glViewport偏移和正射投影

Question

在所有教程中，我发现基于视口大小创建投影矩阵，所有这些都假设视口的左下角坐标为（0,0）。

现在我想绘制到屏幕的不同部分，为此我想相应地切换视口：

glViewport(0,0,windowWidth/2, windowHeight/2); //left bottom
glViewport(0,windowHeight/2,windowWidth/2, windowHeight/2);//left top
glViewport(windowWidth/2,0,windowWidth/2, windowHeight/2);//right bottom
glViewport(windowWidth/2, windowHeight/2,windowWidth/2, windowHeight/2);//right top

现在我在定义投影矩阵方面遇到了问题。没有任何（x，y）offest我使用此代码来计算我的正投影矩阵：

if (m_WindowWidth > m_WindowHeight)
{
    auto viewportAspectRatio = (float)m_WindowWidth / (float)m_WindowHeight;

    m_ProjectionMatrix.m_fLeft = (-1.0f) * m_fWindowSize * viewportAspectRatio;
    m_ProjectionMatrix.m_fRight = m_fWindowSize * viewportAspectRatio;
    m_ProjectionMatrix.m_fBottom = (-1.0f)*m_fWindowSize;
    m_ProjectionMatrix.m_fTop = m_fWindowSize;
    m_ProjectionMatrix.m_fNear = -(10.0f)*m_fWindowSize;
    m_ProjectionMatrix.m_fFar = (10.0f)*m_fWindowSize;

    m_fMoveSpeed = static_cast<GLfloat>(m_fWindowSize * 2 / static_cast<float>(m_WindowHeight));
}
else
{
    auto viewportAspectRatio = (float)m_WindowHeight / (float)m_WindowWidth;

    m_ProjectionMatrix.m_fLeft = (-1.0f)*m_fWindowSize;
    m_ProjectionMatrix.m_fRight = m_fWindowSize;
    m_ProjectionMatrix.m_fBottom = (-1.0f)*m_fWindowSize * viewportAspectRatio;
    m_ProjectionMatrix.m_fTop = m_fWindowSize * viewportAspectRatio;
    m_ProjectionMatrix.m_fNear = -(10.0f)*m_fWindowSize;
    m_ProjectionMatrix.m_fFar = (10.0f)*m_fWindowSize;

    m_fMoveSpeed = static_cast<GLfloat>(m_fWindowSize * 2 / static_cast<float>(m_WindowWidth));
}

这样可以正常工作 UNTIL 我会在视口中添加任何（x，y）偏移量。使用glViewport(0, m_WindowHeight/2, m_WindowWidth/2, m_WindowHeight/2）后效果如下：

How to obtain the RevCommit or ObjectId from a SHA1 ID string with JGit?

使用glViewport(0, 0, m_WindowWidth/2, m_WindowHeight/2)：

如何让它发挥作用？

Answer 1

首先，宽高比始终是宽度/高度。

然后我认为你在寻找的是：

m_ProjectionMatrix.m_fLeft = x;
m_ProjectionMatrix.m_fRight = x + m_WindowWidth;
m_ProjectionMatrix.m_fBottom = y;
m_ProjectionMatrix.m_fTop = y + m_WindowHeight;
m_ProjectionMatrix.m_fNear = -(10.0f)*m_fWindowSize;
m_ProjectionMatrix.m_fFar = (10.0f)*m_fWindowSize;

看看这个wiki page

Answer 2

我找到了解决这个问题的方法并将其发布在gamedev论坛上： https://gamedev.stackexchange.com/questions/122284/glviewport-offset-and-ortho-projection/122289#122289

简而言之： 我正在将整个场景绘制到帧缓冲区，然后我将生成的纹理渲染到屏幕上。此操作导致不需要的glViewport（）转换累积。

解决这个问题的方法是将glViewport（）设置为原始befrore渲染到帧缓冲区，然后在将纹理渲染到屏幕时设置偏移。