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MEL How-To #87

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How do I determine the coordinates for an object relative to the camera's viewport?

Firstly, I must acknowledge that this tutorial is based almost exclusively on the "screenSpace.mel" script generously distributed by 185vfx. Whereas their script determines a screen-space coordinate from a selected point, this tutorial outlines the steps required for going the other way. I am using their math, simply solved for the inverse of the equation.

When finding a world location to match a 2D coordinate for a camera view one must accept there is an infinite number of solutions to the problem. You can stretch a line from the camera to infinity where the entire line is represented by a single pixel on the screen. Thus, when plotting from screen-space to world-space you need to make a choice for where on that line you wish to solve. A common choice for this is a distance equal to (or just slightly greater than) the camera's near clip plane.

For this example Maya's default perspective camera - "persp" - will be used for the calculations. Here's the camera's values in the Attribute Editor. I've modified some attributes - such as Camera Scale - to show how they apply to the calculation; you wouldn't necessarily use the values I have set.

camera attributes
film back

And here's what it looks like. The dotted lines encompassing the larger area represent the camera's film gate, whereas the solid line represents the resolution gate. These two ratios - both different than the area that is being displayed by Maya - are significant depending on whether you wish to plot a point within the camera frustum, a point within the rendered image, or a point that you can see in Maya's viewport.

persp camera

The camera's scale and lens squeeze ratio will influence our results, so let's grab those first.

// Get the camera's scale and lens squeeze ratio
float $cs = `camera -q -cs persp`;
// Result: 1.616 //

float $lsr = `camera -q -lsr persp`;
// Result: 2 //

The camera's film aspect, expressed as a ratio between its horizontal and vertical fields of view, represents the area shown as the film gate. These two angles start our calculation.

// Get the camera's horizontal field of view
float $hfv = `camera -q -hfv persp`;
// Result: 27.407221 //

// Get the camera's vertical field of view
float $vfv = `camera -q -vfv persp`;
// Result: 23.344729 //

The angles are halved, as they represent the full viewing angle across the frame. We're interested in the angle from the center of the view to the edge of the frame, which is exactly half of the full viewing angle. Our example plots a point at the top-right corner of the view; to calculate a different position, use a portion of this angle. Negate the horizontal angle for the left side of the screen; negate the vertical for the bottom half.

Note: These values will be returned in the unit as defined by your Angular Unit Preferences. For consistency, we will represent all angle values in radians, and all linear values in centimeters. To assist us in this quest, we define three convenience procedures.

proc float angle_to_internal( float $angle )
{
  string $pref = `currentUnit -q -a`;
  if ( "deg" == $pref )
  {
    $angle = $angle * 0.0174532925;
  }

  return $angle;
}

proc float linear_to_internal( float $linear )
{
  float $factor = 1.0;
  string $pref = `currentUnit -q -l`;
  if ( "mm" == $pref ) $factor = 0.1;
  if ( "m" == $pref ) $factor = 100.0;
  if ( "inch" == $pref ) $factor = 2.54;
  if ( "ft" == $pref ) $factor = 30.48;
  if ( "yard" == $pref ) $factor = 91.44;

  return ( $linear * $factor );
}

proc float linear_to_ui( float $linear )
{
  float $factor = 1.0;
  string $pref = `currentUnit -q -l`;
  if ( "mm" == $pref ) $factor = 10.0;
  if ( "m" == $pref ) $factor = 0.01;
  if ( "inch" == $pref ) $factor = 0.3937007874;
  if ( "ft" == $pref ) $factor = 0.03280839895;
  if ( "yard" == $pref ) $factor = 0.01093613298;

  return ( $linear * $factor );
}

Now use these to express the field of view angles in radians.

$hfv = angle_to_internal( $hfv );
// Result: 0.478346 //

$vfv = angle_to_internal( $vfv );
// Result: 0.407442 //

Use the tan() trigonometric function to obtain the X and Y components of our camera-space vector.

Note also that we apply the camera's lens squeeze ratio here, modifying the X coordinate accordingly.

// Determine the X coordinate in 2D
float $x = tan($hfv/2.0);
// Result: 0.24384 //

// Apply the camera's lens squeeze ratio
$x = $x * $lsr;
// Result: 0.487681 //

// Apply the camera's scale
$x = $x * $cs;
// Result: 0.788092 //
// Determine the Y coordinate in 2D
float $y = tan($vfv/2.0);
// Result: 0.206587 //

// Apply the camera's scale
$y = $y * $cs;
// Result: 0.333845 //

The Z component for the vector will be 1.0, which is one unit in front of the camera. If Z was not 1.0, then the X and Y coordinates would have to be multiplied by the Z coordinate. For example, if we used a Z of l.618034 then the X and Y coordinates we calculate would both be multiplied by l.618034.

// Decide how far from the camera our point will be.
// This value can be anything, but if you need an object at the final
// coordinates to display in the viewport then this value must be
// within the camera's near and far clip planes.
float $z = 1.0;

// Maintain homogenous coordinates.
$x = $x * $z;
$y = $y * $z;

Here's our vector. This vector represents the position - in camera-space - at the top-right corner of the camera's film gate. Initially this vector is expressed using our UI units, so we use the procedure we defined earlier to convert it to Maya's internal (cm) units.

// Z is negated here because the camera's local view axis is {0.0,0.0,-1.0}.
vector $camera_space = << $x, $y, -z >>;
// Result: <<0.788092, 0.333845, -1>>  //

$camera_space = $camera_space * linear_to_internal( 1.0 );
// Result: <<78.809246, 33.384472, -100>>  //

Now we'll employ a couple of procedures borrowed from 185vfx. These take care of querying our camera's matrix attributes, and multiplying our vector by this matrix.

// Get a matrix
proc matrix screenSpaceGetMatrix(string $attr)
{
  float $v[]=`getAttr $attr`;
  matrix $mat[4][4]=<< $v[ 0], $v[ 1], $v[ 2], $v[ 3];
                       $v[ 4], $v[ 5], $v[ 6], $v[ 7];
                       $v[ 8], $v[ 9], $v[10], $v[11];
                       $v[12], $v[13], $v[14], $v[15] >>;
 return $mat;
}

// Multiply the vector v by the 4x4 matrix m.
proc vector screenSpaceVecMult(vector $v, matrix $m)
{
  matrix $v1[1][4]=<<$v.x, $v.y, $v.z, 1>>;
  matrix $v2[1][4]=$v1*$m;
  return <<$v2[0][0], $v2[0][1],  $v2[0][2]>>;
}

To move our camera-space coordinate to the corresponding location in the world we need simply transform it by the same matrix as applied to the camera.

Note: Here is where our consistency with Maya's (cm) units becomes important. Maya will always return the '.worldMatrix' attribute using its internal linear unit. If our own vector was not in (cm) we'd be mixing units and our results wouldn't be very useful.

// Query the camera's inclusive matrix.
matrix $camera_matrix[4][4] = screenSpaceGetMatrix("persp.worldMatrix");

// And apply it to our vector.
vector $world_space = screenSpaceVecMult( $camera_space, $camera_matrix );
// Result: <<78.809253, 247.057664, 509.212149>>  //

Convert our vector to our linear unit preference:

// Convert back from (cm) units to Maya's unit preference.
$world_space = $world_space * linear_to_ui( 1.0 );
// Result: <<0.788093, 2.470577, 5.092121>>  //

Let's plot a locator at this point to test our results:

spaceLocator -p ($world_space.x) ($world_space.y) ($world_space.z);

persp camera with locator


Acknowledgements

14 Oct 2003