Camera and projector have many purposes.
A common use of the camera is for panning and zooming still images in order to add some movement.
A common use of the projector is for having multiple videos on the same canvas or for a Picture-in-Picture (PiP) effect.
A good way of "feeling" the differences between Camera and Projector is doing the following experiments:
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Load a clip or an image.
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Right click on the picon an select Resize track in the popup menu. Enter Size 500x200 or something like that. Leave Scale 1x1.
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Click on the thick green tick. Only a portion of your video is now shown on the compositor.
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Make sure your resized track is the first armed track of the stack.
(in this screenshot the Compositor window has been moved from its deafult position)
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Drag the image on the Compositor. You can choose what portion of the image you want to be shown just like if you were looking at the display of your videocamera. If you move your camera downward, the image move upward; if you move it to the left the image moves to the right.
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Now enable the projector instead (clicking on the button below the camera one). A red outline will appear.
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Drag the image. You can choose the position of your image inside the canvas just if you were looking at a Cinema screen. If you move the projector downward the light moves downdard and so does the image; if you move it to the left the image moves to the left.
Zooming
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Enable the Camera again and try zooming in and out by dragging the image while holding the Shift key down. You can select the portion of the image to display just if you were looking at the camera viewfinder while zooming in and out.
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Enable the Projector and try zooming in and out with the same command (Shift + drag). You can change the dimension of the image projected on the canvas just like if you were adjusting the display of a video on a cinema screen tweaking with the zoom of the videoprojector.
Experiment #2: The Camera is applied before the effects and the Projector after them
Part a:
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Write a title using the Title Effect (a white and big title, with no motion) an put it on an empty track.
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Make sure the track with the title is is the first armed track of the stack and the insertion point is inside the Title effect brown bar.
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Move and zoom it with the Camera: it has no effect.
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Move and zoom it with the Projector: you can change the position and size of your title.
Part b:
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Load a clip or an image.
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Apply the Wave Video Effect, and set pretty high values for Amplitude, Phase, Wavelength.
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Make sure this track is the first armed track of the stack and the insertion point is inside the edit.
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Drag the image around and zoom in and out (Shift + drag) using the Camera and the Projector to compare the different result in the Compositor. Moving the Camera changes the image you feed the effect with; moving the Projector changes the position of the same image in the canvas.