Speedup image comparision slider during WipeEffect

[Edi61]

As also marked in
WinMerge/winmerge#1596
the slider is very slow.

Here maybe one solution:

• WipeEffect only if position changed (no refreshimage on everywipe -> not sensefull, since during wiping with the mouse
  in any case you are not be able to change any parameters which could lead to an image change)
• Difference based wipeeffect (Only part between old and new position is updated)

Principially it works very fine and smooth, but if the image is changed before wiping with e.g. adjust offset,
the draw events are lost/not perfmored, maybe you have an idea why?

Regards,

[sdottaka]

Thank you for PR. However, it does not seem to work well as shown in the attached GIF animation.

Principially it works very fine and smooth, but if the image is changed before wiping with e.g. adjust offset, the draw events are lost/not perfmored, maybe you have an idea why?

I don't know so far.

[Edi61]

Aah, I have only checked with heic files, did not jet tessted with other image formats.
Then its probably related to something which is special for heic files.
I'll recheck it.

[Edi61]

I have found the problem, it's generally not the image format,
but any transperancy in the image which leads to not working wiping.
It also explains why adjusting/moving image is also not working,
because here we get also some transperancy parts.
Don't know jet how to solve...

[sdottaka]

That certainly seems like it. As of now, I don't know why either.

[Edi61]

I have found the root cause.

It's the function call FreeImage_Composite inside fipWinImage::drawEx:
https://github.com/WinMerge/freeimage/blob/78364ed94b4cb2d830805a26cb65bf5e6f523d58/Wrapper/FreeImagePlus/src/fipWinImage.cpp#L407
in case of the image transparent:
https://github.com/WinMerge/freeimage/blob/78364ed94b4cb2d830805a26cb65bf5e6f523d58/Wrapper/FreeImagePlus/src/fipWinImage.cpp#L399

Problem is that a copy of a new display including transparency is created and this copy
is not updated anymore. Even if it would be updated it would slow down the wipe effect due to recreation of the transparency each time :-(.

[Edi61]

I have found one solution, maybe not the best, but seems principially working.
If there is transparency speed up is a little lower.