Maya Camera Attributes
The following image shows in green which Maya camera attributes are taken into account by Maxwell:
The near and far clipping distances are applied when "Auto Render Clip Plane" is off, just like in the Maya Software renderer; this feature is called Z-clip planes in the main Maxwell documentation. The horizontal and vertical film offsets are used to simulate a Shift Lens. Maxwell can only render perspective cameras, orthographic views are not currently supported.
Maxwell-Specific Camera Attributes
The plug-in adds a group called "Maxwell Render" at the bottom of the camera Attribute Editor. This contains the Maxwell-specific attributes:
Since the Maxwell camera is very similar to a real-life DSLR camera, most of the parameters in this group should be intuitive to anyone familiar with traditional photography: shutter speed, f/stop, ISO, exposure mode, diaphragm. The Camera section in the main Maxwell documentation contains a detailed explanation of how these features work.
The region settings allow you to render only a part of the frame. You can also do this by using Maya's region render feature, but sometimes it's more useful to be able to type in the region extents, rather than use the selection rectangle in the Render View. When Maya region rendering is used, it overrides any region settings defined at the camera level. The "Region Mode" attribute has 3 possible values:
- Full: region rendering is off.
- Region: only the specified region is rendered and the rest of the image is black.
- Blowup: the region is scaled to cover the full render resolution. Black bars are added to maintain the aspect ratio, if needed.
The "Custom Bokeh" settings allow you to change the aspect ratio of the bokeh effect, to simulate an anamorphic lens.
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If this group does not appear in your scenes, make sure "Auto Load" is checked for the Maxwell plug-in in the Plug-in Manager. |
Focus
Unlike other render engines, Maxwell always computes depth of field. This effect is influenced by 4 factors: the size of the objects, the focal length, the f/stop and the focus point. It is important to use realistic camera settings and model sizes in order to obtain correct renders.
The focus plane is always parallel to the camera film plane. By default, it is placed at the center of interest of the camera - the point around which the camera orbits when you manipulate it in the viewport. This point coincides to the aim locator when the camera controls are set to "Camera and Aim" or "Camera, Aim and Up". The f/stop number controls the depth of the focus volume, or how much it extends in front and behind the focus plane.
If you need to control the focus point independently of the aim point, you can check the "Manual Focus Distance" box in the Maxwell attributes group and adjust the "Focus Distance" attribute. The focus plane will still be perpendicular to the camera aim vector, but it will be placed at the specified distance from the camera position.
If you wish to move the focus to a certain object without altering the camera position, you can select both the camera and the object and click the "Focus to selection" button on the Maxwell shelf (
). If Manual Focus Distance is off on that camera, this will move the aim locator so that the focus plane contains the pivot of the selected object. If Manual Focus Distance is on, it will leave the aim locator in place and alter the Focus Distance attribute instead.
Exposure
The exposure of the rendered image is influenced by the shutter time, f/stop number and film ISO rating. The shutter time and f/stop also influence motion blur (if enabled) and depth of field, respectively. The ISO value acts simply as an intensity multiplier and doesn't alter any other aspects of the output (in a real DSLR, increasing the ISO also increases the electronic noise of the sensor, but Maxwell simulates a "perfect" sensor which does not have this unwanted behavior). To simplify set-up, the plug-in offers 4 exposure modes: manual, aperture priority, shutter priority and rotary shutter.
In manual mode, you can control shutter and f/stop independently, as illustrated in these images:
f/16, 1/20 s |
f/8, 1/20 s |
f/16, 1/10 s |
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You can see how going from f/16 to f/8 makes the image brighter, but also produces a more shallow field (more blur). Leaving the f/stop unchanged and increasing the exposure to 1/10 seconds produces a brighter image, but also increases the motion blur on the opening lid.
The priority modes which can be selected from the "Exposure Mode" dropdown allow you to keep the exposure constant while varying either the shutter time or the f/stop, but not both. The exposure is defined by the EV number, a standardized setting found on cameras which also offer priority modes. When using shutter priority, you can change the shutter time to control motion blur and the f/stop is adjusted automatically:
Shutter priority, EV 14, 1/10 s |
Shutter priority, EV 14, 1/20 s |
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Notice how the two images have equal brightness, but the motion blur is less pronounced when the shutter time is halved. However, since the f/stop is adjusted automatically to keep the exposure constant, lowering the shutter time also produces a more shallow field, as you can see by comparing the blur on the foremost lighter.
The situation is reversed when the priority mode is set to aperture: you can control the f/stop directly and the shutter time is computed.
Aperture priority, EV 14, f/18 |
Aperture priority, EV 14, f/10 |
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The brightness is not affected by going from f/18 to f/10, so in this mode you can control the depth of field without affecting exposure. Since in this case the shutter time is slaved to the f/stop, a lower f number also produces shorter exposure, so in the second image there's almost no motion blur.
The fourth mode, rotary shutter, can be configured separately by checking the "Use Rotary Shutter" box. A rotary shutter is a device found on movie cameras which is used to control the shutter time: a semicircular object rotates in front of the film, performing a complete rotation during a frame. The angle of the opening in the shutter determines how long the film is exposed to light: 90 degrees means that the shutter time is a quarter of the frame time, while a value of 360 degrees means there's no shutter at all and the film is exposed for the whole duration of the frame.
In this mode Maxwell overrides the shutter time value input by the user with the value computed from the angle (time = angle / 360 * frame time, with frame time = 1 / FPS). It then adjusts the film ISO value so that you get the same exposure as you would with the shutter, f/stop and ISO values that are specified in the camera parameters. This means that you can set up the exposure and depth of field as you would for a still shot and then control the motion blur independently, without affecting anything else in the render.
f/18, 1/10 s, ISO 500, rotary shutter off |
f/18, 1/10 s, ISO 500, shutter angle 90 |
f/18, 1/10 s, ISO 500, shutter angle 180 |
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Observe how turning on rotary shutter leaves the exposure and depth of field unchanged and allows you to control the motion trails in isolation. This is in contrast with the shutter priority mode, where changing the motion blur also affected the depth of field, and with the manual mode, where changing the blur also changed the image brightness.
The rotary shutter feature is incompatible with the aperture priority mode. If these settings are used in combination, the plug-in will issue a warning at export time and force the exposure mode to shutter priority.
Scripting and Keying the Maxwell Camera Attributes
The plug-in only creates Maxwell camera attributes when they are first set to a non-default value; they do not exist before that. This is done in order to prevent encumbering the scenes with custom attributes, but has the unfortunate side effect that the Attribute Editor controls for them do not behave like regular controls. You cannot right click them to lock the attribute, set keys etc. However, as soon as you change the value of an attribute, it gets created and it also appears under the "Extra Attributes" group, prefixed with "Mx".
The controls in this group behave like regular attribute controls, so you can use them for keying, locking, setting layer overrides etc.
If you want to manipulate these attributes from script, you need to first ensure that they exist. The plug-in provides a function called maxwellSetCustomScalar which creates the attribute if needed and sets it to the specified value, so use that instead of calling setAttr directly:
maxwellSetCustomScalar("double", "cameraShape1", "mxFstop", 8.0);
maxwellSetCustomScalar("long", "cameraShape1", "mxIso", 200);
The first parameter specifies the type of the attribute. Valid values are "long" for integers and enums, "double" for floating point numbers and "bool" for booleans. The following parameters are the node name, attribute name and value. For the complete list of custom attribute names, see the file maxwellCustomAttrs.mel in the scripts/others subdirectory of the Maya installation path.