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Since mozjpeg is now backward ABI-compatible with libjpeg[-turbo], it is now possible to temporarily load mozjpeg into a binary application and cause that application to generate uber-compressed JPEGs (at the expense of an extreme performance loss, of course.) For instance, someone could do LD_LIBRARY_PATH=/opt/mozjpeg/lib convert blah_blah_blah to make ImageMagick use mozjpeg instead of the system's pre-installed JPEG library (libjpeg-turbo, in most cases.) However, this only makes sense if mozjpeg is actually producing different behavior by default than libjpeg-turbo. Currently it isn't. Currently it requires the application to set JBOOLEAN_USE_MOZ_DEFAULTS to TRUE in order to enable the mozjpeg-specific behavior, but of course applications that were built to use libjpeg[-turbo] won't do that. Thus, this patch sets use_moz_defaults to TRUE by default, requiring an application to explicitly set it to FALSE in order to revert to the libjpeg[-turbo] behavior (makes sense, since the only applications that would need to revert to the libjpeg[-turbo] behavior would be mozjpeg-aware applications.) Note that we discussed the possibility of adding a function (jpeg_revert_defaults()), which would act the same as jpeg_set_defaults() does in libjpeg[-turbo]. This is a good solution for implementing the -revert switch in cjpeg, but unfortunately it doesn't work for jpegtran. The reason is that jpeg_set_defaults() is called within the body of jpeg_copy_critical_parameters(), which is part of the API. So yet again, if mozjpeg were loaded into a non-mozjpeg-aware application at run time, it would be desirable for jpeg_copy_critical_parameters() to set the parameters to mozjpeg defaults. That means that, in order to implement the -revert switch in jpegtran, it would be necessary to introduce a new function (jpeg_revert_critical_parameters(), perhaps). It seems cleaner to just keep using the JBOOLEAN_USE_MOZ_DEFAULTS parameter to control the behavior of jpeg_set_defaults(), even though this represents a minor abuse of the libjpeg API (jpeg_set_defaults() is technically supposed to set all of the parameters to defaults, irrespective of any previous state. However, as long as we document that JBOOLEAN_USE_MOZ_DEFAULTS works differently, then it should be OK.)
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.
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.
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