From: Benjamin Otte <otte@gnome.org>

Date: Thu, 09 Oct 2008 20:45:12 +0000

Message-ID: <a4ed255b0810091344n7b23c764x780ebf8524f95055@mail.gmail.com>

To: "Alex Danilo" <alex@abbra.com>

Cc: "SVG Working Group WG" <public-svg-wg@w3.org>

Date: Thu, 09 Oct 2008 20:45:12 +0000

Message-ID: <a4ed255b0810091344n7b23c764x780ebf8524f95055@mail.gmail.com>

To: "Alex Danilo" <alex@abbra.com>

Cc: "SVG Working Group WG" <public-svg-wg@w3.org>

So I've spent the day looking up the bled-mode formulas, implementing them and testing them against various applications to make sure they're correct. It turns out that not only soft-light, but also color-dodge and color-burn are incorrect. I'll post the correct versions, taken from the ISO32000 PDF specification at http://www.adobe.com/devnet/acrobat/pdfs/PDF32000_2008.pdf and adapted to the syntax used in the SVG spec: Color Dodge: if Sca == Sa and Dca == 0 Dca' = Sca.(1 - Da) + Dca.(1 - Sa) otherwise if Sca == Sa Dca' = Sa.Da + Sca.(1 - Da) + Dca.(1 - Sa) otherwise Dca' = Da.Sa. min (Dca / Da / (1 - Sca/Sa)) Note that the first term is not part of the PDF, but implemented by all PDF applications. Color Burn: if Sca. == 0 and Dca == Da Dca' = SaDa + Sca.(1 - Da) + Dca.(1 - Sa) otherwise if Sca == 0 Dca' = Sca.(1 - Da) + Dca.(1 - Sa) otherwise Dca' = Sa.Da.(1 - min (1, (1 - Dca/Da).Sa / Sca)) + Sca.(1 - Da) + Dca.(1 - Sa) Again the first term is not part of the specification, but implemented by PDF applications. Soft Light: if (2.Sca <= Sa) Dca' = Dca.(Sa - (1 - Dca/Da).(2.Sca - Sa)) + Sca.(1 - Da) + Dca.(1 - Sa) otherwise if Dca.4 <= Da Dca' = Dca.(Sa + (2.Sca - Sa).((16.Dca/Da - 12).Dca/Da + 3) + Sc.(1 - Da) + Dca.(1 - Sa) otherwise Dca' = (Dca.Sa + (SQRT (Dca/Da).Da - Dca).(2.Sca - Sa)) + Sca.(1 - Da) + Dca.(1 - Sa) For more details about my experiments and wich apps I tested, see http://lists.freedesktop.org/archives/cairo/2008-October/015362.html Cheers, Benjamin On Thu, Oct 9, 2008 at 12:22 PM, Alex Danilo <alex@abbra.com> wrote: > Hi Benjamin, > > > In that case you should use that as the definitive formula. > As I said, we had to try to work it out by help of others reverse > engineering work. A pixel by pixel comparison of all input values > against the PDF viewer should be used to verify that it's correct. > > _But_, as I said before we had to extend the formulas > to take into account alpha. There's no alpha in the formulas > you quoted, so you will need to derive the correct formulas > based on source and destination alpha to give the correct > result for the general case. Opaque blends are just a subset > of what is needed. > > Alex > > --Original Message--: >>Adobe has published the blend modes used in PDF at >>http://www.adobe.com/devnet/pdf/pdfs/blend_modes.pdf >>It defines soft-light as >> >>if (Sc <= 0.5) >> Dc' = Dc - (1 - 2.Sc).Dc.(1-Dc) >>otherwise >> Dc' = Dc + (2.Sc - 1) . (F(Dc) - Dc) >> >>with >>if (x <= 0.25) >> F(x) = ((16.x - 12).x + 4).x >>otherwise >> F(x) = x >> >>which is slightly different from the formula you presented. >>It is also easier to implement as it doesn't involve square roots. >> >>Cheers, >>Benjamin >> >> >>On Thu, Oct 9, 2008 at 6:30 AM, Alex Danilo <alex@abbra.com> wrote: >>> >>> Hi All, >>> >>> --Original Message--: >>>>ISSUE-2095 (soft-light blend mode): Algorithm for soft-light blend mode [Module: Filters] >>>> >>>>http://www.w3.org/Graphics/SVG/WG/track/issues/2095 >>>> >>>>Raised by: Doug Schepers >>>>On product: Module: Filters >>>> >>>>Benjamin Otte >>>><http://lists.w3.org/Archives/Public/www-svg/2008Oct/0029.html>: >>>>[[ >>>>I'm in the process of implementing bend modes in the Cairo imaging >>>>library[1]. As Cairo is used as the renderer for a wide variety of >>>>formats, I wanted to have an as comprehensive list of blend modes as >>>>possible. And it turns out that the blend mode algorithms used are >>>>almost always equal. However, there is one exception: soft-light. The >>>>latest SVG draft specifies a different algorithm than PDF. >>>> >>>>So I was wondering if it was possible to change the algorithm in SVG >>>>to match the one of PDF, as that would solve a lot of problems. >>>>]] >>> >>> If there is a discrepancy it is a mistake. >>> >>> While I was working on this, we had to deduce the functions from various >>> sources, since the equations were not published. The starting point was >>> some Japanese site that had reverse engineered the equations from >>> looking at Photoshop. We extended the base equations to include >>> alpha correctly. >>> >>> These were always supposed to be compatible with PDF. >>> >>> Implementing this showed up bugs in compatibility which were fixed >>> but probably never made it out to the spec. I believe the correct equations >>> are: >>> >>> if 2.Sca < Sa >>> Dca' = Dca.(Sa + (1 - Dca/Da).(2.Sca - Sa)) + Sca.(1 - Da) + Dca.(1 - Sa) >>> otherwise if 8.Dca <= Da >>> Dca' = Dca.(Sa + (1 - Dca/Da).(2.Sca - Sa).(3 - 8.Dca/Da)) + Sca.(1 - Da) + Dca.(1 - Sa) >>> otherwise >>> Dca' = (Dca.Sa + ((Dca/Da)^(0.5).Da - Dca).(2.Sca - Sa)) + Sca.(1 - Da) + Dca.(1 - Sa) >>> >>> Da' = Sa + Da - Sa.Da >>> >>> which is different to what is in the latest published spec. Adobe had an action to >>> check the equations many years ago which was never acted on. Hopefully the >>> above equation is what you expected, if not please point us to the references that >>> are causing issues for you. >>> >>> Thanks, >>> Alex >>> >>> >> >> > >Received on Thursday, 9 October 2008 21:02:45 GMT

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