Fade in getting applied to some elements, despite no fade in?

roninopf · February 17, 2026, 10:14pm

For this project, I’m not intending to have any fade ins or outs, but, for some reason, a fade in is getting applied to SOME elements but not all of them. It’s most obviously with the blue background, which is a long image that has a keyframe on scrolling it across. I also don’t see it in the preview within the editor. Is there another spot I can check for how this is happening, or other info I can provide?

roninopf · February 22, 2026, 12:35am

I redid/recreated this as another project, and I’m finding that it’s specifically the Vertigo filter that’s causing the odd fade in. This isn’t reflected in the preview player, and it doesn’t seem like it should be a part of what this filter does. Is this a bug? Maybe an incorrect starting value for the Vertigo filter?

brian · February 22, 2026, 6:33pm

I’ve watched the 3 second video over and over. But I’m not seeing anything “fade in” - for the blue background or anything else. Can you explain in more detail what you are not expecting? Maybe something is being lost in translation.

shotcut · February 22, 2026, 7:01pm

Vertigo inherently does not alter the alpha channel. Here is the source code that shows it:

github.com/dyne/frei0r

src/filter/vertigo/vertigo.c

master


      
          for(y=h; y>0; y--) 
          {
            ox = inst->sx;
            oy = inst->sy;
            for(x=w; x>0; x--) 
            {
              i = (oy>>16)*w + (ox>>16);
              if(i<0) i = 0;
              if(i>=inst->pixels) i = inst->pixels;
              v = inst->current_buffer[i] & 0xfcfcff;
              alpha = *src & 0xff000000;
              v = (v * 3) + ((*src++) & 0xfcfcff);
              *dst++ = (v>>2) | alpha;
              *p++ = (v>>2);
              ox += inst->dx;
              oy += inst->dy;
            }
            inst->sx -= inst->dy;
            inst->sy += inst->dx;
          }

I think the nature of the effect lends itself to some unpredictability.

roninopf · February 22, 2026, 7:51pm

I took out all of the other elements, this shows it much more clearly. It’s going “from” black, but otherwise never does so during the rest of the video. That was my question, really.

Thanks for clipping that!

shotcut · February 22, 2026, 8:15pm

That is the nature of the effect, unfortunately. It factors in previous frames, but it does not know what to do at the beginning and uses black. When you look at this in preview, you can see this the very first time it plays after you start Shotcut. Otherwise, you are seeking and it has previous frames in its state. It is stupid and does not start with its first frame for the first couple of frames, and it does not know how or when to reset its state such as when playing or seeking in reverse in preview.

You can report it as a bug to GitHub - dyne/frei0r: A large collection of free and portable video plugins if you want to. Otherwise, you need to workaround it probably by applying it with and without certain layers or other filters, extending it a few frames, exporting it to an intermediate, import the intermediate, and trim a few frames from its beginning.

roninopf · February 22, 2026, 9:15pm

That makes sense. I appreciate the response! I’ll be thinking about those options. Consider this closed.