OpenGL中的FBO如何使用模板緩沖區

FBO常用框架

一般需求只有color buffer，depth buffer，前者一般用紋理，後者一般用rbo。標准程序如下：

// 創建
// frame buffer object
glGenFramebuffers(1, &fboId);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);

// color buffer with texture object
glGenTextures(1, &color_rboId);
glBindTexture(GL_TEXTURE_2D, color_rboId);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, targetWidth, targetHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);

// depth buffer with render buffer object
glGenRenderbuffers(1, &depth_rboId);
glBindRenderbuffer(GL_RENDERBUFFER, depth_rboId);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, targetWidth, targetHeight);

// Attach color buffer to FBO
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, color_rboId, 0);

//Attach depth buffer to FBO
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depth_rboId);


//Also attach as a stencil
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT, GL_RENDERBUFFER, depth_stencil_rb);

// 刪除
if (fboId != 0)
{
glDeleteFramebuffers(1, &fboId);
fboId = 0;
}

if (depth_rboId != 0)
{
glDeleteRenderbuffers(1, &depth_rboId);
depth_rboId = 0;
}

if (color_rboId)
{
glDeleteTextures(1, &color_rboId);
color_rboId = 0;
}

FBO中如何增加stencil buffer

正常人都會認為stencil buffer 跟depth buffer類似，於是乎寫出一套類似depth buffer的框架：

// 創建stencil buffer以RBO形式
glGenRenderbuffers(1, &stencil_rboId);
glBindRenderbuffer(GL_RENDERBUFFER, stencil_rboId);
glRenderbufferStorage(GL_RENDERBUFFER, GL_STENCIL_INDEX, targetWidth, targetHeight);

// 關聯stencil buffer 和FBO
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT, GL_RENDERBUFFER, stencil_rboId);

// 刪除stencil buffer
glDeleteRenderbuffers(1, &stencil_rboId);

但實際測試，發現stencil buffer根本不起作用，下面是一個測試函數：

void stencilRenderTest()
{
GLdouble dRadius = 0.1; // Initial radius of spiral
GLdouble dAngle; // Looping variable
float x = 100;
float y = 100;
float rsize = 25;

// Clear blue window
glClearColor(0.0f, 0.0f, 1.0f, 0.0f);

// Use 0 for clear stencil, enable stencil test
glClearStencil(0.0f);
glEnable(GL_STENCIL_TEST);

// Clear color and stencil buffer
glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

// All drawing commands fail the stencil test, and are not
// drawn, but increment the value in the stencil buffer.
glStencilFunc(GL_NEVER, 0x0, 0x0);
glStencilOp(GL_INCR, GL_INCR, GL_INCR);

// Spiral pattern will create stencil pattern
// Draw the spiral pattern with white lines. We
// make the lines white to demonstrate that the
// stencil function prevents them from being drawn
glColor3f(1.0f, 1.0f, 1.0f);
glBegin(GL_LINE_STRIP);
for(dAngle = 0; dAngle < 400.0; dAngle += 0.1)
{
glVertex2d(dRadius * cos(dAngle), dRadius * sin(dAngle));
dRadius *= 1.002;
}
glEnd();

// Now, allow drawing, except where the stencil pattern is 0x1
// and do not make any further changes to the stencil buffer
glStencilFunc(GL_NOTEQUAL, 0x1, 0x1);
glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);

// Now draw red bouncing square
// (x and y) are modified by a timer function
glColor3f(1.0f, 0.0f, 0.0f);
glRectf(x, y, x + rsize, y - rsize);

}

stencil正確時，結果顯示是一個藍色背景，紅色小矩形塊，矩形內部可以看到圓弧，下圖右。

stencil不正確結果是藍色背景同心圓環 以及一個矩形紅色塊，圖左。