Problem with 2nd LUT when using .png overlay

I’m not sure if this is a bug, or if I’m exposing the limitations of LUT’s here. As I’m not a programmer, it’s difficult for me to tell what’s going on.

I’m using a sequence of filters, including a couple of LUT’s that you can see here. The first is a generic LUT for correction purposes; the second is is a creative LUT.


The text used in this video is a simple .png overlay with transparent background. As you can see, when I bring it in, the contrast of the overall video increases, particularly in regards to the shadow areas. The anomaly goes away after the overlay fades out. I’ve used .png overlays for the purpose several times now and haven’t noticed a problem like this before now.

The problem seems to be with that second LUT. When I disable it, the problem goes away. And it doesn’t matter what LUT I use. I have been collecting them from several sources now and it happens no matter which one I select. Deselecting the first one seems to have no effect. For the record, the 2nd LUT used in this instance is one converted from the HaldCLUT collection that was shared with me earlier.

Can anyone give an explanation as to what is going on here, and possibly how to remedy it? Using Text:Simple seems to be the easiest work around (I haven’t tried it yet), but I’d like to know what is causing it in the first place. TIA.

EDIT: I should add, I’m using 20.7.11 on a 64-bit Win10 machine.

It’s probably irrelevant, but just in case: Have you tried to change the order of the filters?

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I’ve tried moving those around with no change. However, I have noticed that it’s not just limited to that 2nd LUT. It seems to happen whenever any kind of LUT is applied.

Hey @shotcut, I’m wondering if this needs to be a report? I’ve been able to isolate the problem and re-create it consistently with different videos. Unless there’s an overlay mode, or some other setting that I’m not seeing, I’m ready to call this a bug. What do you think? Is there anything else that I need to do before that happens?

You can convert this into a bug report, but I will not be personally looking into this anytime soon.

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Thanks Dan. I can certainly appreciate the amount of time and energy it takes to build and improve Shotcut, and that you can’t necessarily track down every issue that comes up. For the record, this problem persists even when I use Text:Simple to generate text within a clip. So, any kind of transparent overlay affects the overall contrast levels when combined with a LUT. When the question comes up again (and I have to believe it will), I guess we just have to call it a known bug.

Just curious, what would it take to elevate the importance of this problem?

UPDATE: This problem goes away when you move Color Grading after the LUT in the Filters list. Of course, that creates additional problems if you’ve already color graded before adding the LUT. So moving forward, it’s important to do that step if combining transparency and a LUT.

I was not able to reproduce this using the description and screenshot provided and the LUTs I have. Even if I were able to, there is a strong probability I could not debug and fix it due to the complexity of image alterations and how they each interact. Sometimes you need to workaround and bake some things into an intermediate file.

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