Camera gets offset from viewport

[casey_hawley]

Max 2025.3 

When 'Tilt & Shift' is used in camera (VRay Physical Camera), the camera grid/view plane gets offset sometimes. See the pic (private info blurred). Even though my model is aligned in the viewport, when I comp the renders into the background image, the render is offset by that same amount. It doesn't match like it does in the viewport and I have to nudge things up or down.

Camera matching to g o o g l e street views can be a pain because they are stitched together and displayed as hemispherical, making them awful for 'Perspective Matching'. I end up having to fake the camera position to fit the model into the image. I don't always need to Tilt or Shift, so I'm not entirely clear why or under what circumstances it causes the viewport frame and camera plane to mismatch when relatively more extreme adjustments are made.

The camera in this scene has been physically tilted down, then both camera height and vertical shift were adjusted until the perspective mostly matched (mostly). This is far from the actual street view camera, which is typically at ~10' high and gyroscopically level. The view ratio is the same, I just can't figure out why this happens. 

Would ultimately like to find out if there are ways to avoid it.

[RobH2]

It's hard to discern exactly what the issue you are experiencing is because it sounds like it's difficult to explain fully. What you may be experiencing is just a byproduct of what happens when you try to match to background imagery that has been altered. Once it's been stretched, altered, warped, or whatever has occurred to it, perspective does not work anymore. 

If you ever took or studied "perspective drawing," you'll recall that there are vanishing points and the geometry has to meet them, or else the perspective is off or incorrect. For instance, if you go out and take a photo, don't alter it in any way; you can generally perspective match a camera to it really nicely. But if you take that same image and use "tilt/shift" to make the verticals parallel (which is frequently done for architectural images) you'll break the perspective, and there is no way to accurately do a camera match to it. Becasue, the perspective is broken. The camera can't figure out what to do if the real perspective rules are destroyed. So never correct the perspective of your images in Photoshop until after you've done all of your compositing. 

Therefore, it's unlikely you'll be able to get a good perspective match to Google Earth or Streetview imagery. It's been altered, and the perspective is broken. So I think the best you can expect is what you are doing now, you'll have to eyeball it and nudge it. 

[casey_hawley]

Thanks for your reply. Yes, under more precise circumstances everything works pretty well. But my backgrounds are typically street views, which we all know have broken perspective. This isn't about preserving correct perspective.

This is about how the camera frame can shift away from being aligned with the viewport frame, making it necessary to nudge the rendering when compositing, rather than a simple 1 to 1 overlay.

After looking up vertical shift, it is just that.. a shifting of the image plane away from the camera sensor IRL, so when pushed too far, things get more and more offset. I will need to figure out how to fake perspective matching without it. Sometimes the Perspective Match tools work on these types of images, but I usually need finer control and end up manually adjusting anyway.

But it sure would be nice to have a toggle to "fake" keeping the two frames aligned for situations like this!

[RobH2]

I'm glad you have zeroed in what might be causing the issues.

If you need more help post more info and I'll try but it looks like you now
know what's causing it.

Your suggestion is good. I'd like that "fake" feature as well. If you get
time, post that in the "Ideas" forum. Things get picked up from there and
implemented. And it's not only the popular ones.

[MartinBeh]

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@casey_hawley wrote:

Thanks for your reply. Yes, under more precise circumstances everything works pretty well. But my backgrounds are typically street views, which we all know have broken perspective. This isn't about preserving correct perspective.

As you say: Stitched images might be hard/impossible to match to a conventional camera, especially if they have been stitched badly.

But: if the image really is a well-behaved 360*180 latlong panorama with leveled horizon, you would not really need to match a camera, as the image could be directly mapped to the inside of a perfect sphere centered on the camera. Or am I missing something here?

Could you share one image for us to test?

[casey_hawley]

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@MartinBeh wrote:
...Or am I missing something here?

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Yes, I'm only talking about the viewport and camera frame mismatch that vertical shift causes.

I think there should be an option to force these two frames of reference to stay locked together because shifting the camera frame off of the 'sensor' (the viewport represents the camera sensor) is actually a hindrance to compositing. Since it's all logical inside software, an option to keep them locked together seems like an easy solution. However, if I can 'fake' vertical shift for compositing purposes without actually using it, then perhaps having this option isn't necessary.

I have been matching to these street views for over a year and my techniques have been evolving as I get better at it. Indeed, the matching is never 100% on stitched backgrounds, but it doesn't need to be for my work. As close as possible is good enough, but there is a limit. 😉 Vertical shift was useful for me in some instances, but it didn't seem to cause a problem before. I probably just didn't notice that it was only slightly offset. I only recently noticed this as something to avoid for compositing.