从像素坐标到opengl坐标

Question

正如我在opengl中看到的那样，坐标在-1和1之间，我想使用opengl渲染对象，我在像素坐标中渲染对象的第一个位置，但是我不知道如何将像素坐标转换为opengl坐标。我正在使用Java开发Android应用程序。

公共类FirstOpenGLProjectRenderer实现GLSurfaceView.Renderer {

private static final String A_POSITION = "a_Position";
private static final String A_COLOR = "a_Color";
private static final int POSITION_COMPONENT_COUNT = 4;
private static final int COLOR_COMPONENT_COUNT = 3;
private static final int BYTES_PER_FLOAT = 4;
private static final int STRIDE =
        (POSITION_COMPONENT_COUNT + COLOR_COMPONENT_COUNT) * BYTES_PER_FLOAT;

private final FloatBuffer vertexData;
private final Context context;

private int program;
private int aPositionLocation;
private int aColorLocation;

private static final String U_MATRIX = "u_Matrix";
private final float[] projectionMatrix = new float[16];
private int uMatrixLocation;
private final float[] modelMatrix = new float[16];



public FirstOpenGLProjectRenderer(Context context) {
    this.context = context;


 //
    // Vertex data is stored in the following manner:
    //
    // The first two numbers are part of the position: X, Y
    // The next three numbers are part of the color: R, G, B
    //
    float[] tableVerticesWithTriangles = {
            // Order of coordinates: X, Y, R, G, B


            // Triangle Fan
            0f, 0f, 0f, 1.5f, 1f, 1f, 1f,
            -0.5f, -0.8f, 0f, 1f, 0.7f, 0.7f, 0.7f,
            0.5f, -0.8f, 0f, 1f, 0.7f, 0.7f, 0.7f,
            0.5f, 0.8f, 0f, 2f, 0.7f, 0.7f, 0.7f,
            -0.5f, 0.8f, 0f, 2f, 0.7f, 0.7f, 0.7f,
            -0.5f, -0.8f, 0f, 1f, 0.7f, 0.7f, 0.7f,
        // Line 1
            -0.5f, 0f, 0f, 1.5f, 1f, 0f, 0f,
            0.5f, 0f, 0f, 1.5f, 1f, 0f, 0f,
        // Mallets
            0f, -0.4f, 0f, 1.25f, 0f, 0f, 1f,
            0f, 0.4f, 0f, 1.75f, 1f, 0f, 0f

    };

    vertexData = ByteBuffer
            .allocateDirect(tableVerticesWithTriangles.length * BYTES_PER_FLOAT)
            .order(ByteOrder.nativeOrder()).asFloatBuffer();

    vertexData.put(tableVerticesWithTriangles);
}

@Override
public void onSurfaceCreated(GL10 glUnused, EGLConfig config) {
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

    String vertexShaderSource = TextResourceReader
            .readTextFileFromResource(context, R.raw.imple_vertex_shader);
    String fragmentShaderSource = TextResourceReader
            .readTextFileFromResource(context, R.raw.simple_fragment_shader);

    int vertexShader = ShaderHelper.compileVertexShader(vertexShaderSource);
    int fragmentShader = ShaderHelper
            .compileFragmentShader(fragmentShaderSource);

    program = ShaderHelper.linkProgram(vertexShader, fragmentShader);

    if (LoggerConfig.ON) {
        ShaderHelper.validateProgram(program);
    }

    glUseProgram(program);

    uMatrixLocation = glGetUniformLocation(program, U_MATRIX);

    aPositionLocation = glGetAttribLocation(program, A_POSITION);
    aColorLocation = glGetAttribLocation(program, A_COLOR);

    // Bind our data, specified by the variable vertexData, to the vertex
    // attribute at location A_POSITION_LOCATION.
    vertexData.position(0);
    glVertexAttribPointer(aPositionLocation, POSITION_COMPONENT_COUNT, GL_FLOAT,
            false, STRIDE, vertexData);

    glEnableVertexAttribArray(aPositionLocation);

    // Bind our data, specified by the variable vertexData, to the vertex
    // attribute at location A_COLOR_LOCATION.
    vertexData.position(POSITION_COMPONENT_COUNT);
    glVertexAttribPointer(aColorLocation, COLOR_COMPONENT_COUNT, GL_FLOAT,
            false, STRIDE, vertexData);

    glEnableVertexAttribArray(aColorLocation);

}


@Override
public void onSurfaceChanged(GL10 glUnused, int width, int height) {
    // Set the OpenGL viewport to fill the entire surface.
    glViewport(0, 0, width, height);

    MatrixHelper.perspectiveM(projectionMatrix, 45, (float) width
            / (float) height, 1f, 10f);



    setIdentityM(modelMatrix, 0);

    translateM(modelMatrix, 0, 0f, 0f, -2.5f);
    rotateM(modelMatrix, 0, -60f, 1f, 0f, 0f);

    final float[] temp = new float[16];
    multiplyMM(temp, 0, projectionMatrix, 0, modelMatrix, 0);
    System.arraycopy(temp, 0, projectionMatrix, 0, temp.length);

/}

@Override
public void onDrawFrame(GL10 glUnused) {
    // Clear the rendering surface.
    glClear(GL_COLOR_BUFFER_BIT);

    // Assign the matrix
    glUniformMatrix4fv(uMatrixLocation, 1, false, projectionMatrix, 0);

    // Draw the table.
    glDrawArrays(GL_TRIANGLE_FAN, 0, 6);

    // Draw the center dividing line.
    glDrawArrays(GL_LINES, 6, 2);

    // Draw the first mallet.
    glDrawArrays(GL_POINTS, 8, 1);

    // Draw the second mallet.
    glDrawArrays(GL_POINTS, 9, 1);
}

}

Answer 1

首先，如果只想在屏幕上绘制2D图元，则不希望应用任何3D模型，视图或投影矩阵（即使用仅将位置不变地传递位置或将这些矩阵设置为标识的着色器） 。然后，您基本上可以指定要直接在剪贴空间中绘制的东西的坐标。clip-space实际上是一个投影空间，clip-space坐标实际上是四维齐次坐标，但是您现在不必真正担心这一点。只要将第四个坐标w设置为1，剪辑空间坐标将直接对应于规范化的设备坐标，这就是您似乎已经熟悉的-1到1坐标系。

那么，从像素坐标到归一化的设备坐标，我们如何解决呢？好吧，我们需要做的是将x和y像素索引的[0 .. w）和[0 .. h）范围映射到[-1，1]范围内的位置（其中w和h是视口/屏幕分别在x和y方向上的像素）。这里要注意的一件事是，OpenGL中的像素位置对应于采样网格的单元中心。这意味着像素（0，0）不会落在左下角，而是会偏移“ 1/2像素”。如果您原谅我的ASCII艺术：

    |     |     |     |   
    +-----+-----+-----+---
    |  o  |  o  |  o  |   
    +-----+-----+-----+---
    |  o  |  o  |  o  |   
    +-----+-----+-----+---
    |  X  |  o  |  o  |   
    +-----+-----+-----+---
(-1, -1)

像素位置之间的距离在x方向上为1 / w，在y方向上为1 / h 。因此，像素（x，y）在规范化设备坐标中的位置为：

x_ndc = 2.0f * (x + 0.5f) / w - 1.0f;
y_ndc = 2.0f * (y + 0.5f) / h - 1.0f;

并且要放置在该点的顶点的齐次坐标为(x_ndc, y_ndc, -1.0f, 1.0f)。请注意，z坐标可以是您要将2D元素放置在的任何深度（-1对应于近平面，+ 1对应于远平面）。