Houdini uses a mathematical pinhole camera to simulate a camera. Because a pinhole camera does not have in-camera effects such as depth of field and bokeh, you must be explicitly tell mantra to simulate them (see below).

Unlike some other packages, Houdini’s viewport view does not necessarily correspond to a camera. You can lock the viewport to a camera (so changing the view moves/reorients the camera), but normally the current view in the viewport is separate from any cameras. There are also many options to render the current view without creating a camera.

There are three types of blur.

Transformation blur blurs moving objects overall.

To blur the moving pieces of deforming geometry, there are two options.

Velocity blur uses the v velocity attribute on points in deforming geometry. This requires that the velocity attribute exist on the geometry. Since the attribute is not recalculated at each sub-frame, this method cannot blur deformation along a curve. This method works even if the topology of the geometry changes between frames (as with particle systems, where new points may be born or die between frames).

Geometry blur sends separate copies of the geometry to mantra for each sub-frame. This requires that the topology of the geometry does not change between frames. Since this method requires sending multiple copies of the geometry to mantra, it is necessarily slower.

See the parameter descriptions on the mantra render node for more information.

Bokeh simulates how different lenses, apertures, and reflections inside the camera body affect how elements blur outside the depth of field. It makes bright points in the out-of-focus background appear as large transparent discs, boxes, or hexagons (depending on the camera).

Mantra lets you use radial (circular) or box bokeh, or specify an image representing the shape to use for blurred bright spots.

To control bokeh on a camera:

1. Select the camera object.

2. In the parameter editor, click the Sampling tab.

3. Use the bokeh parameters to control the look of the out of focus elements.

   • Bokeh

   • Bokeh image file

   • Bokeh rotation

When specifying a bokeh image, it should be a gray scale image where white pixels are the shape and black pixels are the background. (You can use a color image, but mantra will automatically convert it to gray scale.)

This is useful for overlaying information, for example to display a slate over top of the view for flipbooking. This does not actually gate or gobo the camera/light, it only affects the viewer.

1. Create a geometry (SOP) network to generate the graphics.

   Houdini scales the portion of the mask geometry between 0 and +1 X and Y to the viewport size to create the overlay.

2. Create a camera and choose Edit Rendering Parameters from the gear drop-down menu.

3. In the Edit Parameter Interface window, expand the Viewport Display folder, select the View Mask folder, and click the add arrow. This will add the View Mask tab to the parameter editor.

4. In the camera node's parameter editor, click the View Mask tab.

5. Set the Mask aspect to control the gating of the view.

6. Set the Mask opacity.

7. If you created overlay geometry, click the + button next to the Mask overlay SOP parameter and choose the node containing the overlay geometry.

8. To see the overlay, look through the camera or light, and turn on Camera mask on the Guides and Markers tab of the display options window.

• To make the camera appear to shake during an animation, try these expressions in the camera’s Translate-Z parameter:

  Shake on 2s

  if($F%2, 1, -1) * easeout()

  Shake on 4s

  if( $F/2%2, if($F%2, 1, 0.5), if($F%2, -1, -0.5) ) * easeout()