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194 lines
8.8 KiB
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194 lines
8.8 KiB
Text
Mozilla JPEG Encoder Project
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============================
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mozjpeg is a fork of libjpeg-turbo that aims to speed up load times of web
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pages by reducing the size (and, by extension, the transmission time) of JPEG
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files. It accomplishes this by enabling optimized Huffman trees and
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progressive entropy coding by default in the JPEG compressor, as well as
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splitting the spectrum of DCT coefficients into separate scans and using
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Trellis quantisation.
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Although it is based on libjpeg-turbo, mozjpeg is not intended to be a
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general-purpose or high-performance JPEG library. Its performance is highly
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"asymmetric". That is, the JPEG files it generates require much more time to
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compress than to decompress. When the default settings are used, mozjpeg is
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considerably slower than libjpeg-turbo or even libjpeg at compressing images.
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Thus, it is not generally suitable for real-time compression. It is best used
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as part of a web encoding workflow.
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libjpeg API Extensibility Framework
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===================================
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mozjpeg's implementation of the libjpeg API includes an extensibility framework
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that allows new features to be added without modifying the transparent libjpeg
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compress/decompress structures (which would break backward ABI compatibility.)
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Extension parameters are placed into the opaque jpeg_comp_master structure, and
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a set of accessor functions and globally unique tokens allows for
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getting/setting those parameters without directly accessing the structure.
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Currently, only the accessor functions necessary to support the mozjpeg
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extensions are implemented, but the framework can be easily extended in the
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future to accommodate additional simple parameter types, complex or
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multi-valued parameters, or decompressor extensions.
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The currently-implemented accessor functions are as follows:
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boolean jpeg_c_bool_param_supported (j_compress_ptr cinfo,
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J_BOOLEAN_PARAM param)
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Returns TRUE if the given boolean extension parameter is supported by
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this implementation of the libjpeg API, or FALSE otherwise.
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void jpeg_c_set_bool_param (j_compress_ptr cinfo,
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J_BOOLEAN_PARAM param, boolean value);
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Set the given boolean extension parameter to the given value (TRUE or
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FALSE.)
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boolean jpeg_c_get_bool_param (j_compress_ptr cinfo, J_BOOLEAN_PARAM param)
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Get the value of the given boolean extension parameter (TRUE or FALSE.)
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boolean jpeg_c_float_param_supported (j_compress_ptr cinfo,
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J_FLOAT_PARAM param)
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Returns TRUE if the given floating point extension parameter is
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supported by this implementation of the libjpeg API, or FALSE
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otherwise.
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void jpeg_c_set_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param,
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float value)
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Set the given floating point extension parameter to the given value.
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float jpeg_c_get_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param);
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Get the value of the given floating point extension parameter.
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boolean jpeg_c_int_param_supported (j_compress_ptr cinfo,
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J_INT_PARAM param)
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Returns TRUE if the given integer extension parameter is supported by
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this implementation of the libjpeg API, or FALSE otherwise.
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void jpeg_c_set_int_param (j_compress_ptr cinfo, J_INT_PARAM param,
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int value)
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Set the given integer extension parameter to the given value.
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int jpeg_c_get_int_param (j_compress_ptr cinfo, J_INT_PARAM param)
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Get the value of the given integer extension parameter.
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Boolean Extension Parameters Supported by mozjpeg
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-------------------------------------------------
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* JBOOLEAN_OPTIMIZE_SCANS (default: TRUE)
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Specifies whether scan parameters should be optimized. Parameter
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optimization is done as in jpgcrush. jpeg_simple_progression() should be called
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after setting JBOOLEAN_OPTIMIZE_SCANS.
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When disabling JBOOLEAN_OPTIMIZE_SCANS, cinfo.scan_info should additionally be
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set to NULL to disable use of the progressive coding mode, if so desired.
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* JBOOLEAN_TRELLIS_QUANT (default: TRUE)
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Specifies whether to apply trellis quantization. For each 8x8 block, trellis
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quantization determines the best tradeoff between rate and distortion.
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* JBOOLEAN_TRELLIS_QUANT_DC (default: TRUE)
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Specifies whether to apply trellis quantization to DC coefficients.
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* JBOOLEAN_TRELLIS_EOB_OPT (default: FALSE)
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Specifies 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.
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* JBOOLEAN_USE_LAMBDA_WEIGHT_TBL currently has no effect.
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* JBOOLEAN_USE_SCANS_IN_TRELLIS (default: FALSE)
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Specifies whether multiple scans should be considered during trellis
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quantization.
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* JBOOLEAN_TRELLIS_Q_OPT (default: FALSE)
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Specifies whether to optimize the quantization table after trellis
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quantization. If enabled, then a revised quantization table is derived so
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as to minimize the reconstruction error of the quantized coefficients.
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* JBOOLEAN_OVERSHOOT_DERINGING (default: TRUE)
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Specifies whether overshooting is applied to samples with extreme values
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(for example, 0 and 255 for 8-bit samples). Overshooting may reduce ringing
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artifacts from compression, in particular in areas where black text appears
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on a white background.
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Floating Point Extension Parameters Supported by mozjpeg
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--------------------------------------------------------
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* JFLOAT_LAMBDA_LOG_SCALE1 (default: 14.75)
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JFLOAT_LAMBDA_LOG_SCALE2 (default: 16.5)
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These parameters specify the lambda value used in trellis quantization. The
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lambda value (Lagrange multiplier) in the
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R + lambda * D
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equation is derived from
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lambda = 2^s1 / ((2^s2 + n) * q^2),
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where s1 and s2 are the values of JFLOAT_LAMBDA_LOG_SCALE1 and
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JFLOAT_LAMBDA_LOG_SCALE2, n is the average of the squared unquantized AC
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coefficients within the current 8x8 block, and q is the quantization table
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entry associated with the current coefficient frequency. If
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JFLOAT_LAMBDA_LOG_SCALE2 is 0, then an alternate form is used that does not
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rely on n:
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lambda = 2^(s1-12) / q^2.
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* JFLOAT_TRELLIS_DELTA_DC_WEIGHT (default: 0.0)
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This parameter controls how distortion is calculated in DC trellis quantization
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(enabled with JBOOLEAN_TRELLIS_QUANT_DC). It defines weighting between distortion
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of the DC coefficient and distortion of the vertical gradient of DC coefficients.
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The value of the parameter corresponds to the weight applied to the distortion
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of the vertical gradient.
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Integer Extension Parameters Supported by mozjpeg
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-------------------------------------------------
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* JINT_COMPRESS_PROFILE (default: JCP_MAX_COMPRESSION)
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Select a compression profile, which is a set of default parameters that will
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achieve a desired compression goal. This parameter controls the behavior of
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the jpeg_set_defaults() function. Thus, setting JINT_COMPRESS_PROFILE does
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not cause any other parameters to be modified until jpeg_set_defaults() is
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called. The following compression profiles are supported:
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- JCP_MAX_COMPRESSION (default)
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Increase the compression ratio as much as possible, at the expense of
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increased encoding time. This enables progressive entropy coding and all
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mozjpeg extensions.
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- JCP_FASTEST
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Use the libjpeg[-turbo] defaults (baseline entropy coding, no mozjpeg
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extensions enabled.)
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* JINT_TRELLIS_FREQ_SPLIT (default: 8)
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Specifies the position within the zigzag scan at which the split between
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scans is positioned in the context of trellis quantization.
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JBOOLEAN_USE_SCANS_IN_TRELLIS must be enabled for this parameter to have any
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effect.
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* JINT_TRELLIS_NUM_LOOPS (default: 1)
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Specifies the number of trellis quantization passes. Huffman tables are
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updated between passes.
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* JINT_BASE_QUANT_TBL_IDX (default: 3)
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Specifies which quantization table set to use. The following options are
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available:
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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
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(1992) Klein, Silverstein and Carney
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6 = Table from: DCTune Perceptual Optimization of Compressed Dental X-Rays
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(1997) Watson, Taylor, Borthwick
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7 = Table from: A Visual Detection Model for DCT Coefficient Quantization
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(12/9/93) Ahumada, Watson, Peterson
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8 = Table from: An Improved Detection Model for DCT Coefficient Quantization
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(1993) Peterson, Ahumada and Watson
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* JINT_DC_SCAN_OPT_MODE (default: 1)
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Specifies the DC scan optimization mode. The following options are
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available:
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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 the first
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component plus one scan for the remaining components
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