"Phil Endecott" <spam_from_boost_dev@chezphil.org> wrote in message news:1286804807448@dmwebmail.dmwebmail.chezphil.org...
I've had a quick look at your libjpeg interface. Two features that I often use are dct_method=JDCT_IFAST and scaling down 2x or 4x using scale_num and scale_denom. It would be interesting to know whether (a) your existing code lets me use these via a "native handle", or (b) they could be added easily.
Well, w/o trying, I think you should be able to do it straight away with option a:
gil::libjpeg_image::reader_for<...>::type your_image( ... ); your_image.lib_object().dct_method = JDCT_IFAST; your_image.lib_object().scale_num = your_scale_num; your_image.copy_to.....(...);
Ok, w/ trying it did not work if you tried to copy to an image with 'dimension synchronization' (resizing the target GIL image to fit the JPEG image) because: a) the input/original/unscaled dimensions were used to resize the image/report the JPEG dimensions b) even if output/scaled dimensions were used it still would not work because the target image is resized _before_ the start of the JPEG decompression (when the output dimensions are updated)... So I just committed a fix to account for both of those issues. The key issue was to cause the output dimensions to be updated/recalculated (to call jpeg_calc_output_dimensions()) if the user changes the scale_num value. There are several ways that this can be handled: - add an internal flag to libjpeg_image that will be set every time the user calls lib_object() and will be checked (and cleared) in dimensions() (where jpeg_calc_output_dimensions() will be called if the flag is set) - instead of a plain jpeg_decompress_struct reference make lib_object() return a proxy reference class that will call jpeg_calc_output_dimensions() in its destructor - expose a new 'update_output_dimensions()' like member function through which the user can (and must if relevant changes are made using lib_object()) deterministically cause the update... ...all of these of course have different compromises between overhead, 'cleanness' and safety...currently I used the first option... -- "What Huxley teaches is that in the age of advanced technology, spiritual devastation is more likely to come from an enemy with a smiling face than from one whose countenance exudes suspicion and hate." Neil Postman