parameter
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is disabled.
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* JBOOLEAN_USE_LAMBDA_WEIGHT_TBL has currently no effect.
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* JBOOLEAN_USE_SCANS_IN_TRELLIS indicates whether multiple scans are considered during trellis
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quantization. By default this parameter is disabled.
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* JBOOLEAN_TRELLIS_Q_OPT indicates whether to optimize the quantization table after trellis quantization.
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If enabled a revised quantization table is derived such as to minimize the reconstruction error
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given the quantized coefficients. By default this parameter is disabled.
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* JBOOLEAN_OVERSHOOT_DERINGING indicates whether overshooting is applied to samples with extreme
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values (e.g., 0 and 255 for 8-bit samples). Overshooting may reduce ringing artifacts from
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compression, in particular in areas where black text appears on a white background. By default
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this parameter is enabled.
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Floating-point parameters
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-------------------------
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* JFLOAT_LAMBDA_LOG_SCALE1 and JFLOAT_LAMBDA_LOG_SCALE2 determine the lambda value used in
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trellis quantization. By default these parameters are set to 14.75 and 16.5. The lambda value
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(Lagrance multiplier) in the R + lambda * D equation is derived from
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lambda = 2^s1 / ((2^s2 + n) * q^2) where s1 and s2 are the values of JFLOAT_LAMBDA_LOG_SCALE1
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and JFLOAT_LAMBDA_LOG_SCALE2, n is the average of the squared unquantized AC coefficients
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within the current 8x8 block, and q is the quantization table entry associated with the
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current coefficient frequency. If JFLOAT_LAMBDA_LOG_SCALE2 is 0, an alternate form is used that
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does not rely on n: lambda = 2^(s1-12) / q^2.
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Integer parameters
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------------------
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* JINT_TRELLIS_FREQ_SPLIT determines the position within the zigzag scan at which the split between
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scans is positioned in the context of trellis quantization. JBOOLEAN_USE_SCANS_IN_TRELLIS must
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be enabled for this parameter to take effect. By default this parameter is set to value 8.
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* JINT_TRELLIS_NUM_LOOPS determines the number of trellis quantization passes. Huffman tables are
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updated between passes. By default this parameter is set to value 1.
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* JINT_BASE_QUANT_TBL_IDX determines which quantization table set to use. Multiple sets are defined
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as below. By default this parameter is set to value 3.
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- 0 Tables from JPEG Annex K
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- 1 Flat table
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- 2 Table tuned for MSSIM on Kodak image set
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- 3 Table from http://www.imagemagick.org/discourse-server/viewtopic.php?f=22&t=20333&p=98008#p98008
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- 4 Table tuned for PSNR-HVS-M on Kodak image set
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- 5 Table from Relevance of human vision to JPEG-DCT compression (1992) Klein, Silverstein and Carney
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- 6 Table from DCTune perceptual optimization of compressed dental X-Rays (1997) Watson, Taylor, Borthwick
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- 7 Table from A visual detection model for DCT coefficient quantization (12/9/93) Ahumada, Watson, Peterson
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- 8 Table from An improved detection model for DCT coefficient quantization (1993) Peterson, Ahumada and Watson
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* JINT_DC_SCAN_OPT_MODE determines the DC scan optimization mode. Modes are defined as below. By default
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this parameter is set to value 1.
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- 0 One scan for all components
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- 1 One scan per component
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- 2 Optimize between one scan for all components and one scan for 1st component plus one scan for
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remaining components
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