深入浅出Java3D编程：打造丰富3D应用

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"Java3D编程是Sun Microsystems开发的一种客户端Java应用程序接口（API），用于使用Java渲染交互式3D图形。通过Java3D，开发者能够创建丰富多样的互动3D应用，从沉浸式游戏到科学可视化应用都有涵盖。本书适合中级至高级的Java开发者阅读，虽然之前的图形编程经验（如OpenGL和Swing）会非常有用，但并不是必需的。书中内容自成体系，每章都相对独立或明确引用相关章节，让读者可以快速定位到与问题相关的资料。对于新接触Java3D的读者，建议下载Sun提供的优秀（免费）Java3D教程。本书旨在作为Sun API文档和Java3D教程的补充。全书包含18章，外加3个附录和一个参考文献列表，内容安排有序，即便你是刚开始一个‘从零开始’的项目，也能循序渐进地学习。" Java3D编程的核心知识点包括： 1. **Java3D API基础**：理解Java3D API的基本结构，包括场景图、几何对象、变换、材质、光照和视图等元素。场景图是Java3D中的核心概念，用于组织3D模型和它们之间的关系。 2. **几何对象**：学习如何创建和操作3D几何形状，如立方体、球体、锥体、圆柱体等，以及自定义几何对象的方法。 3. **变换**：掌握平移、旋转和缩放等基本变换，以及组合变换，了解如何通过变换组来实现复杂的运动效果。 4. **材质和光照**：学习如何给3D对象添加颜色、纹理和透明度，以及设置光源以创建逼真的视觉效果。 5. **视图控制**：理解视图坐标系统和相机的概念，如何调整视口和相机位置以改变用户观察3D世界的视角。 6. **动画和交互性**：学习如何实现动态更新和时间依赖的动画，以及如何通过键盘、鼠标等输入设备与3D场景进行交互。 7. **碰撞检测**：了解如何检测3D对象之间的碰撞，这对于游戏开发和交互式应用尤为重要。 8. **性能优化**：探讨如何通过缓存、批处理和减少不必要的计算来提高Java3D应用的性能。 9. **用户界面集成**：学习将Java3D与Swing或JavaFX等用户界面库结合，创建混合2D/3D的复杂应用。 10. **应用案例**：通过实际案例，如游戏、虚拟现实环境、数据可视化等，学习如何将上述技术应用于实际项目。 11. **扩展和自定义**：了解如何编写Java3D扩展，以满足特定需求，例如新的几何形状、渲染算法等。 12. **调试和测试**：学习有效的调试技巧和测试策略，确保3D应用的稳定性和正确性。 13. **资源管理**：理解如何有效地加载、管理和释放3D模型、纹理和其他资源，以避免内存泄漏和性能问题。 14. **社区和资源**：利用书中的附录和参考文献，探索Java3D的在线资源、论坛、示例代码和开源项目，持续学习和解决问题。 15. **版本更新和兼容性**：关注Java3D的版本更新，了解新功能和改进，同时考虑不同Java版本和平台的兼容性问题。通过这本书，读者将获得构建复杂3D应用程序所需的知识和技能，并能够适应不断发展的Java3D生态系统。

From AwtRenderingEngine.java

private int xScreenCenter = 320/2;

private int yScreenCenter = 240/2;

private Vector3d screenPosition = new Vector3d( 0, 0, 20 );

private Vector3d viewAngle = new Vector3d( 0, 90, 180 );

private static final double DEG_TO_RAD = 0.017453292;

private double modelScale = 10;

CT = Math.cos( DEG_TO_RAD * viewAngle.x );

ST = Math.sin( DEG_TO_RAD * viewAngle.x );

CP = Math.cos( DEG_TO_RAD * viewAngle.y );

SP = Math.sin( DEG_TO_RAD * viewAngle.y );

public void projectPoint( Point3d input, Point3d output )

{

double x = screenPosition.x + input.x * CT − input.y * ST;

double y = screenPosition.y + input.x * ST * SP + input.y * CT * SP

+ input.z * CP;

double temp = viewAngle.z / (screenPosition.z + input.x * ST * CP

+ input.y * CT * CP − input.z * SP );

output.x = xScreenCenter + modelScale * temp * x;

output.y = yScreenCenter − modelScale * temp * y;

output.z = 0;

}

Let’s quickly project some points using this routine to see if it makes sense. The result of running seven 3D

points through the projectPoint method is listed in table 2.1.

CT: 1

ST: 0

SP: 1

CP: 0

Table 2.1 Sample output from the projectPoint method to project points from 3D−world coordinates to 2D−screen

coordinates

WX WY WZ SX SY

1 1 0 250 30

−1 1 0 70 30

1 −1 0 250 210

−1 −1 0 70 210

0 0 0 160 120

1 1 1 255 25

−1 −1 1 65 215

Figure 2.4 Compare the output from Java 3D (left) with the output from MyJava3D (right)

2.2.3 Drawing filled triangles

Java 3D rendered the hand as an apparently solid object. We cannot see the triangles that compose the hand,

and triangles closer to the viewer obscure the triangles further away.

You could implement similar functionality within MyJava3D in several ways:

Hidden surface removal

You could calculate which triangles are not visible and exclude them from rendering. This is typically

performed by enforcing a winding order on the vertices that compose a triangle. Usually vertices are

connected in a clockwise order. This allows the graphics engine to calculate a vector that is normal

(perpendicular) to the face of the triangle. The triangle will not be displayed if its normal vector is pointing

away from the viewer.

This technique operates in object space—as it involves mathematical operations on the objects, faces, and

edges of the 3D objects in the scene. It typically has a computational complexity of order n

where n is the

number of faces.

This quickly becomes complicated however as some triangles may be partially visible. For partially visible

triangles, an input triangle has to be broken down into several new wholly visible triangles. There are many

good online graphics courses that explain various hidden−surface removal algorithms in detail. Use your

favorite search engine and search on “hidden surface removal” and you will find lots of useful references.

Depth sorting (Painter’s algorithm)

The so−called Painter’s algorithm also operates in object space; however, it takes a slightly different

approach. The University of North Carolina at Chapel Hill Computer Science Department online course

Introduction to Computer Graphics (http://www.cs.unc.edu/~davemc/Class/136/) explains the Painter’s

algorithm (http://www.cs.unc.edu/~davemc/Class/136/Lecture19/Painter.html).

The basic approach for the Painter’s algorithm is to sort the triangles in the scene by their distance from the

viewer. The triangles are then rendered in order: triangle furthest away rendered first, closest triangle rendered

last. This ensures that the closer triangles will overlap and obscure triangles that are further away.

An uncomplicated depth sort is easy to implement; however, once you start using it you will begin to see

strange rendering artifacts. The essential problem comes down to how you measure the distance a triangle is

from the viewer. Perhaps you would

Take the average distance of each of the three vertices•

Take the distance of the centroid of the triangle•

With either of these simple techniques, you can generate scenes with configurations of triangles that render

incorrectly. Typically, problems occur when:

Triangles intersect•

Centroid or average depth of the triangle is not representative of the depth of the corners•

Complex shapes intersect•

Shapes require splitting to render correctly•

For example, figure 2.5 shows some complex configurations of triangles that cannot be depth sorted using a

simple algorithm.

Figure 2.5 Interesting configurations of triangles that are challenging for depth−sorting algorithms

The depth of an object in the scene can be calculated if the position of the object is known and the position of

the viewer or image plane is known. It would be computationally intensive to have to re−sort all the triangles

in the scene every time an object or the viewer’s position changed. Fortunately, binary space partition (BSP)

trees can be used to store the relative positions of the object in the scene such that they do not need to be

re−sorted when the viewpoint changes. BSP trees can also help with some of the complex sorting

configurations shown earlier.

Depth buffer (Z−buffer)

In contrast to the other two algorithms, the Z−buffer technique operates in image space. This is conceptually

the simplest technique and is most commonly implemented within the hardware of 3D graphics cards.

If you were rendering at 640 × 480 resolution, you would also allocate a multidimensional array of integers of

size 640 × 480. The array (called the depth buffer or Z−buffer) stores the depth of the closest pixel rendered

into the image.

As you render each triangle in your scene, you will be drawing pixels into the frame−buffer. Each pixel has a

color, and an xy−coordinate in image space. You would also calculate the z−coordinate for the pixel and

update the Z−buffer. The values in the Z−buffer are the distance of each pixel in the frame from the viewer.

Before actually rendering a pixel into the frame−buffer for the screen display, inspect the Z−buffer and notice

whether a pixel had already been rendered at the location that was closer to the viewer than the current pixel.

If the value in the Z−buffer is less than the current pixel’s distance from the viewer, the pixel should be

obscured by the closer pixel and you can skip drawing it into the frame−buffer.

It should be clear that this algorithm is fairly easy to implement, as long as you are rendering at pixel level;

and if you can calculate the distance of a pixel from the viewer, things are pretty straightforward. This

algorithm also has other desirable qualities: it can cope with complex intersecting shapes and it doesn’t need

to split triangles. The depth testing is performed at the pixel level, and is essentially a filter that prevents some

pixel rendering operations from taking place, as they have already been obscured.

The computational complexity of the algorithm is also far more manageable and it scales much better with

large numbers of objects in the scene. To its detriment, the algorithm is very memory hungry: when rendering

at 1024 × 800 and using 32−bit values for each Z−buffer entry, the amount of memory required is 6.25 MB.

The memory requirement is becoming less problematic, however, with newer video cards (such as the nVidia

Geforce II/III) shipping with 64 MB of memory.

The Z−buffer is susceptible to problems associated with loss of precision. This is a fairly complex topic, but

essentially there is a finite precision to the Z−buffer. Many video cards also use 16−bit Z−buffer entries to

conserve memory on the video card, further exacerbating the problem. A 16−bit value can represent 65,536

values—so essentially there are 65,536 depth buckets into which each pixel may be placed. Now imagine a

scene where the closest object is 2 meters away and the furthest object is 100,000 meters away. Suddenly only

having 65,536 depth values does not seem so attractive. Some pixels that are really at different distances are

going to be placed into the same bucket. The precision of the Z−buffer then starts to become a problem and

entries that should have been obscured could become randomly rendered. Thirty−two−bit Z−buffer entries

will obviously help matters (4,294,967,296 entries), but greater precision merely shifts the problem out a little

further. In addition, precision within the Z−buffer is not uniform as described here; there is greater precision

toward the front of the scene and less precision toward the rear.

When rendering using a Z−buffer, the rendering system typically requires that you specify a near and a far

clipping plane. If the near clipping plane is located at z = 2 and the far plane is located at z = 10, then only

objects that are between 2 and 10 meters from the viewer will get rendered. A 16−bit Z−buffer would then be

quantized into 65,536 values placed between 2 and 10 meters. This would give you very high precision and

would be fine for most applications. If the far plane were moved out to z = 50,000 meters then you will start

to run into precision problems, particularly at the back of the visible region.

In general, the ratio between the far and near clipping (far/near) planes should be kept to below 1,000 to avoid

loss of precision. You can read a detailed description of the precision issues with the OpenGL depth buffer at

the OpenGL FAQ and Troubleshooting Guide (http://www.frii.com/~martz/oglfaq/depthbuffer.htm).

2.3 Lighting effects

MyJava3D includes some simple lighting calculations. The lighting equation sets the color of a line to be

proportional to the angle between the surface and the light in the scene. The closer a surface is to being

perpendicular to the vector representing a light ray, the brighter the surface should appear. Surfaces that are

perpendicular to light rays will absorb light and appear brighter. MyJava3D includes a single white light and

uses the Phong lighting equation to calculate the intensity for each triangle in the model (figure 2.6).

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