[Cin] oops apparently 'subtract' mode is broken!

[Phyllis Smith]

> may be Phyllis can replace our original red-green square  images with
>>>> full-color gradients + photo, so effect will be more natural? Or add this
>>>> into new appendix .....
>>>>
>>> Good idea to add in appendix but I am not sure I could get it right.
>>
>
>
> may be download images from said website via "download" button, then place
> them on tracks and render single frame for each of those  30 different
> composition modes on top track? sorry if this sounds big, I can try this
> idea  myself, just then I will generate whole folder of images for you to
> pick up and integrate into manual. < oh Andrew, come on, you just too lazy
> for even minimal work - my conscience >
>

It does sound big but quite interesting to me and not a problem with TeX
editing.  I think I will have time in October to get serious and then have
you proofread?

>
> or you mean whole TeX editing? I should try to retest if my pdf generating
> setup still work ....
>
>
>
>>
>>> more documentation, it seems Adobe products use shared code for this ....
>>>
>>>
>>> https://www.rippletraining.com/blog/final-cut-pro-x/understanding-blend-modes-final-cut-pro/
>>>
>> Another good URL to look at.  When coding all of the Overlay modes,
>> Porter Duff explanations and example were used as the basis.  Not sure if
>> Gimp and PhotoShop use the same basis.
>>
>>>
>>>
>>> --quote---
>>>
>>> Subtract
>>>
>>> Subtract does what it advertises; the color values of one layer are
>>> subtracted from the other layer. This tends to have a net darkening effect.
>>> Pixel values can’t fall below zero, so negative numbers are rendered to
>>> black.
>>>
>>>
>>> ---end quote---
>>>
>>>
>>> in older cingg code (from 4.6.mod era - earliest public code on Google
>>> code) I see such construct :
>>>
>>>
>>> /data/data/com.termux/files/home/cingg-g~nelerra-4.6.mod/cinelerra/overlayframe.C
>>>      4762/69317  UTF-8        6%
>>>
>>>                         break; \
>>>
>>>                 case TRANSFER_SUBTRACT: \
>>>
>>>                         r = (temp_type)output[0] - (temp_type)input1; \
>>>
>>>                         g = ((temp_type)output[1] -
>>> (temp_type)chroma_offset) - \
>>>
>>>                                 ((temp_type)input2 -
>>> (temp_type)chroma_offset) + \
>>>
>>>                                 (temp_type)chroma_offset; \
>>>
>>>                         b = ((temp_type)output[2] -
>>> (temp_type)chroma_offset) - \
>>>
>>>                                 ((temp_type)input3 -
>>> (temp_type)chroma_offset) + \
>>>
>>>                                 (temp_type)chroma_offset; \
>>>
>>>                         if(r < 0) r = 0; \
>>>
>>>                         if(g < 0) g = 0; \
>>>
>>>                         if(b < 0) b = 0; \
>>>
>>>                         r = (r * opacity + output[0] * transparency) /
>>> max; \
>>>
>>>                         g = (g * opacity + output[1] * transparency) /
>>> max; \
>>>
>>>                         b = (b * opacity + output[2] * transparency) /
>>> max; \                                                              break; \
>>>
>>>
>>> ---
>>>
>>>
>>> so it basically had specific guard against going negative. Not sure if
>>> our implementation does the same just in different place?
>>>
>>>
>>> I see no specific handling of it in macros ...
>>>
>>>
>>>
>>> overlayframe.h     [----] 38 L:[ 53+ 0  53/615] *(1572/17155b) 001[*][X]
>>>
>>> // ADDITION<--->[(Sa + Da), (Sc + Dc)]
>>>
>>> #define ALPHA_ADDITION(mx, Sa, Da) (Sa + Da)
>>>
>>> #define COLOR_ADDITION(mx, Sc, Sa, Dc, Da) (Sc + Dc)
>>>
>>> #define CHROMA_ADDITION COLOR_ADDITION
>>>
>>>
>>> // SUBTRACT<--->[(Sa - Da), (Sc - Dc)]
>>>
>>> #define ALPHA_SUBTRACT(mx, Sa, Da) (Sa - Da)
>>>
>>> #define COLOR_SUBTRACT(mx, Sc, Sa, Dc, Da) (Sc - Dc)
>>>
>>> #define CHROMA_SUBTRACT COLOR_SUBTRACT
>>>
>>>
>>> but apparently you can add some comparing here ...just .. it went from
>>> two-stage process to single stage for untrained eye and I am not sure where
>>> to put those ifs?
>>>
>>>
>>>
>>>
>>>
>>>
>>>>
>>>> вс, 25 сент. 2022 г., 11:06 Andrew Randrianasulu <
>>>> randrianasulu at gmail.com>:
>>>>
>>>>> I found Graphics-softlight mode behaving visually close...but not sure
>>>>> how close to CinHV/CV subtract mode ...
>>>>>
>>>>>
>>>>>
>>>>> вс, 25 сент. 2022 г., 10:24 Andrew Randrianasulu <
>>>>> randrianasulu at gmail.com>:
>>>>>
>>>>>> I was trying to repeat tuto at
>>>>>>
>>>>>> https://linuxvideoediting.blogspot.com/2022/01/transparent-text-effect-in-Cinelerra-part1.html?m=1
>>>>>>
>>>>>>
>>>>>> so I loaded video, created empty new vid track, applied 'titles',
>>>>>> 'blur' and 'invert video' to _empty track_, then set track's mode to
>>>>>> subtract .... result was color inversion of original video on second vid
>>>>>> track (
>>>>>>
>>>>>
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