java - OpenGL 透视图沿 z 轴扭曲场景远超预期

Question

我在 LWJGL 工作，并且一直在努力编写一个允许简单 3D 查看场景的程序。我遇到的主要问题是，每当我应用透视时，我的场景都会在 z 轴上拉伸(stretch)非常非常远。

中间的正方形是用正投影绘制的立方体(这里立方体的意思是“所有边都相等”)。从左下角到中心的形状也是一个立方体，但用透视投影绘制!

显然，这看起来不像一个立方体。这是我的代码(它有点像一堵墙，所以我把它打破了):

public class OpenGL
{
    //width and height of the screen, plus the depth of the 3d space
    public static final int WIDTH = 800, HEIGHT = 600, DEPTH = 1000;

    //called once at startup
    private static void initGl()
    {
        GL11.glEnable(GL11.GL_TEXTURE_2D);
        GL11.glEnable(GL11.GL_BLEND);
        GL11.glEnable(GL11.GL_DEPTH_TEST);

        GL11.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
        GL11.glViewport(0,0,WIDTH,HEIGHT);
        GL11.glDepthFunc(GL11.GL_LESS);
        GL11.glPolygonMode(GL11.GL_FRONT_AND_BACK, GL11.GL_FILL);
        GL11.glClearColor(0,0,0,0);
        GL11.glShadeModel(GL11.GL_SMOOTH);

        GL11.glMatrixMode(GL11.GL_PROJECTION);
        GL11.glLoadIdentity();
        GL11.glOrtho(0, WIDTH, HEIGHT, 0, DEPTH, -DEPTH);

        GL11.glMatrixMode(GL11.GL_MODELVIEW);

        DEFAULT_FONT.addAsciiGlyphs();
        DEFAULT_FONT.addGlyphs(400, 600);
        DEFAULT_FONT.getEffects().add(new ColorEffect(java.awt.Color.WHITE));
        try
        {
            DEFAULT_FONT.loadGlyphs();
        }
        catch (SlickException e)
        {
            e.printStackTrace();
        }
    }
}

这些都在另一个类中，每秒调用 30 次。

//main rendering function.
public void render()
{   
        //these two just input values for different variables, allowing for 
        //movement of the camera and testing different values of znear and zfar.  
        //nothing OpenGL-related is changed.
        update();
        updatePerspectiveTest();

        GL11.glMatrixMode(GL11.GL_MODELVIEW);

        p.render(); //draws the normal cube, the "player"

        if (perspective)
        {
            perspective();
        }

        GL11.glMatrixMode(GL11.GL_MODELVIEW);

        loadRotationalMatrix(yaw,pitch,roll);
        loadPositionalMatrix(-y,-x,-z);

        env.render(); //draws the distorted cube, the "environment"

        GL11.glLoadIdentity();

        ortho();

        //render debug data
}


//resets to orthographic projection
public static void ortho()
{
        int i = GL11.glGetInteger(GL11.GL_MATRIX_MODE);
        GL11.glMatrixMode(GL11.GL_PROJECTION);
        GL11.glLoadIdentity();
        GL11.glOrtho(-OpenGL.WIDTH/2, OpenGL.WIDTH/2, 
                OpenGL.HEIGHT/2, -OpenGL.HEIGHT/2, OpenGL.DEPTH/2, -OpenGL.DEPTH/2);
        GL11.glMatrixMode(i);
}

这是切换视角的功能。这可能就是问题所在。

//activate perspective projection. Maybe something wrong here?
public static void perspective()
{
    int i = GL11.glGetInteger(GL11.GL_MATRIX_MODE);
    GL11.glMatrixMode(GL11.GL_PROJECTION);
    GL11.glLoadIdentity();
    GL11.glFrustum(-OpenGL.WIDTH/2, OpenGL.WIDTH/2, 
                OpenGL.HEIGHT/2, -OpenGL.HEIGHT/2, 
                pzNear, pzFar);
    GL11.glMatrixMode(i);
}

这些函数用于转换。它们可能不相关，但我认为包含它们是件好事。

//translates the camera via matrix multiplication.
public static void loadPositionalMatrix(float x, float y, float z)
{
    FloatBuffer m = BufferUtils.createFloatBuffer(16);

    m.put(new float[]
            {
            1, 0, 0, 0,
            0, 1, 0, 0,
            0, 0, 1, 0,
            x, y, z, 1
            });

    m.flip();

    GL11.glMultMatrix(m);
}

    //sets yaw, pitch and roll using rotational matrices. Not really being used now.
public static void loadRotationalMatrix(double pitch, double yaw, double roll)
{
        FloatBuffer Ry = BufferUtils.createFloatBuffer(16);
        FloatBuffer Rx = BufferUtils.createFloatBuffer(16);
        FloatBuffer Rz = BufferUtils.createFloatBuffer(16);

        Rx.put(new float[]
                {
                1, 0, 0, 0,
                0, (float) cos(pitch), (float) sin(pitch), 0,
                0, (float) -sin(pitch), (float) cos(pitch), 0,
                0, 0, 0, 1
                });
        Ry.put(new float[]
                {
                (float) cos(yaw), 0, (float) -sin(yaw),  0,
                0, 1, 0, 0,
                (float) sin(yaw), 0, (float) cos(yaw), 0,
                0, 0, 0, 1
                });
        Rz.put(new float[]
                {
                (float) cos(roll), (float) sin(roll), 0, 0,
                (float) -sin(roll), (float) cos(roll), 0, 0, 
                0, 0, 1, 0,
                0, 0, 0, 1
                });

        Rx.flip();
        Ry.flip();
        Rz.flip();

        GL11.glMultMatrix(Rz);
        GL11.glMultMatrix(Ry);
        GL11.glMultMatrix(Rx);
    }

我已经尝试过 zNear 和 zFar，但到目前为止我尝试过的值都没有产生正确的投影。

具体问题是:为什么第二个立方体看起来如此可怕地扭曲，我能做些什么来纠正它？

Answer 1

矩阵使用剪裁平面从 3D 环境投影到屏幕。因此，x 和 y 剪裁平面需要考虑窗口的纵横比和 FOV。所以你应该有这样的东西:

...
double fovyInDegrees = 50; // Change this if you want.
double ymax, xmax, aspectRatio;
aspectRatio = OpenGL.WIDTH / OpenGL.HEIGHT;
ymax = znear * Math.tan(fovyInDegrees * Math.PI / 360);
xmax = ymax * aspectRatio;
GL11.glFrustum(-xmax, xmax, -ymax, ymax, znear, zfar);
...

请注意，我刚刚将一些 C++ 代码更改为 Java，因此未经测试。不过它在我的代码中有效，所以应该没问题。