CS452/552; EE465/505. Image Formation

Transcription

1 CS452/552; EE465/505 Image Formation

2 Outline! Image Formation! Introduction to WebGL, continued Draw a colored triangle using WebGL Read: Angel, Chapters 2 & 3 Homework #1 will be available on github tomorrow at noon 2

3 Image Formation! Fundamental imaging notions! Physical basis for image formation Light Color Perception! Synthetic camera model! Other models

4 Image Formation! In computer graphics, we form images which are generally two dimensional using a process analogous to how images are formed by physical imaging systems Cameras Microscopes Telescopes Human visual system

5 Elements of Image Formation! Objects! Viewer! Light source(s)! Attributes that govern how light interacts with the materials in the scene! Note the independence of the objects, the viewer, and the light source(s)

6 Light! Light is the part of the electromagnetic spectrum that causes a reaction in our visual systems! Generally these are wavelengths in the range of about nm (nanometers)! Long wavelengths appear as reds and short wavelengths as blues

7 Luminance and Color Images! Luminance Image Monochromatic Values are gray levels Analogous to working with black and white film or television! Color Image Has perceptional attributes of hue, saturation, and lightness Do we have to match every frequency in visible spectrum? No!

8 Three-Color Theory! Human visual system has two types of sensors Rods: monochromatic, night vision Cones Color sensitive Three types of cones Only three values (the tristimulus values) are sent to the brain! Need only match these three values Need only three primary colors

9 Additive and Subtractive Color! Additive color Form a color by adding amounts of three primaries CRTs, projection systems, positive film Primaries are Red (R), Green (G), Blue (B)! Subtractive color Form a color by filtering white light with Cyan (C), Magenta (M), and Yellow (Y) filters Light-material interactions Printing Negative film

10 Pinhole Camera Use trigonometry to find projection of point at (x,y,z) x p = -x/z/d y p = -y/z/d z p = d These are equations of simple perspective

11 Synthetic Camera Model projector p image plane projection of p center of projection

12 Advantages! Separation of objects, viewer, light sources! Two-dimensional graphics is a special case of three-dimensional graphics! Leads to simple software API Specify objects, lights, camera, attributes Let implementation determine image! Leads to fast hardware implementation

13 Global vs Local Lighting! Cannot compute color or shade of each object independently Some objects are blocked from light Light can reflect from object to object Some objects might be translucent

14 Ray Tracing and Geometric Optics! One way to form an image is to follow rays of light from a point source finding which rays enter the lens of the camera. However, each ray of light may have multiple interactions with objects before being absorbed or going to infinity. Can handle global effects: Multiple reflections, Translucent objects Slow Must have whole data base available at all times

15 Why not ray tracing?! Ray tracing seems more physically based so why don t we use it to design a graphics system?! Possible and is actually simple for simple objects such as polygons and quadrics with simple point sources! In principle, can produce global lighting effects such as shadows and multiple reflections but ray tracing is slow and not well-suited for interactive applications! Ray tracing with GPUs is close to real time

16 Practical Approach! Process objects one at a time in the order they are generated by the application Can consider only local lighting! Pipeline architecture application program! All steps can be implemented in hardware on the graphics card display 16

17 Immediate Mode Graphics! Geometry specified by vertices Locations in space( 2 or 3 dimensional) Points, lines, circles, polygons, curves, surfaces! Immediate mode Each time a vertex is specified in application, its location is sent to the GPU Old style uses glvertex Creates bottleneck between CPU and GPU Removed from OpenGL 3.1 and OpenGL ES 2.0

18 Retained Mode Graphics! Put all vertex attribute data in array! Send array to GPU to be rendered immediately! Almost OK but problem is we would have to send array over each time we need another render of it! Better to send array over and store on GPU for multiple renderings

19 Execution in Browser

20 Example: red triangle, revisited! How to draw lines? points?! How is the data sent to the GPU, and interpreted as the variables in the shaders?

21 Shaders! We assign names to the shaders that we can use in the JS file! These are trivial pass-through (do nothing) shaders that which set the two required built-in variables gl_position gl_fragcolor! Note both shaders are full programs! Note vector type vec4! Must set precision in fragment shader

22 Files../Common/webgl-utils.js: Standard utilities for setting up WebGL context in Common directory on website../common/initshaders.js: contains JS and WebGL code for reading, compiling and linking the shaders../common/mv.js: our matrix-vector package square.js: the application file

23 Example Code <!DOCTYPE html> <html> <head> <script id="vertex-shader" type="x-shader/x-vertex"> attribute vec4 vposition; void main(){ gl_position = vposition; } </script> <script id="fragment-shader" type="x-shader/x-fragment"> precision mediump float; void main(){ gl_fragcolor = vec4( 1.0, 0.0, 0.0, 1.0 ); } </script>

24 Notes onload: determines where to start execution when all code is loaded canvas gets WebGL context from HTML file vertices use vec4 type in MV.js JS array is not the same as a C or Java array object with methods vertices.length // 4 Values in clip coordinates

25 Notes initshaders used to load, compile and link shaders to form a program object Load data onto GPU by creating a vertex buffer object on the GPU Note use of flatten() to convert JS array to an array of float32 s Finally we must connect variable in program with variable in shader need name, type, location in buffer

26 JS File (cont) // Configure WebGL gl.viewport( 0, 0, canvas.width, canvas.height ); gl.clearcolor( 1.0, 1.0, 1.0, 1.0 ); // Load shaders and initialize attribute buffers var program = initshaders( gl, "vertex-shader", "fragment-shader" ); gl.useprogram( program ); // Load the data into the GPU var bufferid = gl.createbuffer(); gl.bindbuffer( gl.array_buffer, bufferid ); gl.bufferdata( gl.array_buffer, flatten(vertices), gl.static_draw );

27 JS File (cont) // Associate our shader variables with our data buffer }; var vposition = gl.getattriblocation( program, "vposition" ); gl.vertexattribpointer( vposition, 2, gl.float, false, 0, 0 ); gl.enablevertexattribarray( vposition ); render(); function render() { gl.clear( gl.color_buffer_bit ); gl.drawarrays( gl.triangles, 0, 3 ); }

28 Example: colored triangle! Let s make each vertex of the triangle a different color: red, green and blue what will this look like? 28

29 Example: colored triangle! Let s make each vertex of the triangle a different color: red, green and blue 29

30 (-1, 1) (0, 1) Normalized Device Coordinate System (1, 1) (-1, 0) (0, 0) (1, 0) (-1, -1) JEFF CHASTINE (0, -1) (1, -1) 30

31 Coordinates of our triangle (0.0f, 0.5f, 0.0f) (-0.5f, -0.5f, 0.0f) (0.5f, -0.5f, 0.0f) JEFF CHASTINE 31

32 ColorFillTriangle.html // ColorFillTriangle.html based on (c) 2012 matsuda <!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8" /> <title>draw a triangle with different vertex colors</title> <script id="vertex-shader" type= x-shader/x-vertex"> // on next slide </script> <script id="fragment-shader" type= x-shader/x-fragment"> // next slide </script> <script type="text/javascript" src="../common/webgl-utils.js"></script> <script type="text/javascript" src="../common/initshaders.js"></script> <script type="text/javascript" src="colorfilltriangle.js"></script> </head>

33 Vertex Shader <script id="vertex-shader" type="x-shader/x-vertex"> attribute vec4 a_position; attribute vec4 a_color; varying vec4 v_color; void main() { } gl_position = a_position; v_color = a_color; </script>

34 Fragment Shader <script id="fragment-shader" type= x-shader/x-fragment"> precision mediump float; varying vec4 v_color; void main() { gl_fragcolor = v_color; } </script>

35 ColorFillTriangle.html, cont. <!DOCTYPE html> <html lang="en"> <head> // on previous slide </head> <body> <canvas id="gl-canvas" width="400" height="400"> Please use a browser that supports "canvas" </canvas> </body> </html>

36 ColorFillTriangle.js // ColorFillTriangle.js based on (c) 2012 matsuda window.onload = function init() { // Retrieve <canvas> element var canvas = document.getelementbyid( "gl-canvas" ); // Get the rendering context for WebGL var gl = WebGLUtils.setupWebGL(canvas); if (!gl) { console.log('failed to get the rendering context for WebGL'); return; } gl.viewport( 0, 0, canvas.width, canvas.height ); gl.clearcolor( 0.0, 0.0, 0.0, 1.0 );

37 ColorFillTriangle.js, cont. // Create shading program var program = initshaders( gl, "vertex-shader", "fragment-shader" ); gl.useprogram( program ); gl.program = program; // initvertexbuffers() is on the next slide var n = initvertexbuffers(gl); if (n < 0) { console.log('failed to set the vertex information'); return; } } // Specify the color for clearing <canvas> gl.clearcolor(0.0, 0.0, 0.0, 1.0); // Clear <canvas> gl.clear(gl.color_buffer_bit); // Draw the rectangle gl.drawarrays(gl.triangles, 0, n);

38 ColorFillTriangle.js, cont. function initvertexbuffers(gl) { var verticescolors = new Float32Array([ // Vertex coordinates and color 0.0, 0.5, 1.0, 0.0, 0.0, -0.5, -0.5, 0.0, 1.0, 0.0, 0.5, -0.5, 0.0, 0.0, 1.0, ]); var n = 3; // Create a buffer object var vertexcolorbuffer = gl.createbuffer(); if (!vertexcolorbuffer) { console.log('failed to create the buffer object'); return false; } // continued on next slide

39 ColorFillTriangle.js, cont. // Bind the buffer object to target gl.bindbuffer(gl.array_buffer, vertexcolorbuffer); gl.bufferdata(gl.array_buffer, verticescolors, gl.static_draw); var FSIZE = verticescolors.bytes_per_element; //Get the storage location of a_position, assign & enable buffer var a_position = gl.getattriblocation(gl.program, 'a_position'); if (a_position < 0) { console.log('failed to get the storage location of a_position'); return -1; } gl.vertexattribpointer(a_position, 2, gl.float, false, FSIZE * 5, 0); gl.enablevertexattribarray(a_position); // Enable the assignment of the buffer object

40 ColorFillTriangle.js, cont. // Get the storage location of a_color, assign buffer and enable var a_color = gl.getattriblocation(gl.program, 'a_color'); if(a_color < 0) { console.log('failed to get the storage location of a_color'); return -1; } gl.vertexattribpointer(a_color, 3, gl.float, false, FSIZE * 5, FSIZE * 2); gl.enablevertexattribarray(a_color); // Enable the assignment of the buffer object // Unbind the buffer object gl.bindbuffer(gl.array_buffer, null); } return n;

41 Vertex Processing! Much of the work in the pipeline is in converting object representations from one coordinate system to another Object coordinates Camera (eye) coordinates Screen coordinates! Every change of coordinates is equivalent to a matrix transformation! Vertex processor also computes vertex colors

42 Projection! Projection is the process that combines the 3D viewer with the 3D objects to produce the 2D image Perspective projections: all projectors meet at the center of projection Parallel projection: projectors are parallel, center of projection is replaced by a direction of projection

43 Primitive Assembly Vertices must be collected into geometric objects before clipping and rasterization can take place Line segments Polygons Curves and surfaces

44 Clipping Just as a real camera cannot see the whole world, the virtual camera can only see part of the world or object space Objects that are not within this volume are said to be clipped out of the scene

45 Rasterization! If an object is not clipped out, the appropriate pixels in the frame buffer must be assigned colors! Rasterizer produces a set of fragments for each object! Fragments are potential pixels Have a location in frame bufffer Color and depth attributes! Vertex attributes are interpolated over objects by the rasterizer

46 Fragment Processing! Fragments are processed to determine the color of the corresponding pixel in the frame buffer! Colors can be determined by texture mapping or interpolation of vertex colors! Fragments may be blocked by other fragments closer to the camera Hidden-surface removal

47 Lack of Object Orientation! All versions of OpenGL are not object oriented so that there are multiple functions for a given logical function! Example: sending values to shaders gl.uniform3f gl.uniform2i gl.uniform3dv! Underlying storage mode is the same

48 WebGL function format function name dimension gl.uniform3f(x,y,z) belongs to WebGL canvas x,y,z are variables gl.uniform3fv(p) p is an array

49 WebGL constants! Most constants are defined in the canvas object In desktop OpenGL, they were in #include files such as gl.h! Examples desktop OpenGL glenable(gl_depth_test); WebGL gl.enable(gl.depth_test) gl.clear(gl.color_buffer_bit)

50 WebGL and GLSL! WebGL requires shaders and is based less on a state machine model than a data flow model! Most state variables, attributes and related pre 3.1 OpenGL functions have been deprecated! Action happens in shaders! Job of application is to get data to GPU

51 GLSL! OpenGL Shading Language! C-like with Matrix and vector types (2, 3, 4 dimensional) Overloaded operators C++ like constructors! Similar to Nvidia s Cg and Microsoft HLSL! Code sent to shaders as source code! WebGL functions compile, link and get information to shaders

52 Coordinate Systems! The units in points are determined by the application and are called object, world, model or problem coordinates! Viewing specifications usually are also in object coordinates! Eventually pixels will be produced in window coordinates! WebGL also uses some internal representations that usually are not visible to the application but are important in the shaders! Most important is clip coordinates

53 Coordinate Systems and Shaders! Vertex shader must output in clip coordinates! Input to fragment shader from rasterizer is in window coordinates! Application can provide vertex data in any coordinate system but shader must eventually produce gl_position in clip coordinates! Simple example uses clip coordinates

54 WebGL Camera! WebGL places a camera at the origin in object space pointing in the negative z direction! The default viewing volume is a box centered at the origin with sides of length 2

55 Viewports! Do not have use the entire window for the image: gl.viewport(x,y,w,h)! Values in pixels (window coordinates)

56 Next Topic! Transformations & Viewing! User Interaction