Merge pull request #126 from dcommander/doc-tweaks
Tweak mozjpeg README file
This commit is contained in:
Josh Aas
@@ -1,109 +1,178 @@
|
||||
Mozilla JPEG Encoder Project
|
||||
============================
|
||||
|
||||
This project's goal is to reduce the size of JPEG files without reducing
|
||||
quality or compatibility with the vast majority of the world's deployed decoders.
|
||||
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.
|
||||
|
||||
The idea is to reduce transfer times for JPEGs on the Web, thus reducing page load times.
|
||||
|
||||
'mozjpeg' is not intended to be a general JPEG library replacement. It makes tradeoffs that
|
||||
are intended to benefit Web use cases and focuses solely on improving encoding. It is best
|
||||
used as part of a Web encoding workflow. For a general JPEG library (e.g. your system libjpeg),
|
||||
especially if you care about decoding, we recommend libjpeg-turbo.
|
||||
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.
|
||||
|
||||
|
||||
Data structures
|
||||
===============
|
||||
libjpeg API Extensibility Framework
|
||||
===================================
|
||||
|
||||
New parameters introduced by the Mozilla JPEG encoder are placed into the jpeg_comp_master data
|
||||
structure which his not directly accessible. Several functions are introduced to get and set these
|
||||
parameters:
|
||||
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.
|
||||
|
||||
EXTERN(boolean) jpeg_c_bool_param_supported (j_compress_ptr cinfo, J_BOOLEAN_PARAM param);
|
||||
EXTERN(void) jpeg_c_set_bool_param (j_compress_ptr cinfo, J_BOOLEAN_PARAM param, boolean value);
|
||||
EXTERN(boolean) jpeg_c_get_bool_param (j_compress_ptr cinfo, J_BOOLEAN_PARAM param);
|
||||
|
||||
EXTERN(boolean) jpeg_c_float_param_supported (j_compress_ptr cinfo, J_FLOAT_PARAM param);
|
||||
EXTERN(void) jpeg_c_set_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param, float value);
|
||||
EXTERN(float) jpeg_c_get_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param);
|
||||
|
||||
EXTERN(boolean) jpeg_c_int_param_supported (j_compress_ptr cinfo, J_INT_PARAM param);
|
||||
EXTERN(void) jpeg_c_set_int_param (j_compress_ptr cinfo, J_INT_PARAM param, int value);
|
||||
EXTERN(int) jpeg_c_get_int_param (j_compress_ptr cinfo, J_INT_PARAM param);
|
||||
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.
|
||||
|
||||
|
||||
Boolean parameters
|
||||
------------------
|
||||
The currently-implemented accessor functions are as follows:
|
||||
|
||||
* JBOOLEAN_USE_MOZ_DEFAULTS indicates whether the Mozilla default settings should be used. Otherwise
|
||||
the behavior reverts to the default from libjpeg-turbo. Note that this parameter should be set
|
||||
before calling jpeg_set_defaults(). By default this parameter is enabled.
|
||||
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.
|
||||
|
||||
* JBOOLEAN_OPTIMIZE_SCANS indicates whether to optimize scan parameters. Parameter optimization is
|
||||
done as in jpgcrush. By default this parameter is enabled.
|
||||
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.)
|
||||
|
||||
* JBOOLEAN_TRELLIS_QUANT indicates whether to apply trellis quantization. For each 8x8 block trellis
|
||||
quantization determines the best trade-off between rate and distortion. By default this parameter
|
||||
is enabled.
|
||||
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.)
|
||||
|
||||
* JBOOLEAN_TRELLIS_QUANT_DC indicates whether to apply trellis quantization to DC coefficients. By
|
||||
default this parameter is enabled.
|
||||
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.
|
||||
|
||||
* JBOOLEAN_TRELLIS_EOB_OPT indicates whether to optimize runs of zero blocks in trellis quantization.
|
||||
This is applicable only when JBOOLEAN_USE_SCANS_IN_TRELLIS is enabled. By default this parameter
|
||||
is disabled.
|
||||
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.
|
||||
|
||||
* JBOOLEAN_USE_LAMBDA_WEIGHT_TBL has currently no effect.
|
||||
float jpeg_c_get_float_param (j_compress_ptr cinfo, J_FLOAT_PARAM param);
|
||||
Get the value of the given floating point extension parameter.
|
||||
|
||||
* JBOOLEAN_USE_SCANS_IN_TRELLIS indicates whether multiple scans are considered during trellis
|
||||
quantization. By default this parameter is disabled.
|
||||
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.
|
||||
|
||||
* JBOOLEAN_TRELLIS_Q_OPT indicates whether to optimize the quantization table after trellis quantization.
|
||||
If enabled a revised quantization table is derived such as to minimize the reconstruction error
|
||||
given the quantized coefficients. By default this parameter is disabled.
|
||||
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.
|
||||
|
||||
* JBOOLEAN_OVERSHOOT_DERINGING indicates whether overshooting is applied to samples with extreme
|
||||
values (e.g., 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. By default
|
||||
this parameter is enabled.
|
||||
int jpeg_c_get_int_param (j_compress_ptr cinfo, J_INT_PARAM param)
|
||||
Get the value of the given integer extension parameter.
|
||||
|
||||
Floating-point parameters
|
||||
-------------------------
|
||||
|
||||
* JFLOAT_LAMBDA_LOG_SCALE1 and JFLOAT_LAMBDA_LOG_SCALE2 determine the lambda value used in
|
||||
trellis quantization. By default these parameters are set to 14.75 and 16.5. The lambda value
|
||||
(Lagrance 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, an alternate form is used that
|
||||
does not rely on n: lambda = 2^(s1-12) / q^2.
|
||||
Boolean Extension Parameters Supported by mozjpeg
|
||||
-------------------------------------------------
|
||||
|
||||
Integer parameters
|
||||
------------------
|
||||
* JBOOLEAN_USE_MOZ_DEFAULTS (default: TRUE)
|
||||
This parameter controls the behavior of the jpeg_set_defaults() function and
|
||||
should thus be set prior to calling that function. If this parameter is
|
||||
TRUE, then jpeg_set_defaults() will configure the library to use the mozjpeg
|
||||
defaults (which will enable settings that increase the compression ratio as
|
||||
much as possible, at the expense of increased encoding time.) If this
|
||||
parameter is FALSE, then jpeg_set_defaults() will configure the library to
|
||||
use the libjpeg[-turbo] defaults (baseline entropy coding, no mozjpeg
|
||||
extensions enabled.)
|
||||
|
||||
* JINT_TRELLIS_FREQ_SPLIT determines 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 take effect. By default this parameter is set to value 8.
|
||||
* JBOOLEAN_OPTIMIZE_SCANS (default: TRUE)
|
||||
Specifies whether scan parameters should be optimized. Parameter
|
||||
optimization is done as in jpgcrush.
|
||||
|
||||
* JINT_TRELLIS_NUM_LOOPS determines the number of trellis quantization passes. Huffman tables are
|
||||
updated between passes. By default this parameter is set to value 1.
|
||||
* 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.
|
||||
|
||||
* JINT_BASE_QUANT_TBL_IDX determines which quantization table set to use. Multiple sets are defined
|
||||
as below. By default this parameter is set to value 3.
|
||||
- 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
|
||||
* JBOOLEAN_TRELLIS_QUANT_DC (default: TRUE)
|
||||
Specifies whether to apply trellis quantization to DC coefficients.
|
||||
|
||||
* JINT_DC_SCAN_OPT_MODE determines the DC scan optimization mode. Modes are defined as below. By default
|
||||
this parameter is set to value 1.
|
||||
- 0 One scan for all components
|
||||
- 1 One scan per component
|
||||
- 2 Optimize between one scan for all components and one scan for 1st component plus one scan for
|
||||
remaining components
|
||||
* 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.
|
||||
|
||||
|
||||
Integer Extension Parameters Supported by mozjpeg
|
||||
-------------------------------------------------
|
||||
|
||||
* 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: 1)
|
||||
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
|
||||
|
||||
Reference in New Issue
Block a user