# Re: ISSUE-2095 (soft-light blend mode): Algorithm for soft-light blend mode [Module: Filters]

From: Alex Danilo <alex@abbra.com>
Date: Thu, 09 Oct 2008 21:22:58 +1100
Message-Id: <AIVG8K.HSES4CUU8INM@abbra.com>
To: "Benjamin Otte" <otte@gnome.org>
Cc: "SVG Working Group WG" <public-svg-wg@w3.org>
```
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
>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 10:23:47 UTC

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