2 Rendering 2.1 Overview 2.3 The Renderer Interface

2.2 Viewers

This section summarizes viewer functionality as defined by the Viewer interface. Note that specific viewer implementations may provide significant additional functionality, such as interactive view control, keyboard and mouse event handling, or graphical fixtures such as coordinate axes, grids or transformers. The description of these extra features is beyond the scope of this document.

Rendering is triggered within a viewer by calling its rerender() method, which then initiates two rendering steps:

1.

prerendering: The viewer calls the prerender() method for all its renderables, and adds those which are visible into a render list;
2.

repainting: All components in the render list are redrawn using their render() method;

Another viewer method, repaint(), can subsequently be called to initiate repainting without invoking prerendering.

In general, rerender() should be called whenever there is a change in the graphical state of the renderables. This includes changes in geometry, color, or visibility. In the context of simulation, rerender() should be called after every simulation step.

Otherwise, repaint() should be called when the graphical state of the scene has not changed but the final screen display has, such as when the viewpoint is changed, or the display window is unhidden or resized. This is more efficient than calling rerender() because it avoids the overhead of the prerendering step.

The prerendering step is invoked directly within the rerender() method, whereas the repainting step is called in whatever thread implements the graphical display. Since, as described below, one of the functions of the prerendering step is to cache rendering information, rerender() should be called in synchronization with both the graphical display thread and whatever thread(s) (such as a simulation thread) might be altering the state of the renderables.

2.2.1 Render lists

A render list is implemented by the class RenderList and sorts renderable components into separate sublists depending on whether they are primarily opaque, transparent, 2d opaque, and 2d transparent. These designations are determined by examining the flags returned by the renderable’s getRenderHints() method, with TWO_DIMENSIONAL indicating a two dimensional component and TRANSPARENT indicating a transparent one. These sublists assist the viewer in rendering a scene more realistically. For example, in OpenGL, better results are obtained if opaque objects are drawn before transparent ones, and two dimensional objects are drawn after three dimensional ones, with the depth buffer disabled.

A viewer maintains its own internal render list, and rebuilds it once during every prerendering step, using the following algorithm:

⬇

RenderList list;

for (each renderable component r) {

list.addIfVisible (r);

}

The list’s addIfVisible() method calls the component’s prerender() method, and then adds it to the appropriate sublist if it is visible:

⬇

addIfVisible (IsRenderable r) {

r.prerender();

if (r is visible) {

add r to appropriate sublist;

}

A renderable’s visibility is determined as follows:

•

Any object implementing IsRenderable is visible by default.
•

If the object also implements HasRenderProps (as described in Section 2.3.9), then it is visible only if the RenderProps returned by getRenderProps() is non-null and the associated visible property is true.

As discussed in Section 2.2.3, a viewer can also be provided with an external render list, which is maintained by the application. It is the responsbility of the application, and not the viewer, to rebuild the external render list during the prerendering step. However, in the repainting step, the viewer will handle the redrawing of all the components in both its internal and external render lists.

2.2.2 Prerendering and Rendering

Prerendering allows renderables to

1.

update data structures anc cached data needed for rendering
2.

add additional renderables to the render list.

The caching of graphical state is typically necessary when rendering is performed in a thread separate from the main application, which can otherwise cause synchronization and consistency problems for renderables which are changing dynamically. For example, suppose we are simulating the motion of two objects, A and B, and we wish to render their positions at a particular time t. If the render thread is allowed to run in parallel with the thread computing the simulation, then A and B might be drawn with positions corresponding to different times (or worse, positions which are indeterminate!).

Synchronizing the rendering and simulation threads will aleviate this problem, but that means foregoing the speed improvement of allowing the rendering to run in parallel. Instead, renderables can use the prerender() method to cache their current state for later use in the render() method, in a manner analagous to double buffering. For example, suppose a renderable describes the position of a point in space, inside a member variable called myPos. Then its prerender() method can be constructed to save this into another another member variable myRenderPos for later use in render():

⬇

import maspack.geometry.Vector3d;

import maspack.render.*;

...

Vector3d myPos; // point position

float[] myRenderPos; // cached value for rendering

void prerender (RenderList list) {

...

myPos.get (myRenderPos); // save cached value

...

}

In the example above, the cached value is stored using floating point values, since this saves space and usually provides sufficient precision for rendering purposes.

As described in Section 2.4, objects can sometimes make use of render objects when rendering themselves. These can result in improved graphical efficieny, and also provide an alternate means for caching graphical information. If render objects are being used, it is recommmended that they be created or updated within prerender().

The prerender() method can also be used to add additional renderables to the render list. This is done by recursively calling the list’s addIfVisible() method. For example, if a renderable has two subcomponents, A and B, which also need to be rendered, then it can add them to the render list as follows:

⬇

void prerender (RenderList list) {

IsRenderable A, B;

...

// add both A and B to the render list

list.addIfVisible (A);

list.addIfVisible (B);

}

In addition to adding A and B to the render list if they are visible, addIfVisible() will also call their prerender() methods, which will in turn give them the opportunity to add their own sub components to the render list. Note that prerender() is always called for the specified renderable, whether it is visible (and added to the list) or not (since even if a renderable is not visible, it might have subcomponents which are). This allows an entire hierarchy of renderables to be rendered by simply adding the root renderable to the viewer.

Note that any renderables added to the render list within prerender() are not added to the primary list of renderables maintained by the viewer.

Because of the functionality outlined above, calls to prerender() methods, and the viewer rerender() method that invokes them, should be synchronized with both the graphical display thread and whatever thread(s) might be altering the state of the renderables.

As indicated above, actual object rendering is done within its render() method, which is called during the repaint step, within whatever thread is responsible for graphical display. The render() method signature,

⬇

void render (Renderer renderer, int flags)

provides a Renderer interface (Section 2.3) which the object uses to draws itself, along with a flags argument that specifies additional rendering conditions. Currently only one flag is formally supported, Renderer.HIGHLIGHT, which requests that the object be rendered using highlighting (see Section 2.3.3.1).

2.2.3 Viewer renderables and external render lists

Renderables can be added or removed from a viewer using the methods

⬇

void addRenderable (IsRenderable r);

void removeRenderale (IsRenderable r);

void clearRenderables();

If renderables are arranged in a hierarchy, and add their own subcomponents to the render list during prerender(), as described in Section 2.2.2, then it may only be necessary to add top level renderable components to the viewer.

It is also possible to assign a viewer an external render list, for which the prerendering step is maintained by the application. This is useful in situations where multiple viewers are being used to simultaneously render a common set of renderables. In such cases, it would be wasteful for each viewer to repeatedly execute the prerender phase on the same renderable set. It may also lead to inconsistent results, if the state of renderables changes between different viewers’ invocation of the prerender phase.

To avoid this problem, an application may create its own render list and then give it to multiple viewers using setExternalRenderList(). A code sample for this is as follows:

⬇

import maspack.render.*;

...

Viewer viewer1;

Viewer viewer2;

List<IsRenderable> renderables;

RenderList extlist = new RenderList();

// assign external list to the viewers

viewer1.setExternalRenderList (extlist);

viewer2.setExternalRenderList (extlist);

for (each simulation step) {

// execute the pre-render phase

syncrhonized (extlist) {

for (IsRenderable r : renderables) {

extlist.addIfVisibleAll (r);

}

viewer1.rerender();

viewer2.rerender();

}

The statement synchronize(extlist) ensures that calls to extlist.addIfVisible(r) (and the subsequent internal calls to prerender()) are synchronized with render() method calls made by the viewer. This works because the viewer also wraps its usage of extlist inside synchronize(extlist) statements.

Once the viewers have been assigned an external render list, they will handle the repainting step for its renderables, along with their own internal renderables, every time repainting is invoked through a call to either rerender() (as in the above example) or repaint().

2.2.4 Coordinate frames and view point control

A viewer maintains three primary coordinate frames for describing the relative locations and orientations of scene objects and the observing “eye” (or camera). These are the eye, model, and world frames.

The eye frame (sometimes also known as the camera frame) is a right-handed frame located at the eye (or camera) focal point, with the axis pointing toward the observer. The viewing frustum is located in the half space associated with the negative axis of the eye frame (and usually centered on said axis), with the near and far clipping planes designated as the view plane and far plane, respectively. The viewer also maintains a viewing center, typically located along the negative axis, and which defines the point about which the camera pivots in response to interactive view rotation.

The viewing frustum is defined by the view and far planes, in combination with a projection matrix that transforms eye coordinates into clip coordinates. Most commonly, the projection matrix is set up for perspective viewing (Figure 2.3), but orthographic viewing (Figure 2.4) is sometimes used as well.

Figure 2.3: Primary coordinate frames associated with the renderer, along with the viewing frustum and center point.

Figure 2.4: Primary coordinate frames with the projection matrix set up for orthographic viewing.

The model frame is the coordinate frame in which geometric information for rendered objects is specified, and the world frame is the base with respect to which the model and eye frames are defined. The model matrix is the $4\times 4$ homogeneous affine transform ${\bf X}_{MW}$ from model to world coordinates, while the view matrix is a $4\times 4$ homogeneous rigid transform ${\bf T}_{WE}$ from world to eye coordinates. The composition of ${\bf T}_{WE}$ and ${\bf X}_{MW}$ transforms a point from model coordinates ${}^{M}{\bf p}$ into eye coordinates ${}^{E}{\bf p}$ , according to:

$\left(\begin{matrix}{}^{E}{\bf p}\\ 1\end{matrix}\right)\;=\;{\bf T}_{WE}\,{\bf X}_{MW}\left(\begin{matrix}{}^{M}{% \bf p}\\ 1\end{matrix}\right).$

Note: the renderer assumes that points and vectors are column-based and the coordinate transforms work by pre-multiplying these column vectors. This is in constrast to some computer graphics conventions in which vectors are row based. Our transformation matrices are therefore the transpose of those defined with respect to a row-based convention.

Initially the world and model frames are coincident, so that ${\bf X}_{MW}={\bf I}$ . Rendering methods often redefine the model matrix, allowing existing object geometry or built-in drawing methods to be used at different scales and poses throughout the scene. The methods available for querying and controlling the model matrix are described in Section 2.3.8.

Figure 2.5: Determining the pose of the eye frame from the eye and center points, and an up vector parallel to the frame’s

axis.

Meanwhile, changing the view matrix allows the scene to be observed from different view points. A viewer provides the following direct methods for setting and querying the view matrix:

⬇

void setViewMatrix (RigidTransform3d TWE);

RigidTransform3d getViewMatrix();

void getViewMatrix (RigidTransform3d TWE);

where RigidTransform3d is defined in maspack.matrix and represents a homogenous rigid transformation. Instead of specifying the view matrix, it is sometimes more convenient to specify its inverse, the eye-to-world transform ${\bf T}_{EW}$ . This can be done with

⬇

void setEyeToWorld (RigidTransform3d TEW);

void setEyeToWorld (Point3d eye, Point3d center, Vector3d up);

void getEyeToWorld (RigidTransform3d TEW);

where Point3d and Vector3d are also defined in maspack.matrix and represent 3 dimensional points and vectors. The method setEyeToWorld(eye,center,up) sets ${\bf T}_{EW}$ according to legacy OpenGL conventions so that (with respect to world coordinates) the eye frame’s origin is defined by the point eye, while its orientation is defined such that the axis points from eye to center and the axis is parallel to up (see Figure 2.5).

Point3d is a subclass of Vector3d used to describe points in space. The only difference between the two is that they transform differently in response to rigid transforms (described by RigidTransform3d) or affine transforms (described by AffineTransform3d): point transformations include the translational component, while vector transformations do not.

The viewer also maintains a center position and an up vector that are used to modify ${\bf T}_{EW}$ in conjunction with the following:

⬇

void setEye (Point3d eye);

Point3d getEye();

void setCenter (Point3d center);

Point3d getCenter();

void setUpVector (Vector3d up);

Vector3d getUpVector();

Again with respect to world coordinates, setEye(eye) sets the origin of the eye frame while recomputing its orientation from the current values of center and up, while setCenter(center) and setUpVector(up) set center and up and recompute the eye frame’s orientation accordingly.

It is also possible to specify axis-aligned views, so that the axes of the eye frame are exactly aligned with the axes of the world frame. This can be done using

⬇

void setAxialView (AxisAlignedRotation REW);

AxisAlignedRotation getAxialView();

setAxialView() sets the rotational component of ${\bf T}_{EW}$ to REW, and moves the eye position so that the viewer’s center lies along the new axis. It also sets the up vector to the axis of REW, and stores REW as the viewer’s nominal axial view which can be used for determining default orientations for fixtures such as grids. AxisAlignedRotation defines 24 possible axis-aligned orientations, and so there are 24 possible axis-aligned views. Some of those commonly used in association with setAxialView() are:

X_Y	eye frame and world frame have the same orientaion
X_Z	eye frame - axes correspond to world frame -
Y_Z	eye frame - axes correspond to world frame -

There are several methods available to reset the viewing frustum:

⬇

void setPerspective (

double left, double right, double bottom, double top, double near, double far);

void setPerspective (

double fieldOfView, double near, double far);

void setOrthogonal (

double left, double right, double bottom, double top, double near, double far);

void setOrthogonal (

double fieldHeight, double near, double far);

The setPerspective methods create a perspective-based frustum (Figure 2.3). The first methods explicitly sets the left, right, bottom and top edges of the view plane, along with the (positive) distances to the near (i.e., view) and far planes, while the second method creates a frustum centered on the axis, using a specified vertical field of view. The setOrthogonal methods create an orthographic frustum (Figure 2.4) in a similar manner.

Information about the current frustum can be queried using

⬇

double getViewPlaneWidth();

double getViewPlaneHeight();

double getViewPlaneDistance();

double getFarPlaneDistance();

Vector3d getEyeZDirection();

boolean isOrthogonal();

For convenience, the viewer can also automatically determine appropriate values for the center and eye positions, and then fit a suitable viewing frustum around the scene. This is done using the renderables’ updateBounds() method to estimate a scene center and radius, along with the current value of the up vector to orient the eye frame. The auto-fitting methods are:

⬇

void autoFitOrtho(); // fit orthographic frustum

void autoFitPerspective(); // fit perspective frustum

void autoFit(); // fit frustum of the current type

These auto-fit methods also make use of a default vertical field-of-view, which is initially 35 degrees and which can be controlled using

⬇

double getVerticalFieldOfView();

void setVerticalFieldOfView (double fov);

Finally, the viewer’s background color can be controlled using

⬇

void setBackgroundColor (float[] rgba)

void setBackgroundColor (Color color);

Color getBackgroundColor();

float[] getBackgroundColor(float[] rgba);

2.2.5 Lights

Viewers also maintain scene lighting. Typically, a viewer will be initialized to a default set of lights, which can then be adjusted by the application. The existing light set can be queried using the methods

⬇

int numLights(); // number of lights currently in use

Light getLight (int idx); // get information for the idx-th light

int getIndexOfLight (Light light); // return index for light, or -1

where Light is a class containing parameters for the light. Lights are described using the same parameters as those of OpenGL, as described in Chapter 5 of the OpenGL Programming Guide (Red book). Each has a position ${\bf p}$ and (unit) direction $\bf d$ in space, a type, colors associated with ambient, specular and diffuse lighting, and coefficients for its attenuation formula. The attenuation formula is

$f=\frac{1}{C+Ld+Dd^{2}}$

where is the light intensity, is the distance between the light and the point being lit, and , , and are the constant, linear and quadratic attentuation factors.

Spot lights have the same properties as other lights, in addition to also having a cutoff angle $\theta$ and an exponent . The cutoff angle is the angle between the direction of the light and the edge of its cone, while the exponent, whose default value is 0, is used to determine how focused the light is. If $\bf{v}$ is the unit direction from the light to the point being lit, and if ${\bf d}\cdot{\bf v}\geq\cos\theta$ , then the point being lit is within the light cone and the light intensity in the above equation is multiplied by the spotlight effect, given by

$({\bf d}\cdot{\bf v})^{e}$

Since $|{\bf d}\cdot{\bf v}|\leq 1$ , a value of gives the least light reductuon, while valuse of increase the intensity of the spot light towards its center.

Information for a specific light is provided by a Light object, which contains the following fields:

enabled: A boolean describing whether or not the light is enabled.
type: An instance of Light.LightType describing the type of the light. Current values are DIRECTIONAL, POINT, and SPOT.
position: A 3-vector giving the position of the light in world coordinates.
direction: A 3-vector giving the direction of the light in world coordinates.
ambient: RGBA values for the ambient light color.
diffuse: RGBA values for the diffuse light color.
specular: RGBA values for the specular light color.
constantAttenuation: Constant term C in the attenuation formula
linearAttenuation: Linear term L in the attenuation formula
quadraticAttenuation: Quadratic term Q in the attenuation formula
spotCosCutoff: For spotlights, the cosine of the cutoff angle $\theta$ .
spotExponent: For spotlights, the exponent .

Each of these fields is associated with accessor methods inside Light.

An application can also add or remove lights using

⬇

void addLight (Light light);

boolean removeLight (Light light)

boolean removeLight (int idx);

To create a new light, the application instantiates a Light object, sets the appropriate fields, and then calls addLight(). Lights can be removed either by reference to the light object or its index.

Lights are updated by the viewer at the beginning of each repaint step. That means that for lights already added to the viewer, any changes made to the fields of the associated Light object will take effect at the beginning of the next repaint step.

2.1 Overview 2.3 The Renderer Interface

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