How to switch shader programs in glsl - opengl

Question

I want to do volume rendering using programmable pipeline (using glsl) and no fixed pipelines. I implement it with 2 passes with 2 shader programs, exitPointProg and rayCastProg. the first pass is to get the exit points (i.e. the back face) of a cube bounding box which will be used as a texture in the next pass which doing the raycasting. the idea to do the raycasting volume rendering comes from here. I think I have done the most of the things right cause I have implemented this with fixed pipeline combined with the programmable pipeline. the things that confused me is that I achieved the result with the very first frame, and with a flash the display on the screen turn totally white (I set the glClearColor(1.0f, 1.0f, 1.0f, 1.0f). I think there maybe some wrong with the switch of the shader programs or the FBO. here is what I do in the render() function.
1st pass, render to a fbo with exitPoints texture bound to it to get the exit points of cube bounding box, using shader program exitPointProg.
2ed pass, mapped the exitPoints texture (bound to GL_TEXTURE1) into the shader program rayCastProg as a uniform sampler2D variable which will used in the shader.
here is the setupFBO and render and two subroutines:
the setupFBO() function:
void SimpleRayCasting::setupFBO()
{
GLuint textureHandles[1];
glGenTextures(1, textureHandles);
entryPoints = textureHandles[0];
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, exitPoints);
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_BORDER);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_BORDER);
glTexImage2D(GL_TEXTURE_2D, 0,GL_RGBA16F, width, height, 0, GL_RGBA, GL_FLOAT, NULL);
GLuint depthRenderBuffer;
glGenRenderbuffers(1, &depthRenderBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, depthRenderBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height);
glGenFramebuffers(1, &frameBuffer);
glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D,
exitPoints, 0/* level */);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER,
depthRenderBuffer);
checkFramebufferState(__FILE__, __LINE__);
GLenum drawbufs[] = {GL_COLOR_ATTACHMENT0};
glDrawBuffers(1, drawbufs);
glBindFramebuffer(GL_FRAMEBUFFER,0);
}
the render() function:
void SimpleRayCasting::render()
{
getExitPoints();
GLUtils::checkForOpenGLError(__FILE__,__LINE__);
rayCasting();
GLUtils::checkForOpenGLError(__FILE__,__LINE__);
}
the getExitPoints() function:
void SimpleRayCasting::getExitPoints()
{
// render to the texture
glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer);
glEnable(GL_DEPTH_TEST);
glViewport(0, 0, width, height);
// checkFramebufferState(__FILE__, __LINE__);
glClearColor(0.2f, 0.2f, 0.2f, 1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
exitPointsProg.use();
// note that the model will recalculated.
model = mat4(1.0f);
model *= glm::rotate(angle , vec3(0.0f,1.0f,0.0f));
model *= glm::rotate(90.0f, vec3(1.0f, 0.0f, 0.0f));
model *= glm::translate(vec3(-0.5f, -0.5f, -0.5f));
view = glm::lookAt(vec3(0.0f,0.0f,2.0f), vec3(0.0f,0.0f,0.0f), vec3(0.0f,1.0f,0.0f));
projection = mat4(1.0f);
projection = glm::perspective(60.0f, (float)width/(float)height,0.01f, 400.0f);
setMatrices(exitPointsProg);
glEnable(GL_CULL_FACE);
glCullFace(GL_FRONT);
drawBoundBox();
glDisable(GL_CULL_FACE);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
}
the rayCasting function:
void SimpleRayCasting::rayCasting()
{
// Directly render to the screen
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glEnable(GL_DEPTH_TEST);
glViewport(0, 0, width, height);
glClearColor(1.0f, 1.0f, 1.0f, 1.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
rayCastProg.use();
model = mat4(1.0f);
model *= glm::rotate(angle, vec3(0.0f,1.0f,0.0f));
model *= glm::rotate(90.0f, vec3(1.0f, 0.0f, 0.0f));
model *= glm::translate(vec3(-0.5f, -0.5f, -0.5f));
view = glm::lookAt(vec3(0.0f,0.0f,2.0f), vec3(0.0f,0.0f,0.0f), vec3(0.0f,1.0f,0.0f));
projection = mat4(1.0f);
projection = glm::perspective(60.0f, (float)width/(float)height,0.01f, 400.0f);
setMatrices(rayCastProg);
rayCastProg.setUniform("StepSize", stepSize);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, exitPoints);
rayCastProg.setUniform("ExitPoints", 1);
glActiveTexture(GL_TEXTURE4);
glBindTexture(GL_TEXTURE_3D, volume_to);
rayCastProg.setUniform("VolumeTex", 4);
glActiveTexture(GL_TEXTURE5);
glBindTexture(GL_TEXTURE_1D, transferFunc_to);
rayCastProg.setUniform("TransferFunc", 5);
glEnable(GL_CULL_FACE);
glCullFace(GL_BACK);
drawBoundBox();
glDisable(GL_CULL_FACE);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
}
I am using a class inherited from QGLWidget in Qt. As I have mentioned early, I got the right result with just the 1st frame and quickly it flash away and turn totally white. I don't know what's wrong with this, is there something wrong with the switch of the shader program or the FBO thing? I have been working on this a long time and can't figure it out, any help will be appreciated!
edit to ask
is there any demo or tutorial demonstrate how to use multiple glsl shader programs with FBO?