FOV not matching projection matrix

Question

I'm trying to write custom OpenGL renderer that produces the exact same images as blender. However I struggle with getting camera right.

I duplicated this code and I managed to produce the exact same matrix as in calc_matrix_camera

render = bpy.context.scene.render
depsgraph = bpy.context.evaluated_depsgraph_get()
proj_matrix = bpy.context.scene.camera.calc_matrix_camera(depsgraph,x=render.resolution_x,y=render.resolution_y,scale_x=render.pixel_aspect_x,scale_y=render.pixel_aspect_y)

I get the model-view matrix identical as in bpy.context.scene.camera.matrix_world but for testing purposes I always place camera at the origin point with no rotation (so model-view matrix is the identity matrix). However I still cannot get the renderings right. What I find suspicious is that

bpy.data.cameras['Camera'].angle_y

returns a different value from

2.0*math.atan( 1.0/proj_matrix[1][1])

(explanation of math here). I suspect there is some extra logic that I'm not taking into account. How do I get aspect ratio and fov from blender? Which of those values is correct?

Answer 1

There is indeed some extra logic. In general you should use camera.angle and take the settings under Camera Properties > Camera > Sensor Fit into account.

Lets set first

camera = bpy.data.cameras['Camera']
aspect = render.resolution_x / render.resolution_y

If the Sensor Fit is set to Horizontal you'll have (equaling angle_x)

camera.angle = 2.0 * math.atan(aspect / proj_matrix[1][1])

If it's set to Vertical you'll get (equaling angle_y)

camera.angle = 2.0 * math.atan( 1.0/ proj_matrix[1][1])

Finally, if it's set to Auto it depends on the aspect ratio:

If aspect > 1 then camera.angle = camera.angle_x = 2.0 * math.atan(aspect / proj_matrix[1][1])

If aspect < 1 then camera.angle = camera.angle_x = 2.0 * math.atan(1.0 / proj_matrix[1][1])

If aspect == 1 then camera.angle = camera.angle_x = 2.0 * math.atan(aspect / proj_matrix[1][1]) = 2.0 * math.atan(1.0 / proj_matrix[1][1])

For an in-depth explanation on what Blender is doing under the hood see my answer here.