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OpenGL Fixed Function to Shaders - Porting a fixed function application to “modern” OpenGL - Webinar Mar 2016

  1. Fixed Function to Shaders Porting a fixed-function application to “modern” Opengl. Watch the video here:
  2. Outline There are many tutorials that introduce you to “modern” OpenGL: (OpenGL 3.3/OpenGL ES 2.0 or greater which is where the fixed-function APIs were removed from the spec.) Here we will compare and contrast old fixed- functionality and it’s new modern replacement. We will cover some basic things you need to get going: Vertex/Attribute data, rendering, and 3- D math.
  3. Geometry Let’s use the famous OpenGL triangle as a platform to talk about geometry/attributes. It’s the probably the very first OpenGL program you saw when learning OpenGL.
  4. Rendering What is the minimum need to “light up” a pixel? First you need a window on your platform with an OpenGL context bound to it. You used to use GLUT and GLU “helper” libraries Here we use Qt to replace both. For window/platform integration we’re using QOpenGLWindow initializeGL(), resizeGL(), paintGL(), keyPressEvent()
  5. GLUT OpenGL Triangle (clip-space) Show code in Qt Creator.
  6. OpenGL 1.1 Vertex Arrays GLUT Triangle Show code in Qt Creator.
  7. Modern Open Vertex Arrays Qt Triangle Show code in Qt Creator.
  8. Doesn’t Work ?? Previous slide will not render a triangle. Why not? Fixed-function example uses “fixed functionality” to render. With modern OpenGL, you have to program that functionality yourself in the form of “shaders”.
  9. Shaders Many kinds of shaders in Modern OpenGL: Vertex Shader Tessellation Control Shader Tessellation Evaluation Shader Geometry Shader Fragment Shader Compute Shader Only two are required.
  10. Vertex Shader Program that runs on the GPU. Invoked once for each vertex in primitive shapes drawn. Input: Attribute data from vertex arrays Output: Clip space position of vertex: gl_Position Data to pixel shader: varying variables.
  11. Fragment Shader Program run on the GPU once for each fragment (pixel-candidate) displayed on screen. Inputs: varying variables from Vertex Shader Outputs: pixel color: gl_FragColor Pixels are produced by “Rasterization”
  12. Rasterization
  13. Program All the shaders are compiled and linked together similar to C++ program. QOpenGLShaderProgram makes it easy Once compiled and linked bind() must be called to make it active.
  14. Hold On ... You may notice the fragment shader is assigning the output color directly from it’s input varying v_color variable set by the Vertex Shader. How is it that the colors are “mixed” inside the triangle? Outputs of the vertex shader (and corresponding inputs to the pixel shader) are interpolated between the vertices. Equivalent to glShadeModel(GL_SMOOTH);
  15. What about GL_FLAT ? Okay so attribute data output from the vertex shader is interpolated to the pixel shader inputs. What about glShadeModel(GL_FLAT)? Use flat attribute on variable declaration in shader code. flat varying vec3 v_color; Default is smooth. These are equivalent: smooth varying vec3 v_color; varying vec3 v_color;
  16. Review: QGLBufferObject Memory buffer on graphics card that holds vertex attribute data. Equivalent to glBegin/glEnd inside a display list Attributes inside glBegin/glEnd copied to video card instead of being rendered. Equivalent to alloc() on QGLBufferObject. Vertex Buffer Objects (VBO) don’t save primitive type. Instead pass as parameter to glDraw() Just like OpenGL 1.1 Vertex Arrays
  17. Review: QGLVertexArrayObject OpenGL 1.1 Vertex Arrays require setting up attribute array specifications each time before calling glDraw(). Modern OpenGL captures attribute array specifications once when data is uploaded to card using Vertex Array Objects (VAO). VAO “remembers” vertex array state and applies this state when .bind() is called. Modern code only needs a vao.bind() before glDraw()
  18. Another thing to note … Fixed-function primitive types: GL_QUAD, GL_QUAD_STRIP, GL_POLYGON have been removed. You must change your geometry to GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, or GL_TRIANGLE.
  19. Math Fixed-function OpenGL had Matrix Stacks built into the API. Used to create concept of a “camera” (GL_MODELVIEW) rendering a world through a window (GL_PROJECTION) that’s painted on your computer screen. Convenience Functions: glLoadIdentity, glTranslate, glRotate, glScale Matrix stack: glMatrixMode, glPushMatrix, glPopmatrix
  20. Sorry... Sorry, that’s all gone now. You, the programmer, have to perform all this math. Recall the vertex shader is responsible for outputting the vertices clip-space position by assigning to gl_Position. It is this math that you use in the vertex shader to perform this conversion. On the CPU the typical thing to do is recreate the camera/window idiom with model transform matrices, a view transform matrix, and a projection matrix. Pass the matrices to the shaders as “uniforms”
  21. Agnostic Modern OpenGL is agnostic about these idioms. But it does help you by providing matrix math operators in the shader language. You, the programmer, get to decide how to transform your vertex positions to clip space. If you can code it, you can use it.
  22. Math Library If you want to use the Model-View-Projection concept in your program you have to perform the math yourself. Qt has a powerful/concise library built-in which supports vectors, matrices, and quaternions. Matrix functions to replace GL & GLU gluPerspective, gluOrtho2D, glFrustum, gluLookAt, glTranslate, glRotate, glScale, etc. Checkout: QVector[2,3,4]D, QQuaterion, QMatrix4x4
  23. Move out of Clip space (fixed-function) Show code in Qt Creator.
  24. Uniforms A uniform is a OpenGL Shading Language (GLSL) constant parameter. Set in CPU code between glDraw() calls. Constant in the fashion that it has a constant value for all shader invocations originating from glDraw() calls until the value is changed. Use QOpenGLProgram.setUniform() to pass Model, View, Projection matrices to shader code before drawing.
  25. Move out of Clip Space (modern GL) Show code in Qt Creator.
  26. User Clip Planes Another thing to note is the glClipPlane() has been removed. Perform point/plane equation tests in your pixel shader and use keyword discard (instead of assigning to gl_FragColor) to inform OpenGL that that particular pixel should not be displayed.
  27. Managing OpenGL State Another thing to note is that glPushAttrib(), glPopAttrib(), glPushClientAttrib() and glPopClientAttrib() have been removed. You have to manually manage your OpenGL state by either keeping track of it in your C++ program (the preferred method) or by using glGet() to read the current state and then restoring it afterwards.
  28. Wrapping Up We were able to cover transitioning from fixed- function Vertex/Attribute data and the built-in Matrix stacks (and associated matrix functionality) to Modern OpenGL. We learned that Modern OpenGL replaced the “fixed” stuff with programmable shaders. We learned about the Vertex and Fragment shaders, what they do and how data flows through them. We learned that using Qt makes is very easy to create cross-platform Modern OpenGL code.
  29. For More Information For more information checkout the four-part blog series I wrote covering this topic. Also, check out our training class coming up in April out in Silicon Valley
  30. Outline For the Blog: Journal Entry Style - Introduction: Spent a lot of time in past life on porting complex, scenegraph based, fixed function OpenGL code to Modern Pipeline Code ... - Three Things that spring out to be addressed - Geometry and Lighting and Texturing - Picking, Text is another one for another day - Explain Geometry and Lighting using a simple scene example
  31. Simple Scene from <insert link here> Screenshot Code: window/context, geometry, drawing, resize, camera modelview, projection, viewport, light Modern Code: QOpenGLWindow.... Geometry: VertexBuffer, IndexBuffer, VertexArrayObjects, ...
  32. GLUT OpenGL Triangle (clip-space) void init(void) { glMatrixMode(GL_MODELVIEW); glLoadIdentity(); } void display(void) { glClear(GL_COLOR_BUFFER_BIT); glBegin(GL_TRIANGLES); glColor3f(1.0, 0.0, 0.0); glVertex3f(-1.0, -1.0, 0.0); glColor3f(0.0, 1.0, 0.0); glVertex3f( 0.0, 1.0, 0.0); glColor3f(0.0, 0.0, 1.0); glVertex3f( 1.0, -1.0, 0.0); glEnd(); glutSwapBuffers(); } void reshape(int w, int h) { glViewport(0, 0, (GLsizei) w, (GLsizei) h); glMatrixMode(GL_PROJECTION); glLoadIdentity(); } void keyboard (unsigned char key, int , int ) { if (key == 27) exit(0); }

Notes de l'éditeur

  1. in OpenGL 1.1, there was the concept of vertex arrays. Modern OpenGL keeps this concept,