How to calculate camera parameters to match a real photo

Discussion created by Sandy Scott on Aug 13, 2020
Latest reply on Aug 17, 2020 by Sandy Scott

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I'd like to place a render into a real photo, replacing an existing version of our product.

So, I'd like to know if any of you have a reliable method for doing this accurately, or a way to calculate the parameters for the (Solidworks) camera. I realise I can position the camera manually, but it's a pretty tedious process to get it close enough that it looks OK.

I've got as far as setting the image as the scene background, then finding the spot on the model that's at the exact centre of the photo, and fixing that as the camera's target, but that's only the start. I'm hoping for something (software? pen & paper algorithm?) that can take into the co-ordinates of a few points, both in the 3D model and their corresponding point in the 2D photos, and crunch the numbers to work out what the camera position, rotation and perspective should be.

Any tips?

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Sandy Scott Aug 17, 2020 4:59 AM
I figured it out - mostly within Solidworks.
1. On your photo, pick 4 points you can match up to points in the 3D model. These should not all be in a line or in the same plane.
2. Measure the co-ordinates of these 4 points with photo editing software. Note that photo co-ordinates are typically measured from the top-left corner.
3. In your 3D model, create a 3D sketch, we're going to draw the camera, it's target, the "Photo" and the 4 points.
  Draw a point for the camera position - I'd recommend putting this in roughly the right place, but don't apply any constraints to it.
  Draw a rectangle to represent the image, again it'll help avoid too much weirdness if you put it in roughly the right place. It should have just enough constraints to keep it a rectangle, but not position or orient it in space.
  Dimension this rectangle with the dimensions of the photo
  Draw a construction line between the midpoints of the two shortest edges.
  For each of the 4 points you found in your photo, we'll draw "virtual image points" in the rectangle.
  Draw the point
  Draw a line from the point to the left edge of the rectangle, parallel to the top edge
  Draw a line from the point to the top edge of the rectangle, parallel to the left edge.
  Add 2 dimensions to each point to match the measurements from the photo, except for the last point which should only have 1 dimension.
  There should be 4 points - 3 of them have 2 dimensions (horizontal & vertical) and 1 point has just one dimension (it doesn't matter if it's horizontal or vertical) for 7 dimensions total.
  Draw a line from the point to the camera point.
  Make a coincident constraint between this line the corresponding vertex on the 3D model.
  At this point the sketch should be fully defined.
  Draw lines from the camera point to both ends the line that goes across the middle. Create a driven dimension to measure the angle between these two lines. This will be the view angle for the camera.
4. Create a plane to define the "up" direction. This will be perpendicular to the left edge of your rectangle, and coincident with it's midpoint.
5. Create a camera:
  In the DisplayManager tab of the left sidebar, under Scenes, Lights and Cameras, right-click on Cameras and choose "Add Camera"
  Camera Type: "Aimed at target"
  Target Point - Target by selection: Checked
  Select the midpoint of the line that crosses the rectangle.
  Camera Position - Position by selection: Checked
  Select the point with all the lines radiating from it.
  Camera Rotation - Set roll by selection: Checked
  Select the plane
  Field of View - Perspective: Checked
  Set the angle to the value from the driven dimension you created.
  Aspect ratio: set to the dimensions of your photo
  Press OK
6. If the viewpoint doesn't switch to the correct camera view, right click on your camera and choose "Camera view"
7. Set Background
  Scenes, Lights and Cameras, right-click on Scene and choose "Edit Scene"
  Background: Image
  Browse for your photo.
  Press OK
8. All done!
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How to calculate camera parameters to match a real photo

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