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Efficiently Pushing Geometry and Color Data to the GPU with OpenGL

This guide explores how to push geometry, color, and vertex data efficiently to the GPU using OpenGL. It outlines the process of creating buffer objects, binding them, and populating them with vertex properties including positions and colors. By understanding the concepts of normalized device coordinates and the RGB color model, you will be equipped to render triangles and other shapes on the screen. Additionally, the guide covers the management of buffer IDs, loading data types with different frequencies, and the relationship between client-side code and shaders.

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Efficiently Pushing Geometry and Color Data to the GPU with OpenGL

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  1. Pushin’ Geo to the GPU As well as colors, normals, and other vertex data Jeff Chastine

  2. This is what we want to make Jeff Chastine

  3. (-1, 1) (0, 1) (1, 1) Normalized Device Coordinate System (-1, 0) (1, 0) (0, 0) (1, -1) (-1, -1) (0, -1) Jeff Chastine

  4. Coordinates of our triangle (0.0f, 0.5f, 0.0f) (-0.5f, -0.5f, 0.0f) (0.5f, -0.5f, 0.0f) Jeff Chastine

  5. QUICK! Color Theory! • Represent almost any color by adding red, green and blue • Alpha is the transparency • Called the primary colors (RGB or RGBA) • 0.0 means “all the way off” • 1.0 means “all the way on” • Examples: • Red (1.0, 0.0, 0.0, 1.0) • Blue (0.0, 0.0, 1.0, 1.0) • Purple (1.0, 0.0, 1.0, 1.0) • Yellow (1.0, 1.0, 0.0, 1.0) • White (1.0, 1.0, 1.0, 1.0) • Black (0.0, 0.0, 0.0, 1.0) • Grey (0.5, 0.5, 0.5, 1.0) • Brown (0.7, 0.5, 0.1, 1.0) Jeff Chastine

  6. Colors of our triangle* (0.0f, 0.0f, 1.0f, 1.0f) (1.0f, 0.0f, 0.0f, 1.0f) (0.0f, 1.0f, 0.0f, 1.0f) *Note the beautiful interpolation of color! Jeff Chastine

  7. Basic Problem • Get the geometry and color to the GPU • Typically also need a normal and texture coordinatefor each vertex! • Ask the OpenGL driver to create a buffer object • This is just a chunk of memory (e.g. array) • Nothing to be afraid of! • Located on the GPU (probably) Jeff Chastine

  8. Working with Buffers • To create a buffer ID: // This will be the ID of the bufferGLuint buffer; // Ask OpenGL to generate exactly 1 unique IDglGenBuffers(1, &buffer); • To set this buffer as the active one and specify which buffer we’re referring to: glBindBuffer(GL_ARRAY_BUFFER, buffer); • Notes: • That buffer is now bound and active! • Any “drawing” will come from that buffer • Any “loading” goes into that buffer Jeff Chastine

  9. Two Approaches to Loading the Buffer with Data • Assume everything is in GLfloat* (called data) • One-shot call to load the buffer with data: glBufferData(GL_ARRAY_BUFFER, sizeof(data), data, GL_STATIC_DRAW); • Other drawing types GL_X_Y: • X • STREAM for infrequent use and changes • STATIC for frequent use and infrequent change • DYNAMIC for frequent use and frequent change • Y could be DRAW, READ or COPY Jeff Chastine

  10. How We’ll do it • Process • Create the buffer and pass no data • Load the geometry • Load the colors (if any) after that • Load the normals after that… • Note: some like to interlace their vertex data… Jeff Chastine

  11. A Side-By-SIDE Comparison • Show the relationship between client code and shader code • Assume we’re still trying to draw 1 triangle… • numVertices – the number of vertices (will be 3 - duh!) • verts – the position information of each vertex (XYZ - array of GLfloats) • colors – the color information of each vertex (RGBA - array of GLfloats) Jeff Chastine

  12. buffer GLuintbuffer; glGenBuffers(1, &buffer); Note: buffer “lives” on the graphics card in a nice, two-bedroom loft… Jeff Chastine

  13. Hey – I’m active now buffer GLuintbuffer; glGenBuffers(1, &buffer); glBindBuffer(GL_ARRAY_BUFFER, buffer); Jeff Chastine

  14. Now I know how big I am! buffer GLuintbuffer; glGenBuffers(1, &buffer); glBindBuffer(GL_ARRAY_BUFFER, buffer); glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW); Allocate how much space (in bytes) we need Where’d the 7 come from? Jeff Chastine

  15. Now I know how big I am! buffer GLuintbuffer; glGenBuffers(1, &buffer); glBindBuffer(GL_ARRAY_BUFFER, buffer); glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW); Allocate how much space (in bytes) we need Where’d the 7 come from? (x, y, z) + (r, g, b, a) = 7 Jeff Chastine

  16. Now I’m putting verts at the beginning buffer verts GLuintbuffer; glGenBuffers(1, &buffer); glBindBuffer(GL_ARRAY_BUFFER, buffer); glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW); glBufferSubData(GL_ARRAY_BUFFER, 0, numVertices*3*sizeof(GLfloat), verts); glBufferSubData (GL_ARRAY_BUFFER, numVertices*3*sizeof(GLfloat), numVertices*4*sizeof(GLfloat), colors); Put verts at 0… it’s pretty bigthough… Jeff Chastine

  17. I’m putting colors next buffer verts colors GLuintbuffer; glGenBuffers(1, &buffer); glBindBuffer(GL_ARRAY_BUFFER, buffer); glBufferData(GL_ARRAY_BUFFER, numVertices*7*sizeof(GLfloat), NULL, GL_STATIC_DRAW); glBufferSubData(GL_ARRAY_BUFFER, 0, numVertices*3*sizeof(GLfloat), verts); glBufferSubData (GL_ARRAY_BUFFER, numVertices*3*sizeof(GLfloat), numVertices*4*sizeof(GLfloat), colors); Put colors startingright after that! It’s pretty bigtoo… Jeff Chastine

  18. What we Have so far… • We have a buffer with an ID • That buffer lives on the graphics card • That buffer is full of vertex position/color data • How do we get that info to our shader? Jeff Chastine

  19. How To link to the Shader • Query the shader program for its variables • The code below goes into the shader program and gets the “vPosition” ID GLuintvpos;vpos = glGetAttribLocation (programID, “vPosition”); • In OpenGL, we have to enable things (attributes, in this case): glEnableVertexAttribArray(vpos); // turn on vPosition • Finally, we set the location and tell it the format of the data in the buffer glVertexAttribPointer(vpos, 3, GL_FLOAT, GL_FALSE, 0, 0); void glVertexAttribPointer(GLuint index, GLint size, Glenum type, GLboolean normalized, GLsizei stride, constGLvoid* offset); Jeff Chastine

  20. GLuintloc = glGetAttribLocation(shaderProgramID, "vPosition"); #version 130 in vec4 vPosition; in vec4 vColor; out vec4 color; void main () { color = vColor; gl_Position = vPosition; } Find the variable “vPosition” inside the shader Jeff Chastine

  21. Guys! I’m still active, remember? buffer GLuintloc = glGetAttribLocation(shaderProgramID, "vPosition");glEnableVertexAttribArray(loc);glVertexAttribPointer(loc, 3, GL_FLOAT, GL_FALSE, 0, 0); #version 130 in vec4 vPosition; in vec4 vColor; out vec4 color; void main () { color = vColor; gl_Position = vPosition; } verts colors Jeff Chastine

  22. Tell vColor where to find its data in me… buffer verts colors GLuintloc2 = glGetAttribLocation(shaderProgramID, "vColor");glEnableVertexAttribArray(loc2);glVertexAttribPointer(loc2, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(numVertices*3*sizeof(GLfloat))); #version 130 in vec4 vPosition; in vec4 vColor; out vec4 color; void main () { color = vColor; gl_Position = vPosition; } Jeff Chastine

  23. GLuintloc = glGetAttribLocation(shaderProgramID, "vPosition");glEnableVertexAttribArray(loc);glVertexAttribPointer(loc, 3, GL_FLOAT, GL_FALSE, 0, 0); GLuintloc2 = glGetAttribLocation(shaderProgramID, "vColor");glEnableVertexAttribArray(loc2);glVertexAttribPointer(loc2, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(numVertices*3*sizeof(GLfloat))); #version 130 in vec4 vPosition; in vec4 vColor; out vec4 color; void main () { color = vColor; gl_Position = vPosition; } Jeff Chastine

  24. One Last thing • Vertex Array Objects (VAOs) • “Pure State” - it remembers almost everything about buffers • Set it up once, then just call it before drawing! • glVertexAttribPointer… • Doesn’t bind the VBO though… • Creating a vertex array object: // This will be the ID of the VAOGLuintvao; // Ask the driver for exactly 1 unique IDglGenVertexArrays(1, &vao); // Everything after this will be part ofthe VAOglBindVertexArray(vao); Jeff Chastine

  25. // Create the "remember all" glGenVertexArrays(1, &vao); glBindVertexArray(vao); // Create a buffer and bind it as active glGenBuffers(1, &vbo); glBindBuffer(GL_ARRAY_BUFFER, vbo); // Create space and load the buffer glBufferData(GL_ARRAY_BUFFER, 7*3*sizeof(GLfloat), NULL, GL_STATIC_DRAW); glBufferSubData(GL_ARRAY_BUFFER, 0, 3*3*sizeof(GLfloat), vertices); glBufferSubData(GL_ARRAY_BUFFER, 3*3*sizeof(GLfloat),3*4*sizeof(GLfloat), colors); // Find the positions of the variables in the shader positionID= glGetAttribLocation(shaderProgramID, "vPosition"); colorID= glGetAttribLocation(shaderProgramID, "vColor"); // Tell the shader variables where to find their data glVertexAttribPointer(positionID, 3, GL_FLOAT, GL_FALSE, 0, 0); glVertexAttribPointer(colorID, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(3*3*sizeof(GLfloat))); glUseProgram(shaderProgramID); glEnableVertexAttribArray(positionID); glEnableVertexAttribArray(colorID); Jeff Chastine

  26. END WITH A UTAH TEAPOT! Jeff Chastine

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