Portals are not lights in the traditional sense, as they produce no illumination by themselves. They are meant more to control lighting, especially that of interior renders that rely mainly on Global Illumination. Think of them as gathering rays and focusing them into a specific area. In this following example, the interior is illuminated solely by the environment, no lights are being used whatsoever. At the default settings, the image on the left exhibits some chattering and splotches, especially in the area of shadow transitions. In the second image on the right, a Portal has been added to the scene and set with 512 light samples. This produces a much smoother result with no other setting or value changes.
No Portal, 512 IC Samples
Portal Added with 512 Samples
The main use for a Portal is not, surprisingly, to be placed at openings; anywhere light would enter into an interior, such as a window, door, or skylight. For best results you should position the portal directly outside the opening itself (or inside depending on the shape of the opening) and scale it to be slightly larger. You can also adjust the shape of the portal to better represent the shape of the window, either as round or rectangular. You should also note it's better to use fewer portals than more, as this can add significantly to render calculation times. For example, when representing a multi-paned window, using a single Portal scaled to the over-all window size would be better than individual portals for each pane. You should also note that portals take their color from the environment behind them and therefore have no light material setting, like those of other direct light sources.
• Name - this data field displays the current item name. You can easily change it by clicking within the field and typing the new name.
The following Transform options are available for Portal:
• Position - allows you to numerically position the light item in XYZ space.
• Rotation - allows you to numerically set the rotation of the light item.
• Order - allows you to set the order that rotations are applied to the light item. Changing the order that rotations are applied can sometimes help to reduce or eliminate "gimbal lock".
• Scale - allows you to numerically set the size of the light item. This scale transform is a multiplier of the Height and Width options.
• Reset - resets the selected transform values to (0,0,0) returning the items back to their default state.
• Zero - resets the chosen transform property values to '0', leaving the Center position and Mesh position intact. This is done by adding an additional transform item to the Mesh Item's channels with an inverted version of the current values. This is useful to allow, for example, a joint to have a base value of 0,0,0 but still be located away from the World Origin.
• Add - adds additional Transform items to the associated item or, if they do not yet exist it simply adds them. Transform items are the channel groups that store the actual transform values controlling any items position, rotation, and/or scale. You can add as many transform items as desired for any transform property desired. Adding additional Transform items produces an additive effect where each transform group is evaluated before the next and so on. Additional item transforms are evaluated in their order in the Channels list, from the bottom upwards.
It should be noted that, by default, new items do not have any transform items associated with them (even though they are visible here within the Properties panel). This is useful as an optimization as only the necessary transforms are created on an as-needed basis, reducing scene overhead. There are several actions that add these base transform items. One is by simply transforming the target item with one of the various transform tools or by editing the values input fields. This action causes the particular transform items to be added automatically to the Channels viewport list. Due to this fact, you may need to specifically create item transforms when Referencing, because in order to override the channels in the master scene, they must first exist.
• Set Target - by selecting the light item, and a single additional item in the item list, and then pressing Set Target, this function allows you the ability to target specific items in a scene, automating the rotation of an item, so that it continuously points toward the targeted item. Once activated, additional options appear.
• Remove Target - removes the target link between the two items.
• Enable - toggling this option off temporarily disables the targeting function while retaining the link between the items.
• Set Focus Distance - sets the focal distance for DOF when targeting camera items to other item.
• Roll - offset the item's rotation angle away from the target.
• Time Offset - time the offset, by a number of frames, to set how the light follows the target item. It can either be delayed behind it with a negative value, or run ahead of it with a positive value.
The following Portal options are available for Portal:
• Render - allows you to select from three choices: when set to Default, you can enable/disable portals using the visibility function of the item list. When the Portal is visible, it contributes to the final rendered scene and when invisible, it does not. In some instances, you may prefer to fix this state, setting the Portal as On (enabled) or Off (disabled) regardless of visibility. Also useful for workflows that auto toggle visibility, saving you from manually enabling portals for test renders.
• Shape - determines the final shape of the Portal, set as either Rectangular or Elliptical. You can then adjust the Width and Height options to further define square, rectangular, circular, or oval type shapes.
• Width/Height - determines the actual size of the Portal. You should size the Portal to approximate the overall size of the opening it's representing.
• Samples - determines the portal's light gathering ability, having greater numbers of Samples increases the overall accuracy of the final rendered image, but also keeps in mind that additional rays also adversely affect render times, so you should adjust the rays to find a balance of quality versus render time.
• Environment Only - when this option is enabled, all sampling is taken from the Environment definition and any direct lights in the scene are ignored.
The following Light Linking options are available for Portal:
• Control Light Linking - illumination on a surface is generally controlled by the Shader item in the Shader Tree. Within the Shader, it is possible to control a lights affect on a surface with Light Linking. As its name describes, it links the illumination affects of a group of lights to specific Items or material groups. When the Control Light Linking option is enabled on a light item, it acts as an individual light-specific override to the Shader, allowing you to Include or Exclude a specific light's illumination on a group of items.
• Item Group - determines the specific group of item layers in the scene that is affected by the Light Linking. The group needs to be defined in the Groups viewport panel. This can be done easily by selecting the target items while in Items mode and then, in the Groups palette, click the New Group button. Define a name for the group in the pop-up dialog and choose the From Selected Items option, then click OK to accept. Once the group is defined, select the named group here.
• Mode - determines whether the light is Included, meaning it only affects the items in the specified item group, and Excluded by all other surfaces, meaning it is ignored by any items in the specified item group.