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95.4501

95.4501. OpenGL Odds and Ends. Odds and Ends. A few odds and ends to help with assignment #3 particularly issues that might trip you. A bit about blending . A bit about OpenGL stacks . A bit about OpenGL shaders .

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95.4501

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  1. 95.4501 OpenGL Odds and Ends

  2. Odds and Ends • A few odds and ends to help with assignment #3 particularly issues that might trip you. • A bit about blending. • A bit about OpenGL stacks. • A bit about OpenGL shaders. Generally, things you SHOULD be able to figure out by yourself but might not have.

  3. 95.4002 Transformation Extensions

  4. A Useful Addition To Transformation • There is a lookAtForObject which can build an entire transformation for you… • If you need the inverse (for something playing the role of a camera), it might be nice to have a version call lookAtForCamera. • The next slide provides it… IF YOU NEED IT.

  5. A Useful Addition To Transformation //In .h file static Transformation lookAtForCamera (const Point &from, const Vector &direction, const Vector &up, const Vector &alternateUp) ; //In .cpp file Transformation Transformation::lookAtForCamera (const Point &from, const Vector &direction, const Vector &up, const Vector &alternateUp) { //This version returns the same thing as gluLookAt but computes it without using the stack... //Note: up is an approximate yAxis. If it is erroneously directed into the z-direction, use alternateUp instead. Vector zAxis = -direction.normalized (); Vector xAxis = (up.cross (zAxis)).normalized (); if (xAxis.isZero ()) xAxis = (alternateUp.cross (zAxis)).normalized (); Vector yAxis = zAxis.cross (xAxis); Transformation result; //Compute the inverse of rotateToAxes (xAxis, yAxis, zAxis, from) which interpreted as RT means computing T-1R-1. result.set ( xAxis.x, yAxis.x, zAxis.x, 0.0, xAxis.y, yAxis.y, zAxis.y, 0.0, xAxis.z, yAxis.z, zAxis.z, 0.0, -from.x*xAxis.x - from.y*xAxis.y - from.z*xAxis.z, -from.x*yAxis.x - from.y*yAxis.y - from.z*yAxis.z, -from.x*zAxis.x - from.y*zAxis.y - from.z*zAxis.z, 1.0); return result; }

  6. 95.4002 Blending

  7. Many Effect Require Blending Techniques • Blending permits the colors of a texture to be merged with the colors already drawn on the back buffer. • To use it, must enable it; disable when done. • glEnable (GL_BLEND); • glDisable (GL_BLEND);

  8. Must Specify Which Blending Factors To Use • result = SourceFactor*S + DestinationFactor*D • S is the input texture (the source) • D is the screen pixel (the destination or what’s already there) glBlendFunc (sourceFactor, destinationFactor); 15 factors available: can get by with 3 or 4. 1. Example Factors, 2. Example Blend functions coming up

  9. What’s The Complete Set of Factors? • GL_ZERO 0 • GL_ONE 1 • GL_SRC_COLOR S • GF_DST_COLOR D • GL_SRC_ALPHA afrom S • GL_DST_ALPHA afrom D • GL_ONE_MINUS_SRC_ALPHA 1-afrom S • GL_ONE_MINUS_DST_ALPHA 1-afrom D

  10. Complete Set... • GL_ONE_MINUS_SRC_COLOR 1-S • GL_ONE_MINUS_DST_COLOR 1-D • There are a few additional rules (available through extensions).. Examples coming up

  11. What the Blending Factors Apply To glBlendFunc (sourceFactor, destinationFactor); • result = SourceFactor*S + DestinationFactor*D • S is the input texture (the source) • D is the screen pixel (the destination or what’s already there) • Note that result has no alpha in it once it’s combined. • Combination applies to R,G,B components independently (each ranges from 0 to 1)…

  12. Useful Combination 1: Brightening • Take a little from each AND ignore transparency. glBlendFunc (GL_ONE, GL_ONE); • Consequences: • Result = 1*S + 1*D (clamped to 1 if > 1) Creates a brightening (whitening) effect… 1 * 0.5 (gray) + 1 * 0.5 (gray) => 1 (white) glBlendFunc (sourceFactor, destinationFactor)

  13. Useful Combination 2: Raw Transparency • Use transparency (called alpha blending) glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); • Result = a*S + (1- a)*D (clamped to 1 if > 1) • where a is the alpha in the textured color If a = 0.7 (mostly opaque), get .7S+.3D glBlendFunc (sourceFactor, destinationFactor)

  14. What Combination Is Like Having Blending Off? • Same as using glBlendFunc (GL_ONE, GL_ZER0); • Result = 1*S + 0*D = S • So pixel behind is overwritten Better to say, glDisable (GL_BLEND) instead. glBlendFunc (sourceFactor, destinationFactor)

  15. Useful Combination 3: Darkening • One that multiplies rather than adds so that white (1) * what is there => stays the same gray (.5) * what is there => goes darker black (0) * what is there => black glEnable(GL_BLEND); glBlendFunc (GL_ZERO, GL_SRC_COLOR); • Consequences: • Result = 0*S + S*D (clamped to 1 if > 1) This simulates MULTIPLY 0.8*0.8=0.64 (darkens)

  16. 95.4002 The OpenGL Stack

  17. Summary: Thinking With the Left To Right Convention MATH works from left to right v * ABC • From most local to most global The OpenGL Stack works from right to left • growth direction A B C • So push C; mult B; mult A; pop How to use the stack? NEXT

  18. How Do I Put Something On The Stack • Want to translate by T= [tx, ty, tz], and scale by S = [sx, sy, sz]. glMatrixMode (GL_MODELVIEW_MATRIX);glPushMatrix ();glTranslated (tx, ty, tz);glScaled (sx, sy, sz);glPopMatrix (); This puts S*T*whateverIsAlreadyThere on the stack (since it works from right to left) If W transforms a vehicle to a place in the world with camera C, the stack should contain W * C-1

  19. 95.4002 Shaders

  20. Questions YOU Should Have Had In Your Mind • If a shader uses a texture variable called “birdTexture” via • uniform sampler2D birdTexture • How does the game engine know what texture to use; e.g., • texture called “turkey” Generally, stuff explained in Shader::example ()

  21. How The Explanation Will Go • Explain how the ambient occlusion demo does all this. • Relate how this is different in the engine…

  22. Aside • A host of routines are used with names that are synonyms… • glCreateProgram versus glCreateProgramObjectARB • glUseProgramversusglUseProgramObjectARB • glUniform1i versus glUniform1iARB

  23. 95.4002 How The Ambient Occlusion Demo Connects shaders with their textures

  24. Everything done via handles //GettingglGenTextures (1, &textureHandle); shaderProgramHandle = glCreateProgram (); //DeletingglDeleteTextures (1, &textureHandle); glDeleteProgram (shaderProgramHandle); //UsingglBindTexture (GL_TEXTURE_2D, textureHandle); orglBindTexture (GL_TEXTURE_RECTANGLE, textureHandle); glUseProgram (shaderProgramHandle); For coordinates in range 0..799 0 means “don’t use a shader”

  25. Everything done via handles //Uploading textures (MESSY) • glBindTexture (GL_TEXTURE_2D, textureHandle); • Supply a host of sampling information; e.g. GL_CLAMP versus GL_REPEAT • Upload the bits to the card… via glTexImage2D or gluBuild2DMipmaps AO Demo has no bits to upload, only game engine which also provides a Texture object to do it all.

  26. Everything done via handles //Uploading shaders (MESSY) • Compile vertex shader, compile pixel shader. • shaderProgramHandle = glCreateProgram (); • Attach the shaders to the program. • Link the program AO Demo provides their code for doing this; game engine provides a Shader object for doing this.

  27. What You Wanted To Know //Connecting shader samplers to textures In shader “lesterLightingDiffuse”uniform sampler2D texture; After creating the shader(do this once)glUseProgram (shaderProgramHandle); glUniform1i ("texture", 0); //a texture unit When you want to draw with a shaderglUseProgram (shaderProgramHandle); glActiveTexture (GL_TEXTURE0); glBindTexture (GL_TEXTURE_2D, textureHandle); OR glBindTexture (GL_TEXTURE_RECTANGLE, textureHandle); 0, 1, 2, 3, … For coordinates in range 0..799

  28. FrameBuffers Are The SAME Only AO used frame buffers • Create via handles • glGenFramebuffers (howMany, frameBufs);Plus code to create z-buffer and textures to draw into and code to attach to frame buffer • Delete via handles glDeleteFramebuffers (howMany, frameBufs); • Activate via handles glBindFramebuffer (GL_FRAMEBUFFER, frameBufs[i]); OR glBindFramebuffer (GL_FRAMEBUFFER, 0); This means now draw into backbuffer

  29. 95.4002 How The Engine Differs From The AO Demo

  30. The Engine • Has texture and shader objects do some work. • Texture provides • test = Texture::read (“turkey”) //Creates handle + path, reads it • texture->load (mipmapping, forceClamp) //Defaults to true, false (to upload onto card) • activate () //To bind to the correct texture handle • Shader provides • activate () //To bind to the correct program handle

  31. Some Things That Can Cause Problems • Shaders are a recent introduction that were meant to be specified in level file “castle4.wrl”. The start of the file contain the list of shaders used • Shaders: 0; //currentlyShader 1 nameShader 2 name … • A shader manager keeps track of the shaders which it associates with an object and the terrain via a shader index… Each object including the terrain has a property "shader" => "-1“ //-1 means no shader

  32. When the Shader Manager is Setup • It creates a default shader via • defaultShaderIndex = addShader ("lesterlightingDiffuse"); • When a object is imported, it executes • if (shaderIndex == -1) shaderIndex = defaultShaderIndex; • When the terrain is imported, it executes //if (shaderIndex == -1) shaderIndex = defaultShaderIndex; Please UNCOMMENT THIS LINE

  33. How Else Does It Work • When drawing an object (the castle) or the terrain, it activates the shader to use activates the texture to use • But it fails to execute glActiveTexture (GL_TEXTURE0); BECAUSE IT DOES THIS ONCE WHEN OPENGL IS SET UP AND ASSUMES IT’S STILL TRUE… The AO Demo code switches all over the place…

  34. Making Ambient Occlusion Work With The Engine • What strategy did I use? • Create one file “ssao.cpp” containing all the ambient occlusion code and comment out stuff not needed. • Add it to the “physicsForStudent” project including the AO shaders. • Find a spot in “game.cpp” to call “WilfDisplay”. • Find a spot in “ssao.cpp” to invoke “world->draw ()” • Find a spot in “ssao.cpp” to blend AO with what game engine already drew… BASIC IDEA: AVOID CHANGING ANYTHING BEFORE IT WORKS.

  35. More Detail Steps • Create one file “ssao.cpp” containing the contents of the ambient occlusion file “main.cpp”, “wilfMain.cpp”, and the routines that compile the shader. Add to project. • Copied over the ambient occlusion shaders into a subdirectory of shaders and called it “AOshaders”.Changes lines from old locations to new “./src/shaders/ortho.vert” //OLD“../shaders/aoshaders/ortho.vert” //NEW • Edited each shader so that the first line was#version 120

  36. Making Ambient Occlusion Work With The Engine • Fixed the "glew" problem by uncommenting"#include glew.h", commenting out 2 redeclaration error messages, and moving "glewInit" to spot after call to "setupOpenGL ()". • Much later, I found out that I would crash in routine glBindFragDataLocation (how ssao routine binds a texture to a texture unit) but that it would work if I added EXT to the routine… INSTEAD, added the following after "#include glew.h", #undefglBindFragDataLocation #define glBindFragDataLocationglBindFragDataLocationEXT

  37. Making Ambient Occlusion Work With The Engine • There were 3 places in ssao.cpp file that draws quads. Not all of them were drawing counterclockwise (it matters because face culling now has to be on)… glBegin(GL_QUADS); //glVertex2d(0, 0); //glVertex2d(0, size); //glVertex2d(size, size); //glVertex2d(size, 0);glVertex2d (0, 0); //WILF: COUNTER-CLOCKWISE SO ALWAYS WORKS. glVertex2d (size, 0); glVertex2d (size, size); glVertex2d (0, size);glEnd();

  38. Making Ambient Occlusion Work With The Engine • Added 2 routines “setupAmbientOcclusion ()” and “wrapupAmbientOcclusion ()” to contain the setup/wrapup code in the ssao.cpp file and called them at the end of “Game::setup ()” and “Game::wrapup ()” • Also added the following at the end of setupAmbientOcclusion and WilfDisplay… • Add the following after call to Render2GBuffer. glBindFramebuffer(GL_FRAMEBUFFER, 0);|glActiveTexture(GL_TEXTURE0);glColor4d (1.0, 1.0, 1.0, 1.0);glUseProgram (0); glDisable (GL_TEXTURE_RECTANGLE); Used to be permanently enabled (wrecking normal draws) glEnable (GL_TEXTURE_RECTANGLE);

  39. Making Ambient Occlusion Work With The Engine • Made resolution, near, far, FOV same for both… In game engine (main.cpp) //glutInitWindowSize (800, 600);glutInitWindowSize (800, 800); //WILF$$ MAKE THE SAME AS AO SHADER In ssao.cpp const float zNear = 1.0; //0.1f; //WILF$$ MAKE NEAR AND FAR THE SAME AS ssao.cppconst float zFar = 2000; //1000.0f; //WILF$$ MAKE NEAR AND FAR THE SAME AS ssao.cppconst float fov = 40; //65.238f; //WILF$$ MAKE THE SAME AS ENGINE...

  40. Making Ambient Occlusion Work With The Engine In ssao.cpp (replaced code that was grabbing the model view matrix mVMatfrom the stack by code that gets it from the existing camera objects). THIS IS AN INVERSE //glGetFloatv(GL_MODELVIEW_MATRIX, mVMat); CopyMemory (mVMat, &camera->inverseCameraMatrix.m11, sizeof (Transformation)); Similarly, code that compute the inverse is obtained from the camera //M3DInvertMatrix44f(mVMat, iMVMat); CopyMemory (iMVMat, &camera->cameraMatrix.m11, sizeof (Transformation));

  41. The Most Important Thing To Understand • Assuming the engine draws the world and the ambient occlusion code draws the world, what’s the difference? • The engine wants to draws with the "lesterlightingDiffuse“ shader… • The ssao.cpp code draws with a bunch of different ambient occlusion shaders… HOW DO WE MAKE THAT WORK?

  42. Controlling The Game booldisableShaders = false; void Shader::activate () { extern booldisableShaders; if (disableShaders) return; if (isBroken) {deactivate (); return;} glUseProgramObjectARB (privateProgramHandle); } void Shader::deactivate () { extern booldisableShaders; if (disableShaders) return; glUseProgramObjectARB (0); } Added a global just before void Game::draw () {…} Changed the routines that activate/deactivate shaders…

  43. REVISED GAME DRAWING booldisableShaders = false; #define DISABLE disableShaders = true; #define ENABLE disableShaders = false; boolactivateAmbientOcclusionBlending = false; void Game::draw () { //If there is no world, draw a teapot; otherwise, draw the world... //Neither the input manager nor the camera draws itself... enum Choice {AOOnly, NormalOnly, AOThenNormal, NormalThenAO}; Choice choice = NormalThenAO; camera->beginCamera (); if (world == NULL) { drawTeapots (); } else { //REVISED PART } player->draw (); camera->endCamera (); drawFrameRate (); drawHelp (); }

  44. REVISED GAME DRAWING #define DISABLE disableShaders = true; #define ENABLE disableShaders = false; activateAmbientOcclusionBlending = choice == NormalThenAO; switch (choice) { case AOOnly: void WilfDisplay (); DISABLE; WilfDisplay (); break; case NormalOnly: ENABLE; world->draw (); break; case AOThenNormal: DISABLE; WilfDisplay (); ENABLE; world->draw (); break; case NormalThenAO: ENABLE; world->draw (); drawTeapots (); DISABLE; WilfDisplay (); break; }

  45. REVISED ssao.cpp DRAWING void DrawModel () { //Used to draw sponza or sibenik models here… //Game engine needs to draw with texture unit 0…glActiveTexture (GL_TEXTURE0); game->world->draw ();} void Render2GBuffer () { …. DrawModel(); ….}

  46. BLENDING AO WITH GAME • Recall: ssao.cpp makes and uses frame buffers to draw with… • Frame buffers draw into textures. So you never see what they draw on the screen… • Ultimately, though, it must draw into a back buffer… • What controls that…

  47. What Was The Last Thing Drawn By SSAO.cpp RUNS IN A LOOP WITH size going from small up to RESOLUTION void RenderAO (int size, int index) { … extern boolactivateAmbientOcclusionBlending; if (size < RESOLUTION) {glBindFramebuffer(GL_FRAMEBUFFER, frameBufs[index]);glDrawBuffer(GL_COLOR_ATTACHMENT2); } else {glBindFramebuffer (GL_FRAMEBUFFER, 0); if (activateAmbientOcclusionBlending) {glEnable (GL_BLEND); glBlendFunc (GL_DST_COLOR, GL_ZERO); } } CODE (NOT SHOWN) TO DRAW ONE QUAD… if (activateAmbientOcclusionBlending) {glDisable (GL_BLEND); } if (size < RESOLUTION && USE_EXTRA_UPSAMPLING_BLUR) Blur (size, index); } LAST ITERATION, size is RESOLUTION

  48. A Glitch • I did observe a glitch which I assumed was due to the temporal coherence feature. When I rotated continuously, I would sometimes see a dark triangle appear. • Hacked the code in the shader not to do it... float temporallyBlendedOcclusion (float occlusion) { return occlusion; //Disabling hack... • return useTemporalSmoothing? occlusion //temporalBlend(...) : occlusion;}

  49. Found a Fix for the ATI DFDY Instruction Bug • Since shader “geometry.frag” is computing the normal in camera space and face culling is on, only faces that face the camera will draw... So the normal MUST be pointing in the +z direction... • If it’s not, make it so... Shader “geometry.frag #version 120#extensionGL_ARB_texture_rectangle: require in vec4 pos; out vec4 Pos; out vec4 Norm; void main() { Pos = pos / pos.w; vec3 n = cross (dFdx (pos.xyz), dFdy (pos.xyz)); if (n.z < 0.0) n = -n; //If differentials have a bug, fix it.. Norm = vec4 (normalize(n), sign (abs (pos.z))); }

  50. Conclusion • This should convince you that it’s doable… • Did not try the shadow algorithm… So don’t know if anything is tricky…

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