清算FBO并不能清除一切

Question

我的代码相当于以下内容：

初始化代码：

glCullFace(GL_FRONT_AND_BACK);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);
//clear to pink
glClearColor(1.0f, 0.0f, 1.0f, 0.0f);

GLuint textureObject;
glGenTextures(1, &textureObject);
glBindTexture(GL_TEXTURE_2D, textureObject);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, 0);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 512, 512, 0, GL_RGBA,
    GL_UNSIGNED_BTYE, NULL);

GLuint fbo;
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D,
    textureObject, 0);
glReadBuffer(GL_COLOR_ATTACHMENT0);
glDrawBuffer(GL_COLOR_ATTACHMENT0);
assert(glCheckFramebufferStatus(GL_DRAW_FRAMEBUFFER)
    == GL_FRAMEBUFFER_COMPLETE);

Object obj = CreateAFullscreenQuad(textureObject);

绘制代码：

glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo);
glViewport(0, 0, 512, 512);
//clear to cyan
GLfloat clear[] = {0, 1, 1, 1};
glClearBufferfv(GL_COLOR, 0, clear);
SetUpPerspectiveAndCameraMatrix();
DrawSomeStuff();

glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glViewport(0, 0, window.width, window.height);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
SetUpPerspectiveAndCameraMatrix();
obj.Draw();
SwapBuffers();

glxinfo摘录：

OpenGL vendor string: VMware, Inc.
OpenGL renderer string: Gallium 0.4 on llvmpipe (LLVM 3.4, 128 bits)
OpenGL version string: 2.1 Mesa 10.1.3

问题是对glClearBufferfv的调用似乎并没有清除整个缓冲区。如您所见，它清除了三角形的形状，但它并没有触及其他区域。它们也不是glClearColor颜色（粉红色）。如果我使用常规glClear调用，它会执行相同的操作，但是粉红色而不是青色。

Answer 1

结果我在绑定FBO时绘制全屏四边形创建了一个反馈循环，将缓冲区的内容绘制到自身。显然，这种效果最终会散布在glClear的影响中。