There are two pins, “opacity” and “opacity mask”
Opacity is for translucent, Opacity Mask is for masked opacity and will function with dither.
Again, there is only one velocity buffer. So when the renderer attempts to compensate for motion (such as when using TAA, motion blur, or any other effect that relies on motion vectors), it can only correctly compensate each pixel for one amount of motion. The glass up close will move only a small amount, while the background moves a lot. This is why the blurry artifact occurs - it is attempting to compensate for large, distant motion when in fact the glass is much closer.
No output of velocity:
Note that the amount of blur is equal to the very distant background. Correct for the mountains, but excessive for the sphere.
By drawing the glass into the velocity buffer as mentioned previously, the opposite will occur. The engine will correctly compensate the motion of the glass, but the motion of the distant background will be undercompensated. However, I think you will find this artifact far less offensive.
Outputting glass velocity:
Note that the background is no longer blurred as much as it should be, because the sphere’s velocity relative to the camera is very low. While technically wrong, this is much less noticeable.
The artifact can also be dramatically reduced by limiting the distance between the glass and it’s background. Your example is a worst case scenario, as the background is essentially infinitely far from the glass pane. Here’s an example:
Above you can see the artifact is much smaller against the nearby cube vs the distant background. This is because the relative motion vectors of these surfaces are much closer to each other.
Alternatively, you could set the translucency pass setting to “after motion blur”
This will cause the material to render after temporal effects. While this will fix all blurring, it will also cause the material to lack any motion blur or anti aliasing. This is usually a better option than disabling those features completely.
Here’s what that looks like:
Note the correct amount of background blur, but lack of any blur on the sphere. Unfortunately, no antialiasing is a pretty big and obvious downside to this method, as you can clearly see the stair stepping artifacts on the edge of the object.
Dithered Opacity with DLAA/DLSS:
The result here is pretty similar to translucent with velocity output with too little blur in the background, but the correct blur on the sphere. The main advantage of dithering is it is less expensive than translucent materials.
Dithered Opacity with TAA:
This tends to result in ugly smearing if there are large depth differences between the object and background.