![OpenGL Global State Machine
ā
The OpenGL interface uses an architecture called a
āglobal state machineā
ā
Instead of doing this sort of thing:
Mesh myMesh;
myMesh.setData (some_array);
opengl.drawMesh (myMesh);
ā
in a G.S.M. we do this sort of thing [pseudo-code]:
unsigned int mesh_handle;
glGenMesh (&mesh_handle); // okay so far, just C style
glBindMesh (mesh_handle); // my mesh is now ātheā mesh
// affects most recently āboundā mesh
glMeshData (some_array);
ā
How this might cause a problem later?](https://image.slidesharecdn.com/013dproganton-170623132654/85/Computer-Graphics-Lecture-01-3D-Programming-I-12-320.jpg)
![File Naming Convention
ā
Upgarde the template ā load shader strings from text files
ā
post-fix with
ā .vert - vetex shader
ā .frag - fragment shader
ā .geom - geometry shader
ā .comp - compute shader
ā .tesc - tessellation control shader
ā .tese - tessellation evaluation shader
ā
allows you to use a tool like Glslang reference compiler to
check for [obvious] errors](https://image.slidesharecdn.com/013dproganton-170623132654/85/Computer-Graphics-Lecture-01-3D-Programming-I-40-320.jpg)
![Second Array
GLfloat points[] = {
0.0f, 0.5f, 0.0f,
0.5f, -0.5f, 0.0f,
-0.5f, -0.5f, 0.0f
};
GLfloat colours[] = {
1.0f, 0.0f, 0.0f,
0.0f, 1.0f, 0.0f,
0.0f, 0.0f, 1.0f
};](https://image.slidesharecdn.com/013dproganton-170623132654/85/Computer-Graphics-Lecture-01-3D-Programming-I-50-320.jpg)
Computer Graphics - Lecture 01 - 3D Programming I
- 1. 3d Programming I Dr Anton Gerdelan [email protected]
- 2. 3d Programming ā 3d programming is very difficult ā 3d programming is very time consuming
- 3. 3d Programming ā Practical knowledge of the latest, low-level graphics APIs is very valuable (CV) ā Good grasp of basic concepts in this semester ā Caveat - You must keep API knowledge up-to- date or it becomes redundant
- 4. Essential Checklist ā always have a pencil and paper ā solve your problem before you start coding ā know how to compile and link against libraries ā know how to use memory, pointers, addresses ā understand the hardware pipeline ā make a 3d maths cheat sheet ā do debugging (visual and programmatic) ā print the Quick Reference Card for OpenGL ā start assignments ASAP
- 5. Quick Background ā What is a vertex? ā Modern graphics hardware is able to draw: ā triangles ā points ā lines (unofficially deprecated) ā How do we define these? ā What is a normal?
- 10. OpenGL ā Open Grapics Library - originally the IrisGL by SGI ā Managed by Khronos Group {lots of big hw and sw companies} ā Vector graphics ā Only the spec. is sort-of open ā Various implementations, lots of platforms ā C API + driver + GLSL shaders ā All the APIs do the same jobs on hardware, but present different interfaces
- 11. OpenGL Caveats ā OpenGL has a very clunky old-fashioned C interface that will definitely confuse ā OpenGL has been modernised but there is a lot of left- over ācruftā that you shouldn't use ā Print the Quick Reference Card and double-check everything that you use. http://www.opengl.org/sdk/docs/ ā The naming conventions are very confusing ā OpenGL is currently being completely re-written to address these problems ā I'll try to help steer around these issues - use the discussion boards if unsure!
- 12. OpenGL Global State Machine ā The OpenGL interface uses an architecture called a āglobal state machineā ā Instead of doing this sort of thing: Mesh myMesh; myMesh.setData (some_array); opengl.drawMesh (myMesh); ā in a G.S.M. we do this sort of thing [pseudo-code]: unsigned int mesh_handle; glGenMesh (&mesh_handle); // okay so far, just C style glBindMesh (mesh_handle); // my mesh is now ātheā mesh // affects most recently āboundā mesh glMeshData (some_array); ā How this might cause a problem later?
- 13. OpenGL Global State Machine ā Operations affect the currently āboundā object ā Be very careful. Write long functions. ā Can sometimes āunbindā by binding to 0 (no object) ā Operations āenableā states like blending/transparency ā They affect all subsequently drawn things ā Don't forget to ādisableā things ā Can get messy in larger programmes ā Write long functions.
- 14. Graphics API Architecture ā Set-up and rendering loop run on CPU ā Copy mesh data to buffers in graphics hardware memory ā Write shaders to draw on GPU (graphics processing unit) ā CPU command queues drawing on GPU with this shader, and that mesh data ā CPU and GPU then run asychronously
- 15. Before We Look at Code ā OpenGL functions preceded by āgl...()ā ā OpenGL data types preceded by āGL...ā ā OpenGL constants preceeded by āGL_...ā ā Find each feature used in man pages http://www.opengl.org/sdk/d ocs/man/ ā Parameters, related functions
- 16. Walk-Through āHello Triangleā ā Upgrade graphics drivers to download latest OpenGL libraries ā GLEW or GL3W - extensions and newest GL.h ā GLFW or FreeGLUT or SDL or Qt - OS window/context
- 17. Start GLFW (or FreeGLUT) ā Start GLFW ā We use GLFW to start an OpenGL context ā Can use FreeGLUT, SDL, Qt, etc. Instead ā Can also do manually, but ugly
- 18. Hint to Use a Specific OpenGL Version ā Leave commented the first time, it will try to run latest v. ā We can print this out to see what you have on the machine ā Uncomment and specify to force a specific version (good, safe practice) ā CORE and FORWARD mean ādon't allow any old, deprecated stuffā ā Apple only has 3.2 and 4.1? core, forward implemented
- 19. Create a Window on the OS ā GLFW/FreeGLUT etc. do this in a platform- independent way ā Tie OpenGL context to the window ā Refer to GLFW docs for each function
- 20. Start GLEW (or GL3W) ā Windows has a 1992? Microsoft gl.h in the system folder ā Makes sure you are using the latest gl.h ā Makes extensions available (experimental/new feature plug-ins) ā Ask OpenGL to print the version running (check in console) ā 3.2+ is fine for this course
- 22. Create Vertex Buffer Object (VBO) ā We copy our array of points from RAM to graphics memory ā Note ābindingā idea ā The vbo variable is just an unsigned integer that GL will give an unique ID to
- 23. Create Vertex Array Object (VAO) ā āWhatā to draw ā Meta-data with āattributeā pointer that says what data from a VBO to use for a shader input variable
- 24. Vertex Shader and Fragment Shader Strings ā āHowā to draw (style) ā Set vertex positions in range -1 to 1 on X,Y,Z ā gl_Position is the built-in variable to use for final position ā Colour in RGBA (red blue green alpha) range 0.0 to 1.0 ā Like C but more data types.
- 25. Compile and Link Shaders ā Compile each shader ā Attach to shader program (another bad naming convention) ā Link the program ā Should check compile and linking logs for errors
- 26. Drawing Loop ā Clear drawing buffer {colour, depth} ā āUseā our shader program ā āBindā our VAO ā Draw triangles. Start at point 0. Use 3 points. How many triangles? ā Swap buffers. Why?
- 27. Terminate ā Makes sure window closes if your loop finishes first ā Compile, link against GLEW and GLFW and OpenGL ā VS Template on Blackboard ā Makefiles for Linux/Mac/Windows https://github.com/capnramses/antons_opengl_tutorials_book/ ā Don't get stuck on linking/projects - start fiddling now!
- 28. āHello Triangleā ā Getting GL started is a lot of work ā āIf you can draw 1 triangle, you can draw 1000000ā ā Go through some tutorials
- 29. Pause and Review ā What are the main components of a modern 3d graphics programme? ā Where do we store mesh data (vertex points)? ā In what? ā How do we define the format of the data? ā What do we need to do before we tell OpenGL to draw with glDrawArrays() etc.?
- 30. Things to Think About or Try ā Can we change the colour of the triangle drawn? ā How can we extend the triangle into a square? ā Could we load mesh data from a file rather than an array? ā Shaders can be loaded from files. If you change a shader in an external file, do you need to re- compile? ā How would you draw a second, separate shape?
- 31. 3d Programming I Part B
- 32. Graphics System Architecture ā move data to graphics hardware before drawing ā minimise use of the bus
- 33. Asynchronous Processing ā Minimise CPU-GPU bus comm. Overhead ā Maximise parallel processing to reduce overall GPU time ā āClientā gl commands queue up and wait to be run on GPU ā Can glFlush() to say āhurry up, I'm waitingā and glFinish() to actually wait ā don't normally need to use these
- 34. Closer Look at Shaders
- 35. GPU Parallelism ā 1 vertex = 1 shader = 1 core ā 1 fragment = 1 shader = 1 core ā but drawing operations serialise ā .: 1 big mesh draw is faster than many small draws GPU Uniform Shader Cores GeForce 605 48 Radeon HD 7350 80 GeForce GTX 580 512 Radeon HD 8750 768 GeForce GTX 690 1536 Radeon HD 8990 2304
- 36. ā minimum: ā vertex shader ā fragment shader ā also have GPGPU ācomputeā shaders now ā note: Direct3D hw pipeline is the same, but different names
- 37. Difference Between Fragment and Pixels ā Pixel = āpicture elementā ā Fragment = pixel-sized area of a surface ā Each triangle is divided into fragments on rasterisation ā All fragments are drawn, even the obscured ones* ā If depth testing is enabled closer fragments are drawn over farther-away fragments ā .: Huge #s of redundant FS may be executed
- 38. Shader Language OpenGL Version GLSL Version Tag 1.2 none none 2.0 1.10.59 #version 110 2.1 1.20.8 #version 120 3.0 1.30.10 #version 130 3.1 1.40.08 #version 140 3.2 1.50.11 #version 150 3.3 3.30.6 #version 330 4.0 4.00.9 #version 400 4.1 4.10.6 #version 410 4.2 4.20.6 #version 420 4.3 4.30.6 #version 430 4.4 ... #version 440 ... ... ...
- 39. GLSL Operators and Data Types ā Same operators as C ā no pointers ā bit-wise operators since v 1.30 data type detail common use void same as C functions that do not return a value bool, int, float same as C vec2, vec3, vec4 2d, 3d, 4d floating point points and direction vectors mat2, mat3, mat4 2x2, 3x3, 4x4 f.p. matrices transforming points, vectors sampler2D 2d texture texture mapping samplerCube 6-sided texture sky boxes sampler2DShadow shadow projected texture ivec3 etc. integer versions
- 40. File Naming Convention ā Upgarde the template ā load shader strings from text files ā post-fix with ā .vert - vetex shader ā .frag - fragment shader ā .geom - geometry shader ā .comp - compute shader ā .tesc - tessellation control shader ā .tese - tessellation evaluation shader ā allows you to use a tool like Glslang reference compiler to check for [obvious] errors
- 41. Example Vertex Shader #version 400 in vec3 vertex_position; void main() { gl_Position = vec4 (vertex_position, 1.0); } ā Macro to explicitly set GLSL version required. Can also use #defines ā in keyword ā variable from previous stage in pipeline. Q. What is the stage before the vertex shader? ā vec3 - 3d vector. Store positions, directions, or colours. ā Every shader has a main() entry point as in C. ... contd.
- 42. Example Vertex Shader #version 400 in vec3 vertex_position; void main() { gl_Position = vec4 (vertex_position, 1.0); } ā vec4 - has 4th component. gl_Position uses it to determine perspective. Set by virtual cameras. For now leave at 1.0 - "don't calculate any perspective". ā Can also use out keyword to send variable to next stage. Q. What is the next stage? ā Every VS positions one vertex between -1:1,-1:1,-1:1. Q. How does every vertex end up in a different position then?
- 43. ā minimum: ā vertex shader ā fragment shader ā also have GPGPU ācomputeā shaders now ā note: Direct3D hw pipeline is the same, but different names
- 44. Example Fragment Shader #version 400 uniform vec4 inputColour; out vec4 fragColour; void main() { fragColour = inputColour; } ā What important pipeline process happens first? ā A uniform variable is a way to communicate to shaders from the main application in C ā For each fragment set a vec4 to RGBA (range 0.0 to 1.0) Q. What is the next stage? Where does out go? Q. What can the alpha channel do?
- 45. Uniforms // do this once, after linking the shader p. not in the main loop int inputColour_loc = glGetUniformLocation (my_shader_program, āinputColourā); if (inputColour_loc < 0) { fprintf (stderr, āERROR inputColour variable not found. Invalid uniformnā); do something rash; } // do this whenever you want to change the colour used by this shader program glUseProgram (my_shader_program); glUniform4f (inputColour_loc, 0.5f, 0.5f, 1.0f, 1.0f); ā Uniforms are 0 by default i.e. our colour=black ā Unused uniforms are optimised out by shader compiler ā Basic uniforms are specific and persistent to one shader programme ā Uniforms are available to all shaders in programme ā Uniforms are a constant value in all shaders ā You can change a uniform once, every frame, or whenever you like ā Keep uniform updates to a minimum (don't flood the bus)
- 46. Adding Error-Checking Functionality ā After calling glCompileShader() ā Check GL_COMPILE_STATUS with glGetShaderiv() ā If failed, get the log from glGetShaderInfoLog() and print it! ā After calling glLinkProgram() ā Check GL_LINK_STATUS with glGetProgramiv() ā Get the log from glGetProgramInfoLog() and print it! ā Check out the man-pages to find out how to use these functions. ā Add this to your projects! It gives you basic error checking with the line numbers.
- 47. Shadertoy ā WebGL tool to experiment with shaders on-the-fly ā implement entirely in a fragment shader https://www.shadertoy.com/
- 48. Example: Adding Vertex Colours
- 49. Data Layout Options ā We could: ā Concatenate colour data onto the end of our points buffer: array = XYZXYZXYZRGBRGBRGB ā Interleave colours between points: array = XYZRGBXYZRGBXYZRGB ā Just create a new array and new vertex buffer object array1 = XYZXYZXYZ array2 = RGBRGBRGB
- 50. Second Array GLfloat points[] = { 0.0f, 0.5f, 0.0f, 0.5f, -0.5f, 0.0f, -0.5f, -0.5f, 0.0f }; GLfloat colours[] = { 1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f };
- 51. Second VBO GLuint points_vbo = 0; glGenBuffers (1, &points_vbo); glBindBuffer (GL_ARRAY_BUFFER, points_vbo); glBufferData (GL_ARRAY_BUFFER, sizeof (points), points, GL_STATIC_DRAW); GLuint colours_vbo = 0; glGenBuffers (1, &colours_vbo); glBindBuffer (GL_ARRAY_BUFFER, colours_vbo); glBufferData (GL_ARRAY_BUFFER, sizeof (colours), colours, GL_STATIC_DRAW);
- 52. Tell the VAO where to get 2nd variable GLuint vao; glGenVertexArrays (1, &vao); glBindVertexArray (vao); glEnableVertexAttribArray (0); glBindBuffer (GL_ARRAY_BUFFER, points_vbo); glVertexAttribPointer (0, 3, GL_FLOAT, GL_FALSE, 0, NULL); glEnableVertexAttribArray (1); glBindBuffer (GL_ARRAY_BUFFER, colours_vbo); glVertexAttribPointer (1, 3, GL_FLOAT, GL_FALSE, 0, NULL); Second shader input variable Still a vec3 Change these 2 variables if interleaved or concatenated in one VBO
- 53. Modify Vertex Shader #version 400 layout (location=0) in vec3 vp; // point layout (location=1) in vec3 vc; // colour out vec3 fcolour; void main () { fcolour = point; // āpass-throughā output gl_Position = vec4 (vp, 1.0); } Q. Why does the colour input have to start in the vertex shader?
- 54. Modify the Fragment Shader #version 400 in vec3 fcolour; out vec4 frag_colour; void main () { frag_colour = vec4 (fcolour, 1.0); } Q. There are more fragments than vertices. What values will fcolour have in each fragment?
- 55. Interpolation
- 56. Drawing Modes ā Points can be scaled ā Lines are deprecated but still sort-of work ā How would you change your triangles to an edges wireframe?
- 57. Winding Order and Back-Face Culling ā Easy optimisation ā don't waste time drawing the back face or inside-facing parts of a mesh ā How do we define the āfrontā and ābackā ā = order that points are given ā Clock-wise order or Counter clock-wise glEnable (GL_CULL_FACE); // cull face glCullFace (GL_BACK); // cull back face glFrontFace (GL_CW); // usually front is CCW