Placing the Camera

### 6.4.1 Placing the Camera

The POV-Ray camera has ten different models, each of which uses a different projection method to project the scene onto your screen. Regardless of the projection type all cameras use the `location`, `right`, `up`, `direction`, and keywords to determine the location and orientation of the camera. The type keywords and these four vectors fully define the camera. All other camera modifiers adjust how the camera does its job. The meaning of these vectors and other modifiers differ with the projection type used. A more detailed explanation of the camera types follows later. In the sub-sections which follows, we explain how to place and orient the camera by the use of these four vectors and the `sky` and `look_at` modifiers. You may wish to refer to the illustration of the perspective camera below as you read about these vectors.

The perspective camera

#### 6.4.1.1 Location and Look_At

Under many circumstances just two vectors in the camera statement are all you need to position the camera: `location` and `look_at` vectors. For example:

     camera {
      location <3,5,-10>
      look_at <0,2,1>
     }

The location is simply the x, y, z coordinates of the camera. The camera can be located anywhere in the ray-tracing universe. The default location is `<0,0,0>`. The `look_at` vector tells POV-Ray to pan and tilt the camera until it is looking at the specified x, y, z coordinates. By default the camera looks at a point one unit in the z-direction from the location.

The `look_at` modifier should almost always be the last item in the camera statement. If other camera items are placed after the `look_at` vector then the camera may not continue to look at the specified point.

#### 6.4.1.2 The Sky Vector

Normally POV-Ray pans left or right by rotating about the y-axis until it lines up with the `look_at` point and then tilts straight up or down until the point is met exactly. However you may want to slant the camera sideways like an airplane making a banked turn. You may change the tilt of the camera using the `sky` vector. For example:

     camera {
      location <3,5,-10>
      sky   <1,1,0>
      look_at <0,2,1>
     }

This tells POV-Ray to roll the camera until the top of the camera is in line with the sky vector. Imagine that the sky vector is an antenna pointing out of the top of the camera. Then it uses the `sky` vector as the axis of rotation left or right and then to tilt up or down in line with the `sky` until pointing at the `look_at` point. In effect you're telling POV-Ray to assume that the sky isn't straight up.

The `sky` vector does nothing on its own. It only modifies the way the `look_at` vector turns the camera. The default value is `sky<0,1,0>`.

#### 6.4.1.3 Angles

The `angle` keyword followed by a float expression specifies the (horizontal) viewing angle in degrees of the camera used. Even though it is possible to use the `direction` vector to determine the viewing angle for the perspective camera it is much easier to use the `angle` keyword.

When you specify the `angle`, POV-Ray adjusts the length of the `direction` vector accordingly. The formula used is direction_length = 0.5 * right_length / tan(angle / 2) where right_length is the length of the `right` vector. You should therefore specify the `direction` and `right` vectors before the `angle` keyword. The `right` vector is explained in the next section.

There is no limitation to the viewing angle except for the perspective projection. If you choose viewing angles larger than 360 degrees you'll see repeated images of the scene (the way the repetition takes place depends on the camera). This might be useful for special effects.

The `spherical` camera has the option to also specify a vertical angle. If not specified it defaults to the horizontal angle/2

For example if you render an image with a 2:1 aspect ratio and map it to a sphere using spherical mapping, it will recreate the scene. Another use is to map it onto an object and if you specify transformations for the object before the texture, say in an animation, it will look like reflections of the environment (sometimes called environment mapping).

#### 6.4.1.4 The Direction Vector

You will probably not need to explicitly specify or change the camera `direction` vector but it is described here in case you do. It tells POV-Ray the initial direction to point the camera before moving it with the `look_at` or `rotate` vectors (the default value is `direction<0,0,1>`). It may also be used to control the (horizontal) field of view with some types of projection. The length of the vector determines the distance of the viewing plane from the camera's location. A shorter `direction` vector gives a wider view while a longer vector zooms in for close-ups. In early versions of POV-Ray, this was the only way to adjust field of view. However zooming should now be done using the easier to use `angle` keyword.

If you are using the `ultra_wide_angle`, `panoramic`, or `cylindrical` projection you should use a unit length `direction` vector to avoid strange results. The length of the `direction` vector doesn't matter when using the `orthographic`, `fisheye`, or `omnimax` projection types.

#### 6.4.1.5 Up and Right Vectors

The primary purpose of the `up` and `right` vectors is to tell POV-Ray the relative height and width of the view screen. The default values are:

     right 4/3*x
     up y

In the default `perspective` camera, these two vectors also define the initial plane of the view screen before moving it with the `look_at` or `rotate` vectors. The length of the `right` vector (together with the `direction` vector) may also be used to control the (horizontal) field of view with some types of projection. The `look_at` modifier changes both the `up` and `right` vectors. The `angle` calculation depends on the `right` vector.

Most camera types treat the `up` and `right` vectors the same as the `perspective` type. However several make special use of them. In the `orthographic` projection: The lengths of the `up` and `right` vectors set the size of the viewing window regardless of the `direction` vector length, which is not used by the orthographic camera.

When using `cylindrical` projection: types 1 and 3, the axis of the cylinder lies along the `up` vector and the width is determined by the length of `right` vector or it may be overridden with the `angle` vector. In type 3 the `up` vector determines how many units high the image is. For example if you have `up 4*y` on a camera at the origin. Only points from y=2 to y=-2 are visible. All viewing rays are perpendicular to the y-axis. For type 2 and 4, the cylinder lies along the `right` vector. Viewing rays for type 4 are perpendicular to the `right` vector.

Note: that the `up`, `right`, and `direction` vectors should always remain perpendicular to each other or the image will be distorted. If this is not the case a warning message will be printed. The vista buffer will not work for non-perpendicular camera vectors.

##### 6.4.1.5.1 Aspect Ratio

Together the `up` and `right` vectors define the aspect ratio (height to width ratio) of the resulting image. The default values `up<0,1,0>` and `right<1.33,0,0>` result in an aspect ratio of 4 to 3. This is the aspect ratio of a typical computer monitor. If you wanted a tall skinny image or a short wide panoramic image or a perfectly square image you should adjust the `up` and `right` vectors to the appropriate proportions.

Most computer video modes and graphics printers use perfectly square pixels. For example Macintosh displays and IBM SVGA modes 640x480, 800x600 and 1024x768 all use square pixels. When your intended viewing method uses square pixels then the width and height you set with the `Width` and `Height` options or `+W` or `+H` switches should also have the same ratio as the `up` and `right` vectors.

Note: 640/480 = 4/3 so the ratio is proper for this square pixel mode.

Not all display modes use square pixels however. For example IBM VGA mode 320x200 and Amiga 320x400 modes do not use square pixels. These two modes still produce a 4/3 aspect ratio image. Therefore images intended to be viewed on such hardware should still use 4/3 ratio on their `up` and `right` vectors but the pixel settings will not be 4/3.

For example:

     camera {
      location <3,5,-10>
      up    <0,1,0>
      right  <1,0,0>
      look_at <0,2,1>
     }

This specifies a perfectly square image. On a square pixel display like SVGA you would use pixel settings such as `+W480 +H480` or `+W600 +H600`. However on the non-square pixel Amiga 320x400 mode you would want to use values of `+W240 +H400` to render a square image.

The bottom line issue is this: the `up` and `right` vectors should specify the artist's intended aspect ratio for the image and the pixel settings should be adjusted to that same ratio for square pixels and to an adjusted pixel resolution for non-square pixels. The `up` and `right` vectors should not be adjusted based on non-square pixels.

##### 6.4.1.5.2 Handedness

The `right` vector also describes the direction to the right of the camera. It tells POV-Ray where the right side of your screen is. The sign of the `right` vector can be used to determine the handedness of the coordinate system in use. The default value is: `right<1.33,0,0>`. This means that the +x-direction is to the right. It is called a left-handed system because you can use your left hand to keep track of the axes. Hold out your left hand with your palm facing to your right. Stick your thumb up. Point straight ahead with your index finger. Point your other fingers to the right. Your bent fingers are pointing to the +x-direction. Your thumb now points into +y-direction. Your index finger points into the +z-direction.

To use a right-handed coordinate system, as is popular in some CAD programs and other ray-tracers, make the same shape using your right hand. Your thumb still points up in the +y-direction and your index finger still points forward in the +z-direction but your other fingers now say the +x-direction is to the left. That means that the right side of your screen is now in the -x-direction. To tell POV-Ray to act like this you can use a negative x value in the `right` vector such as: `right<-1.33,0,0>`. Since having x values increasing to the left doesn't make much sense on a 2D screen you now rotate the whole thing 180 degrees around by using a positive z value in your camera's location. You end up with something like this.

     camera {
      location <0,0,10>
      up    <0,1,0>
      right  <-1.33,0,0>
      look_at <0,0,0>
     }

Now when you do your ray-tracer's aerobics, as explained in the section "Understanding POV-Ray's Coordinate System", you use your right hand to determine the direction of rotations.

In a two dimensional grid, x is always to the right and y is up. The two versions of handedness arise from the question of whether z points into the screen or out of it and which axis in your computer model relates to up in the real world.

Architectural CAD systems, like AutoCAD, tend to use the God's Eye orientation that the z-axis is the elevation and is the model's up direction. This approach makes sense if you're an architect looking at a building blueprint on a computer screen. z means up, and it increases towards you, with x and y still across and up the screen. This is the basic right handed system.

Stand alone rendering systems, like POV-Ray, tend to consider you as a participant. You're looking at the screen as if you were a photographer standing in the scene. The up direction in the model is now y, the same as up in the real world and x is still to the right, so z must be depth, which increases away from you into the screen. This is the basic left handed system.

#### 6.4.1.6 Transforming the Camera

The various transformations such as `translate` and `rotate` modifiers can re-position the camera once you've defined it. For example:

     camera {
      location < 0, 0, 0>
      direction < 0, 0, 1>
      up    < 0, 1, 0>
      right   < 1, 0, 0>
      rotate  <30, 60, 30>
      translate < 5, 3, 4>
     }

In this example, the camera is created, then rotated by 30 degrees about the x-axis, 60 degrees about the y-axis and 30 degrees about the z-axis, then translated to another point in space.