The camera is the view point of any scene. Using a camera, the view position, orientation, target, and aspect ratio can be set. In HOOPS Visualize, a camera is an attribute of a segment. As is true of any segment attribute, the camera is inherited by the children of the segment, unless they explicitly override it with their own locally set camera. Thus, each segment always has a net value for the camera attribute which used to view the geometry in that segment.
Like any attribute, the camera can be set multiple places in the database. However, since each segment has only one net value for the camera attribute, each piece of geometry in a segment can only be viewed by a single camera. Different segments in a scene can be viewed by different cameras, but each segment will be viewed only once.
There is an exception to the above - if a segment is used as an include segment, it will have its own net camera each time it is included. Thus, geometry in the include segment could be viewed from more than one angle if each had its own net camera.
Unlike most other attributes, the components of a camera inherit as a group. When you set a new camera, it does not inherit individual components from any camera attribute higher up in the database tree. Instead, the new camera completely overrides the inherited camera. A further consequence of this paradigm is that when changing a single camera setting for a segment with an inherited camera, the inherited camera is discarded and the default camera takes its place, incorporating only the new setting. However, this rule does not apply to segments which already have a local camera. If the segment already has its own local camera, its settings can be changed individually using the segment's HPS::CameraControl.
If you unset a camera attribute that was explicitly set on a segment, the segment goes back to inheriting the entire camera. You cannot unset an individual component of a camera.
See also the camera set-up guidelines.
A camera attribute consists of several components: position, target, up vector, field, and projection.
|position||The position of the camera is a point that represents the position of the viewpoint in world coordinates. Normally, the camera position is placed slightly away from the objects that you wish to view.||SegmentKey.GetCameraControl().SetPosition()|
|target||The target is a point, specified in world coordinates, at which the camera is looking. The camera target must not be at the same position as the camera. The vector between the camera position and the camera target is called the "line of sight".||SegmentKey.GetCameraControl().SetTarget()|
The up vector is a vector that defines "which way is up." If you specified only the camera's position and target, the camera's orientation of "up" would be arbitrary relative to the line of sight, so the up vector fixes the orientation of the camera. The up vector must not be all zeros, and cannot be parallel to the line of sight.
Logically, the up vector should be perpendicular to the line of sight. If it is not, the up vector is projected onto a plane perpendicular to the line of sight. The resulting projected vector is used to define the direction of the positive y-axis on the screen.
|field of view||
The field of view of the camera is comprised of two numbers, a width and a height, that define the minimum area around the target that will be visible in the output window. The camera field, along with the distance between the camera position and the camera target, determine - in photographic terms - what kind of lens the camera is using. If the field is larger than the distance from the camera position to the target, then we have the equivalent of a wide-angle lens. If the distance between the camera position and target is much larger than the camera field, then we have the equivalent of a telephoto lens. Changing the size of the field (if the camera position and target remain fixed) is the same as zooming the lens.
The ratio of the width to the height of the field is called the aspect ratio.
The projection determines how Visualize represents the 3D coordinates of a scene using only the 2D coordinates of the screen. A perspective projection scales the x and y coordinates depending on the z coordinate (depth), such that objects that are farther away appear smaller on the screen. In an orthographic projection, the direction of projection is perpendicular to the camera target plane (so, the x and y coordinates aren't scaled). Other projections are stretched, oblique perspective, and oblique orthographic. This section covers oblique projections, and a subsection therein reviews stretched projections.
Reading the camera components
Often it is necessary for an application to inspect the values of the camera components in order to make a decision. To do this, simply "show" the values into a HPS::CameraKit using the HPS::SegmentKey. For example, if you need to get the current location of the camera:
The default camera
Since a camera is an attribute, Visualize supplies a default value for it. The default camera is positioned at [0, 0, -5] with a target of [0, 0, 0] and an up vector of [0, 1, 0]. The field of the camera is 2.0 units wide and 2.0 high, and the projection is perspective. The default camera is inherited down the scene graph by all nodes until a segment's local camera overrides it.
The camera field of 2 units wide by 2 units high, centered at the origin, indicates the limits of the camera's view. In Visualize, this coordinate system is referred to as the window coordinate system. The image below shows how the default camera would map window coordinates onto the screen.
Mapping the camera field to the window
If the camera field is changed, then the portion of world space visible in the window will change. For example, we might have a scene that contains objects that range from +5 to -5 in both x and y (in world coordinates). Using the default camera, we would see only those objects that are within 1 unit of the origin. We can change the camera field such that its width and height are both 10 units (between +5 and -5).
The center of the output window always corresponds to the target position in world coordinates.
Of course, the units/range applied to the world coordinates viewable in the window is completely arbitrary. For example, a molecular-modeling application might use coordinates with magnitudes such as 550 to represent nanometers, and would thus set the camera field to a size appropriately small; a civil engineering application might set the camera field to be 100000 meters wide, so that it could view an area 100 meters across.
It is usually a good idea to scale your application's units such that your coordinates are neither extremely small nor immensely large. For example, it would probably be a bad idea to choose your units to be meters for an astronomy application (so you would have coordinates that were very large) or a molecular-modeling application (so you would have coordinates that were very small). Even though coordinates are floating-point numbers in Visualize, most of the defaults work best with coordinates that range close to +1.0 and -1.0. You can also run into numerical-accuracy problems when you mix numbers with different scales in the same computation.
Changing the camera near plane
By default, Visualize will automatically adjust the near clipping plane to be as close to the camera target as possible, which should generally result in good usage of z-precision. In special circumstances, you may want to favor precision of certain objects at the expense of clipping other objects, and this can be achieved by manually setting the camera's near clipping plane to a positive value (this will disable the default 'auto-adjust' behavior).
You can change the camera near limit by calling the following function:
How you set up the camera in a scene can significantly affect the visual quality of the rendered result. To maximize visual quality and reduce artifacts like edge stitching and shine-through, we recommend that you follow the camera set up guidelines outlined in this section.
When you set up your camera, the distance from the camera position to the camera target should be 2.5 times the field width. This 2.5:1 camera ratio maximizes the z-buffer resolution around the camera target thus reducing the occurrence of edge stitching and shine-through. It also provides a commonly accepted level of foreshortening for perspective projections.
When possible, this general camera set up should be maintained even when zooming in, out, or to the extents of a specific object. For example, when zooming in, you should NOT actually zoom the camera because it effectively modifies the camera field thus changing the 2.5:1 ratio that you want to maintain. Instead, you can create the effect of zooming by dollying the camera. Specifically, you should reset the camera target to the middle of the object that you wish to view, modify the field as desired, and then move the camera position forward or back to maintain the 2.5:1 camera ratio.
If you deliberately want to set up an extreme field of view where the camera ratio is 5:1 or higher, additional steps are required to preserve the visual integrity of your scene. Although Visualize automatically sets the near clip plane to the front most object in the scene to maximize the z-buffer resolution, there are situations where this is not effective. For instance, if you have zoomed into a small part in a complex model, there maybe other parts that are not visible in the view frustum but whose bounding box is still closer to the camera position or possibly behind it. In this case, the automatic near plane adjustment will not be effective. You need to manually reset the near clip plane so that it is closer to the bounding boxes of the objects being viewed.
Even when the automatic near plane adjustment does increase the z-buffer resolution significantly, it cannot remove all the potential edge stitching and edge shine-through at extreme fields of view. In these cases, you may need to modify the face displacement option using the HPS::DrawingAttributeControl. Visualize does not automatically take into account extreme fields of view when normalizing the face displacement value, thus the default value can be too much at extreme field-of-view settings, causing edges to shine through when they should be hidden. You may need to reset the face displacement as the camera view distance gets greater. For example, with a camera ratio of 20:1, resetting the face displacement to 1 removes the edge shine-through. A general guideline is to set the face displacement to 20/camera_ratio.
In addition to tweaking the face displacement value, you can also use the HPS::DrawingAttributeControl to set the vertex displacement. These two set in conjunction with one another can be used to fine tune the visual quality of your scene reducing edge shine-through and stitching.
The ratio of the width to the height of a coordinate system is called the aspect ratio. For example, the default screen-coordinate system has an aspect ratio of 1 to 1 (as defined by the width and height of the camera field). A window on the screen also has an aspect ratio. If the aspect ratio of the screen window exactly matches the aspect ratio of the camera field, then the camera field fits perfectly into the window.
When the output window is resized, the aspect ratio of the screen window may change. If the aspect ratio of the window does not match the aspect ratio of the camera field, Visualize will center the camera field in the screen window, so that the whole camera field is visible. In effect, this means the camera field defines the minimum area around the target in the scene that is guaranteed to be visible in the output window. Visualize pads either the width or the height of the camera field as necessary to make the camera field fit the screen window.
For example, if the output window is resized such that it is 50 percent wider than it is tall (the aspect ratio becomes 1.5 to 1), then the y coordinates will range from -1.0 to 1.0 as before, but the x coordinate will range from -1.5 to 1.5. Visualize does not clip the scene to the camera field, so objects that are slightly outside of the camera field may become visible. Here, the camera field is indicated by dashed lines (these dashed lines do not actually appear in the output window):
As you resize the output window of an Visualize application, the output scene scales as the window gets resized, but the relationship of x to y coordinates does not change. Thus, as the output window gets wider or taller, a circle continues to look circular rather than getting fatter or skinnier (see stretched projections for an example of how to make objects scale nonuniformly to match the window).
It is possible to keep the aspect ratio of the output window constant, using:
When this option is in effect, the user can change the size of the output window, but the ratio of the window's width to height will remain fixed. However, this only applies to Visualize-created windows (HPS::StandAloneWindowKey), and not to windows created by the user and passed to Visualize (HPS::ApplicationWindowKey). Since virtually all applications will take the latter approach, this option is often not applicable.
Once you have a camera set up, you might want to move it around the scene. You can always move it by changing the individual attribute settings, but Visualize also provides a number of high-level routines to make it easier.
Zoom and dolly
Zooming and dollying are accomplished as shown below:
Note that the zoom level is itself not a camera setting. Zooming actually modifies the camera field - a zoom by a factor of 2.0 makes the camera field one-half as big, in both dimensions.
If you are using a perspective projection, then zooming in and out can change how objects look in perspective. Very wide camera angles (which act like a wide-angle lens) accentuate perspective and make objects look strange; very small camera angles (equivalent to a telephoto lens) reduce perspective. If you use a small enough camera angle, perspective will virtually disappear. An orthographic projection can be thought of as a camera infinitely far away with an infinitely large zoom factor.
The dolly command moves both the position of the camera and the camera target but maintains the up vector and field. Thus, the camera produces the same change in the view as would occur if you translated the scene in the opposite direction.
Notes on dolly versus zoom
Dollying the camera forward and back and zooming the camera in and out might seem to have a similar effect on a view, but the effects are actually quite different. If you are using an orthographic projection, then dollying the camera will not make the objects in the scene get larger or smaller. This is because in an orthographic view, the size of an object does not depend on that object's distance from the camera. To make objects larger or smaller in an orthographic projection, you need to zoom the camera (zooming changes the camera field).
In a perspective projection, zooming the camera in and out will make the objects larger or smaller, but it will also change the perspective in the scene. Dollying the camera forward and back will make objects in the scene larger or smaller without changing perspective, but if there is an object close in front of the viewpoint, then dollying the camera forward might put that object behind the camera (or, even more disconcerting, put the camera inside of the object). Likewise, dollying the camera back might put an object that used to be behind the camera in front of it, blocking the view.
Orbiting the camera around the target point is accomplished using the Orbit command. Orbiting produces the same change in the view as would occur if you rotated the scene the opposite direction about the target point.
The two arguments to Orbit are floating-point numbers. The first number is the amount to orbit around to the right (or, if negative, to the left). The second number is the amount to orbit up (or, if negative, down). If both arguments are non-zero, the left-right orbit is performed first. For example, if we start with the default camera, then the code below orbits the camera such that the camera is looking at the scene from the positive x axis.
If you orbit the camera up or down, the up vector is rotated by the same amount, so it remains perpendicular to the new line of sight. If you orbit the camera up 180 degrees (up and over the top), the scene will be upside down (with the up vector pointing in the negative y direction), but if you orbit the camera right 180 degrees, the scene will be right-side up (with no change to the up vector).
Like all calls to the camera movement functions, calls to Orbit work relative to the current camera position, so successive calls are cumulative. Two calls, each of which orbits the camera 10 degrees to the right, will orbit the camera a total of 20 degrees.
Imagine the camera positioned on a tripod. Without changing the position of the tripod, you can swivel the head of the tripod right and left, or up and down. This movement is called panning. Panning the camera changes the camera target, but leaves the camera position unchanged. In addition, if you pan up or down, the camera up vector is rotated an equivalent amount, so it remains perpendicular to the new line of sight.
The two arguments to Pan are the amount in degrees to pan to the right (or, if negative, to the left), and the amount to pan up (or, if negative, down). For example, if someone says "look, up in the sky, it's a...", you probably want the following command:
If both an up-down pan and a right-left pan are specified, then the right-left pan is performed first. Note that pan and orbit both rotate the camera, but orbit rotates the camera about the target point and changes the camera position, whereas pan rotates the camera about the camera position and changes the camera target.
The roll camera command rotates the camera about the line of sight, leaving both the camera position and target unchanged. It is equivalent to rotating the up vector. A positive roll rotates the camera counterclockwise, which makes the scene appear to rotate clockwise. Rolling the camera produces the same change to the view as would occur if you rotated the scene the opposite direction about the line of sight.
The following command rotates the camera 180 degrees, turning the scene upside-down.
In some cases, it may be necessary for certain objects to ignore transforms. Consider the case of an axis triad which is a standard part of any scene. You would normally want the axis triad to rotate along with the coordinate system but not be affected by translations or zooming. In order to do this, HOOPS Visualize provides the concept of transform masks. Setting a transform mask enables a segment to ignore transformations that would otherwise be applied to the entire scene.
In the code sample below, axisSegment is a subsegment of modelSegment. Normally, this would mean that the axis triad would inherit that transformation of its parent segment. However, with transform masks, we can disable the translation and scaling:
The flexibility of transform masks becomes apparent when you have a deep scene hierarchy. Even though a transform mask may be applied to a particular segment, its subsegments can still make use of the original transform if the developer disables the mask on that segment.
So far, we have talked about only orthographic and perspective projections. Visualize also provides a stretched projection, as well as the ability to skew any projection to form an oblique projection.
An orthographic projection is typically used in drafting applications so that objects do not get smaller as they get farther away, and so that parallel lines remain parallel. Unfortunately, a regular orthographic projection can cause some lines to be hidden. For example, in the following figure, viewing a cube straight on in an orthographic view causes it to look like a square.
In an oblique orthographic view, the x and y coordinates are skewed depending on the z coordinate. For example, a typical oblique orthographic view moves objects up and to the right (but does not make them smaller) as they get farther away. We can thus see the sides of the cube, even though we are still viewing it straight on. Oblique projections are created by setting the skew parameters of SetProjection:
An oblique-perspective projection is useful when the target plane of a perspective projection is not perpendicular to the line of sight. There are a few, albeit specialized, situations where this kind of projection can be useful.
For example, consider a graphics system with three display monitors arranged side by side to display a panoramic view of a single scene. Logically, the three monitors are displaying a single view, but physically we need to create three separate views, one for each monitor. For the two side monitors, the screen is not perpendicular to the line of sight, so we must use a target plane (which is always parallel to the screen) that is not perpendicular to the line of sight. The following image shows the situation viewed from above (looking down the y axis).
To make this setup work, the target plane is rotated about the y axis for the side views using an oblique-perspective projection.
To determine the proper angle to rotate the target plane for each monitor, we take the offset from the camera target to the center of the monitor, divided by the distance from the viewpoint to the (entire) target plane, and take the arc tangent of the result. For example, if we are using a default camera for monitor 2, then monitor 1 is offset 2.0 units, and the distance from the viewpoint to the target plane is (the default) 5.0 units, which gives us arctan(2.0 / 5.0) = 21.8 degrees. For the camera corresponding to monitor 1, we issue the following command:
The camera for monitor 3 will have its target plane rotated -21.8 degrees.
The same trick may be useful even if you are not using multiple monitors. For example, consider a flight simulator used for training airplane pilots. Such simulators display the view that the pilot would see out of a window in a monitor positioned where the window would be. Often, these windows are not perpendicular to the pilot's line of sight, so an oblique-perspective view is required.
Another use for oblique-perspective views is for creating stereo images. To create a stereo image, we need to create two views of an object: one from the perspective of each eye. We already know how to create two views of the same object using two cameras. We offset each camera slightly left or right to approximate the position of each eye; however, then the line of sight for each eye is no longer perpendicular to the target. The following image illustrates the situation viewed from above.
We would rotate the target plane slightly for each eye with an oblique-perspective projection. However, it is not necessary for us to manually setup oblique perspective views to create a stereo image because Visualize provides built-in support for stereo viewing.
Stretched projections and 2D scenes
In our discussion on aspect ratio, we saw how Visualize keeps the aspect ratio of a scene constant, even when we change the dimensions of the window. The system keeps the aspect ratio constant by adding extra space to the camera field either on the sides or on the top and bottom.
To create a stretched projection, the camera projection must be set to Stretched.
With a stretched projection, the scene stretches to fit the output window. Why would we want that to happen? Do we not want our circles to remain circular and our squares to remain square? One case where we would use a stretched projection is to draw a border around the inside of a window. The snippet below draws a thick black border by drawing a black edge around the inside of the window.
Since the camera showing the geometry has a perspective projection, and the border has a stretched projection, the result is a scene that looks normal with a border that stretches to match the window size:
Another case where stretched projections are useful is when we want to place an object in a specific position of the output window, even if the output window is resized. For example, to place an object (such as a user-interface gadget) in the upper-right corner of the window, we could position it at x = 1, y = 1. However, it would only appear in the upper-right corner if the output window is square. By using a stretched projection, we can place objects accurately regardless of the aspect ratio of the output window. Another such use would be to place a toolbar along one of the sides of the window. The toolbar itself could then use an unstretched projection, so that the tools (buttons and sliders) would not stretch.
In certain cases, you may want 3D geometry to be rendered on top of other geometry regardless of its world position. For instance, when drawing leader lines, it is usually not desireable to have the leaders obscured. There are a few different ways to do this depending on your situation.
In the leader line case, or when drawing any other 3D geometry that doesn't fall into the categories above, the recommended procedure is to draw the geometry on the camera near plane. Calculating the near plane involves a call to HPS::WindowKey::ConvertCoordinates, using an output space parameter of HPS::Coordinate::Space::InnerWindowNormalized. This will give you the world coordinates of your object in normalized (-1 to 1) space. The X and Y are identical to window space coordinates, so once you have those normalized coordinates, you can simply set the Z value to 0 to make the object appear in front of all other objects in the view.
This camera near plane value is view-dependent, so if you transform your scene in any way, such as performing a rotation, the near plane value will need to be recomputed.
The above process works for both orthographic and perspective projections. Since perspective projections are not linear, a Z value of 1.0 represents an infinite distance from the camera, and therefore cannot be calculated using ConvertCoordinates. Setting the normalized Z coordinate to 0.999999 would cause Visualize to draw the object behind all other objects.