Question

我试图围绕Y轴旋转三角形。当我围绕Z轴旋转时，一切都很好。但是当我尝试围绕Y轴旋转时，我得到的是一个半角三角形，围绕Y轴旋转。我使用的是PowerVRs OpenGL ES 2.0 SDK。我的初始化和绘图功能如下。

int Init(ESContext* esContext)
{
    UserData* userData = (UserData *)esContext->userData;
    const char *vShaderStr =
        "attribute vec4 vPosition;  \n"
        "uniform mat4 MVPMatrix;"
        "void main()                \n"
        "{                          \n"
        "   gl_Position = MVPMatrix * vPosition;\n"
        "}                          \n";

    const char *fShaderStr =
        "precision mediump float;   \n"
        "void main()                \n"
        "{                          \n"
        "   gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0); \n"
        "}                          \n";

    GLuint vertexShader;
    GLuint fragmentShader;
    GLuint programObject;
    GLint linked;
    GLfloat ratio = 320.0f/240.0f;

    vertexShader = LoadShader(GL_VERTEX_SHADER, vShaderStr);
    fragmentShader = LoadShader(GL_FRAGMENT_SHADER, fShaderStr);

    programObject = glCreateProgram();

    if (programObject == 0)
        return 0;

    glAttachShader(programObject, vertexShader);
    glAttachShader(programObject, fragmentShader);

    glBindAttribLocation(programObject, 0, "vPosition");
    glLinkProgram(programObject);
    glGetProgramiv(programObject, GL_INFO_LOG_LENGTH, &linked);

    if (!linked)
    {
        GLint infoLen = 0;
        glGetProgramiv(programObject, GL_INFO_LOG_LENGTH, &infoLen);

        if (infoLen > 1)
        {
            char* infoLog = (char *)malloc(sizeof(char) * infoLen);
            glGetProgramInfoLog(programObject, infoLen, NULL, infoLog);

            free(infoLog);
        }

        glDeleteProgram(programObject);
        return FALSE;
    }

    userData->programObject = programObject;

    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);

    glViewport(0, 0, esContext->width, esContext->height);
    glClear(GL_COLOR_BUFFER_BIT);


    glUseProgram(userData->programObject);

    userData->angle = 0.0f;
    userData->start = time(NULL);
    userData->ProjMatrix = PVRTMat4::Perspective(ratio*2.0f, 2.0f, 3.0f, 7.0f, PVRTMat4::eClipspace::OGL, false, false);
    userData->ViewMatrix = PVRTMat4::LookAtLH(PVRTVec3(0.0f, 0.0f, -3.0f), PVRTVec3(0.0f, 0.0f, 0.0f), PVRTVec3(0.0f, 1.0f, 0.0f));
    return TRUE;
}



void Draw(ESContext *esContext)
{
    GLfloat vVertices[] = {0.0f, 0.5f, 0.0f,
                          -0.5f, -0.5f, 0.0f,
                           0.5f, -0.5f, 0.0f};

    GLint MVPHandle;
    double timelapse;

    PVRTMat4 MVPMatrix = PVRTMat4::Identity();
    UserData* userData = (UserData *)esContext->userData;

    timelapse = difftime(time(NULL), userData->start) * 1000;
    if(timelapse > 16.0f) //Maintain approx 60FPS
    {
        if (userData->angle > 360.0f)
        {
            userData->angle = 0.0f;
        }
        else
        {
            userData->angle += 0.1f;
        }
    }

    userData->ModelMatrix = PVRTMat4::RotationY(userData->angle);

    MVPMatrix = userData->ViewMatrix * userData->ModelMatrix;
    MVPMatrix = userData->ProjMatrix * MVPMatrix;

    MVPHandle = glGetUniformLocation(userData->programObject, "MVPMatrix");
    glUniformMatrix4fv(MVPHandle, 1, FALSE, MVPMatrix.ptr());

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, vVertices);
    glEnableVertexAttribArray(0);

    glClear(GL_COLOR_BUFFER_BIT);
    glDrawArrays(GL_TRIANGLES, 0, 3);
    eglSwapBuffers(esContext->eglDisplay, esContext->eglSurface);
}

Answer 1

PVRTMat4::Perspective(ratio*2.0f, 2.0f, 3.0f, 7.0f, PVRTMat4::eClipspace::OGL, false, false);将近剪裁平面放置在离3.0f单位远离相机的位置（通过the third argument）。

PVRTMat4::LookAtLH(PVRTVec3(0.0f, 0.0f, -3.0f), PVRTVec3(0.0f, 0.0f, 0.0f), PVRTVec3(0.0f, 1.0f, 0.0f));将相机置于（0,0，-3），回头看（0,0,0）。

您使用PVRTMat4::RotationY(userData->angle);直接生成模型矩阵，以便矩阵不进行转换。您正在绘制的三角形根据其几何形状保持在（0,0,0）上。

所发生的事情是，靠近相机的三角形部分比3个单位更接近剪裁平面。近剪裁平面的目的是有效地将相机的镜头相对于图像将被感知的位置放置。或者就像指定用户到屏幕的距离一样，如果您愿意的话。

因此，您需要将近剪裁平面靠近相机或将相机移离三角形。

在OpenGL ES 2.0中旋转时只显示半个三角形

1 个答案: