Android OpenGL ES 2.0-旋转到位

Question

我想在Z轴上旋转三角形，但要绕三角形的中心而不是屏幕的中心。 我对此进行了大量研究，但未找到任何解决方案。

有什么想法可以解决吗？

public class RendererClass implements Renderer {

FloatBuffer bufferObj1;
FloatBuffer bufferObj2;

int programObj1;
int programObj2;

int positionObj1;
int positionObj2;

int rotationMatrixLocationObj1;
int rotationMatrixLocationObj2;

int colorObj1;
int colorObj2;

float[] rotationMatrixObj1 = new float[16];
float[] rotationMatrixObj2 = new float[16];

@Override
public void onSurfaceCreated(GL10 arg0, EGLConfig config) {

    GLES20.glClearColor(0.0f, 1.0f, 1.0f, 1.0f);    

    float[] vertexObj1 = {

            -0.8f, 0.8f,
            -0.8f, 0.0f,
             0.0f, 0.0f

    };

    float[] vertexObj2 = {

             0.8f, 0.8f,
             0.8f, 0.0f,
             0.0f, 0.0f

    };

    bufferObj1 = ByteBuffer.allocateDirect(vertexObj1.length*4).order(ByteOrder.nativeOrder()).asFloatBuffer().put(vertexObj1);
    bufferObj2 = ByteBuffer.allocateDirect(vertexObj2.length*4).order(ByteOrder.nativeOrder()).asFloatBuffer().put(vertexObj2);

    String vssObj1 = "attribute vec4 positionObj1;" +
                     "uniform mat4 rotationMatrixObj1;" +
                     "void main(){" +
                     "gl_Position = rotationMatrixObj1 * positionObj1;" +
                     "}";

    String vssObj2 = "attribute vec4 positionObj2;" +
                     "uniform mat4 rotationMatrixObj2;" +
                     "void main(){" +
                     "gl_Position = rotationMatrixObj2 * positionObj2;" +
                     "}";

    String fssObj1 = "precision mediump float;" +
                     "uniform vec4 colorObj1;" +
                     "void main(){" +
                     "gl_FragColor = colorObj1;" +
                     "}";

    String fssObj2 = "precision mediump float;" +
                     "uniform vec4 colorObj2;" +
                     "void main(){" +
                     "gl_FragColor = colorObj2;" +
                     "}";

    int vsObj1 = glCreateShader(GL_VERTEX_SHADER);
    int vsObj2 = glCreateShader(GL_VERTEX_SHADER);

    int fsObj1 = glCreateShader(GL_FRAGMENT_SHADER);
    int fsObj2 = glCreateShader(GL_FRAGMENT_SHADER);

    glShaderSource(vsObj1, vssObj1);
    glShaderSource(vsObj2, vssObj2);

    glShaderSource(fsObj1, fssObj1);
    glShaderSource(fsObj2, fssObj2);

    glCompileShader(vsObj1);
    glCompileShader(vsObj2);

    glCompileShader(fsObj1);
    glCompileShader(fsObj2);

    programObj1 = glCreateProgram();
    programObj2 = glCreateProgram();

    glAttachShader(programObj1, vsObj1);
    glAttachShader(programObj2, vsObj2);

    glAttachShader(programObj1, fsObj1);
    glAttachShader(programObj2, fsObj2);

    glLinkProgram(programObj1);
    glLinkProgram(programObj2);

    positionObj1 = glGetAttribLocation(programObj1, "positionObj1");
    positionObj2 = glGetAttribLocation(programObj2, "positionObj2");

    rotationMatrixLocationObj1 = glGetUniformLocation(programObj1, "rotationMatrixObj1");
    rotationMatrixLocationObj2 = glGetUniformLocation(programObj2, "rotationMatrixObj2");       

    colorObj1 = glGetUniformLocation(programObj1, "colorObj1");
    colorObj2 = glGetUniformLocation(programObj2, "colorObj2");     

}

@Override
public void onSurfaceChanged(GL10 arg0, int width, int height) {

    Matrix.setIdentityM(rotationMatrixObj1, 0);
    Matrix.rotateM(rotationMatrixObj1, 0, MainActivity.angle, 0, 0, 1);     

    Matrix.setIdentityM(rotationMatrixObj2, 0);
    Matrix.rotateM(rotationMatrixObj2, 0, MainActivity.angle, 0, 0, -1);

}

GL10 gl;

@Override
public void onDrawFrame(GL10 glUnused) {

    GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);     

    onSurfaceChanged(gl, 0, 0);

    glUseProgram(programObj1);
    bufferObj1.position(0);
    glVertexAttribPointer(positionObj1, 2, GL_FLOAT, false, 0, bufferObj1);
    glEnableVertexAttribArray(positionObj1);
    glUniform4f(colorObj1, 0.0f, 0.0f, 1.0f, 1.0f);
    glUniformMatrix4fv(rotationMatrixLocationObj1, 1, false, rotationMatrixObj1, 0);
    glDrawArrays(GL_TRIANGLES, 0, 3);

    glUseProgram(programObj2);
    bufferObj2.position(0);
    glVertexAttribPointer(positionObj2, 2, GL_FLOAT, false, 0, bufferObj2);
    glEnableVertexAttribArray(positionObj2);
    glUniform4f(colorObj2, 1.0f, 0.0f, 0.0f, 1.0f);
    glUniformMatrix4fv(rotationMatrixLocationObj2, 1, false, rotationMatrixObj2, 0);
    glDrawArrays(GL_TRIANGLES, 0, 3);

}   

}

Answer 1

您可以使用glDrawArrays来更改代码中的三角形。 使用简单的三角函数可独立旋转顶点。

Answer 2

旋转矩阵使顶点绕坐标系的中心旋转。 如果要围绕其他点（x0，y0，z0）旋转场景，则必须通过3个简单步骤进行操作：

1. 以这样的方式转换场景，使您的点位于坐标系的中心（通过-x0，-y0，-z0）
2. 旋转
3. 向后翻译（按x0，y0，z0）

当然，不需要3个转换步骤，矩阵乘法可以为我们完成这项工作。