Ah, thanks so much Michael! That explanation helps a great deal; I was always considering things in "straight alpha" format, not even knowing that there was alternative.
I'll play with this tonight; I don't see any problem getting the thing working, though, now that I know what agg expects to see... And yes, alpha support in the image class would be very helpful ;) On Wed, Oct 19, 2011 at 2:16 PM, Michael Droettboom <md...@stsci.edu> wrote: > You are right that Agg is doing the resizing here. Agg expects > premultiplied alpha. See [1] for information about what that means. > > [1] http://en.wikipedia.org/wiki/Alpha_compositing > > After Agg interpolates the pixel values, to prevent oversaturation it > truncates all values to be less than alpha (which makes sense if everything > is assumed to be premultiplied alpha). Arguably, the bug here is that > nearest neighbor (which doesn't have to do any blending) doesn't perform the > truncation step -- then both would look "wrong". > > It happens in this code snippet in span_image_filter_rgba: (base_mask is > 255) > > if(fg[order_type::A] > base_mask) fg[order_type::A] > = base_mask; > if(fg[order_type::R] > fg[order_type::A]) fg[order_type::R] > = fg[order_type::A]; > if(fg[order_type::G] > fg[order_type::A]) fg[order_type::G] > = fg[order_type::A]; > if(fg[order_type::B] > fg[order_type::A]) fg[order_type::B] > = fg[order_type::A]; > > So, the solution to make a partially transparent image is to not do: > > pix[:,:,3] = 127 > > but instead, do > > pix *= 0.5 > > Of course, the real fix here is to support alpha blending properly in the > image class, then the user wouldn't have to deal with such details. A bug > should probably be filed in the matplotlib issue tracker for this. > > Mike > > On 10/19/2011 12:23 PM, Daniel Hyams wrote: > > [Sorry, I keep getting tripped up with HTML mail....resent in ascii, > and resaved one of the attachment png's to make it smaller.] > > > Example script attached (PIL required). Basically, if I impose a > specific value into an image's alpha channel and use any interpolation > scheme other than 'nearest', there appears gray all where the figure > didn't have any color to begin with. I've also attached a screenshot > of the output of the script on my machine. > > Hopefully I'm doing something wrongly? > > I chased the problem and managed to hack in a solution that fixes the > problem, but it's extremely inefficient...basically, in matplotlib's > image.py, routine BboxImage.make_image, you can create two images > there....one with no alpha channel (call it imRGB) and one with (call > it imRGBA). Go through all of the routine, doing exactly the same > things to both of the images *except* for the interpolation, which is > set to 'nearest' for imRGBA. Then, rip the colors out of imRGB, the > alpha channel off of imRGBA, and put them together....go through all > of the routine again with this composited image, and it works. I > know...I told you it was bad ;) > > The problem seems to be in the "resize" call in that routine...resize, > which calls into C code, does not appear to handle things correctly > when the alpha is anything other than 255's across the board. It > might be a problem in the agg routines, but hopefully it is just maybe > a misuse of the agg routines. > > The behavior seems to be backend independent as far as I could test (I > tried with wxagg and tk backends). I am using mpl 1.0.0 on Windows if > it matters. > > > -- > Daniel Hyams > dhy...@gmail.com > > ------------------------------------------------------------------------------ > All the data continuously generated in your IT infrastructure contains a > definitive record of customers, application performance, security > threats, fraudulent activity and more. Splunk takes this data and makes > sense of it. Business sense. IT sense. Common sense. > http://p.sf.net/sfu/splunk-d2d-oct > > _______________________________________________ > Matplotlib-users mailing list > Matplotlib-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/matplotlib-users > > > ------------------------------------------------------------------------------ > The demand for IT networking professionals continues to grow, and the > demand for specialized networking skills is growing even more rapidly. > Take a complimentary Learning@Ciosco Self-Assessment and learn > about Cisco certifications, training, and career opportunities. > http://p.sf.net/sfu/cisco-dev2dev > _______________________________________________ > Matplotlib-users mailing list > Matplotlib-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/matplotlib-users > > -- Daniel Hyams dhy...@gmail.com ------------------------------------------------------------------------------ The demand for IT networking professionals continues to grow, and the demand for specialized networking skills is growing even more rapidly. 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