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I have the following diagram that describes a camera model (I believe it is the pinhole camera model).

enter image description here

Wikipedia has an article about the image plane and virtual image, but these articles do not provide an intuitive description of the two, their differences and purposes.

For example, an image plane is described as follows

that plane in the world which is identified with the plane of the display monitor used to view the image that is being rendered

So, is the image plane just the plane of the camera (or phone, etc.) where the image is displayed?

The virtual image is defined as

In optics, an image is defined as the collection of focus points of light rays coming from an object. A real image is the collection of focus points actually made by converging rays, while a virtual image is the collection of focus points made by extensions of diverging rays. In other words, a virtual image is found by tracing real rays that emerge from an optical device (lens, mirror, or some combination) backward to perceived or apparent origins of ray divergences.

In the diagram above, there are rays coming from the object. The description above seems to be consistent with the virtual image (i.e. the imaginary plane between the pinhole of the camera and the actual 3d real object) in the diagram above.

Because the rays never really converge, a virtual image cannot be projected onto a screen. In contrast, a real image can be projected on the screen as it is formed by rays that converge on a real location.

What the heck is a real image in the diagram above?

After all these conclusions, I still don't get the whole picture (pun intended). What is an image plane? What is a virtual image? What are their differences? What are their purposes?

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After having researched and learned a little bit more about the pinhole camera model, I think I know the difference between the camera (or image) plane and the virtual plane (or image).

The virtual plane (or virtual image) doesn't physically exist, but it's only used to simplify the mathematical modeling and reasoning. How does it do that? The main simplification is that the 3D scene is projected on it without being reversed, while, on the camera (or image) plane, the projected representations of the 3D objects are reversed with respect to their original orientation (and this can actually be seen from the diagram I showed in my question above). The distance between the pinhole and the image plane (i.e. the focal length) and the distance between the pinhole and the virtual plane should be the same.

So, we can think of the virtual plane as a proxy for the image plane, so as to avoid to apply e.g. rotations after the 3d scene has been projected.

This web article may also help.

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