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POHow to switch shader programs in glsl
 

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  1. POHow to switch shader programs in glsl
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    <p>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, <code>exitPointProg</code> and <code>rayCastProg</code>. 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 <a href="http://www.daimi.au.dk/~trier/?page_id=98" rel="nofollow">here</a>. 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 <code>glClearColor(1.0f, 1.0f, 1.0f, 1.0f)</code>. 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.</p> <ol> <li>1st pass, render to a fbo with <code>exitPoints</code> texture bound to it to get the exit points of cube bounding box, using shader program <code>exitPointProg</code>.</li> <li><p>2ed pass, mapped the <code>exitPoints</code> texture (bound to <code>GL_TEXTURE1</code>) into the shader program <code>rayCastProg</code> as a uniform sampler2D variable which will used in the shader. here is the <code>setupFBO</code> and <code>render</code> and two subroutines: the <code>setupFBO()</code> function:</p> <pre><code>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, &amp;depthRenderBuffer); glBindRenderbuffer(GL_RENDERBUFFER, depthRenderBuffer); glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height); glGenFramebuffers(1, &amp;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); } </code></pre> <p>the <code>render()</code> function:</p> <pre><code>void SimpleRayCasting::render() { getExitPoints(); GLUtils::checkForOpenGLError(__FILE__,__LINE__); rayCasting(); GLUtils::checkForOpenGLError(__FILE__,__LINE__); } </code></pre> <p>the <code>getExitPoints()</code> function:</p> <pre><code>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); } </code></pre> <p>the <code>rayCasting</code> function:</p> <pre><code>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); } </code></pre> <p>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!</p></li> </ol> <p><strong>edit to ask</strong> is there any demo or tutorial demonstrate how to use multiple glsl shader programs with FBO?</p>
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    1. COThis code is pretty dirty. However, I suspect the problem concerns the glDrawBuffers(...) calls; you're never resetting them. Also, you're never changing back to the first texture with glActiveTexture(...). Other than that, I recommend modularizing everything so you can see what's going on.
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      1. USimallett
    2. CO@IanMallett It said that `The state set by glDrawBuffers is part of the FBO (or default framebuffer). So you can generally set this up once and leave it set.` frome [opengl official](http://www.opengl.org/wiki/Framebuffer_Object#Multiple_Render_Targets) and I add the `glActiveTexture(GL_TEXTURE0)` at the end of function `getExitPoints()` and `render()`, but nothing differently happens.
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    3. COIt costs me a few days to figure it out, My code works great on my friend's GTX 570 card, but not on my APU 3500 (AMD Radeon HD 6620G). Finally I find it something wrong with my fragment shader corresponding to my `rayCasting()` function. I didn't use the `Vertex Color` in the fragment shader first. After I add `Color` (`Color = VertexColor` in vertex shader)in fragment shader like this: `FragColor = colorAcum + (vec4(Color, 1.0) * 0.0);`. It works and I don't know why!
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