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Introduction to 2D Graphics

Introduction to 2D Graphics. Using OpenGL. Why Learn About OpenGL?. A well-known industry standard for real-time 2D and 3D computer graphics Available on most platforms Desktop operating systems, mobile devices (OpenGL ES* , e.g., Android), browsers (WebGL)

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Introduction to 2D Graphics

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  1. Introduction to 2D Graphics Using OpenGL 2D Graphics using OpenGL – 9/10/2019

  2. Why Learn About OpenGL? • A well-known industry standard for real-time 2D and 3D computer graphics • Available on most platforms • Desktop operating systems, mobile devices (OpenGL ES* , e.g., Android), browsers (WebGL) • Older (OpenGL 1.0) API provided features for rapid prototyping; newer API (OpenGL 2.0 and newer) provides more flexibility and control • Many old features exist in new API as “deprecated” functionality, supported only for backwards-compatibility with legacy apps • We will use the new API exclusively * ES is for “Embedded Systems” 2D Graphics using OpenGL – 9/10/2019

  3. Why Learn 2D first? • A good stepping stone towards 3D – many issues much easier to understand in 2D • no need to simulate lights, cameras, the physics of light interacting with objects, etc. • intro to modeling vs. rendering and other notions • get used to rapid prototyping in OpenGL, both of designs and concepts • 2D is still really important and the most common use of computer graphics, e.g. in UI/UX, documents, browsers 2D Graphics using OpenGL – 9/10/2019

  4. Graphics Platforms (1/4) • We’re writing an interior-design app • Application Model (AM) is the data being represented by a rendered image • manipulated by user interaction with the application • typically a hierarchical model, with components built from lower-level components • In our application, AM contains positions & size of each bed, dresser, and table 2D Graphics using OpenGL – 9/10/2019

  5. Graphics Platforms (2/4) • Graphics Platform runs in conjunction with window manager • Determines what section of the screen is allocated to the application • Handles “chrome” (title bar, resize handles); client area is controlled by application 2D Graphics using OpenGL – 9/10/2019

  6. Graphics Platforms (3/4) • Typically, AM uses client area for: • user interface to collect input to the AM • display some representation of AM in the viewport • This is usually called the scene, in the context of both 2D and 3D applications • Scene is rendered by the scene generator, which is typically separate from the UI generator, which renders rest of UI 2D Graphics using OpenGL – 9/10/2019

  7. Graphics Platforms (4/4) • Early raster graphics packages/libraries/platforms • RamTek library 1981, Apple QuickDraw 1984 • Microsoft's Graphics Display Interface (GDI 1990, now GDI+), Java.awt.Graphics2D • Earliest packages were low-level, closely bound to the h/w, and usually had: • geometric primitives/shapes, appearance attributes specified in attribute bundles • Such attributes might include primitive interior color, stroke width, or line pattern • Attributes applied modally rather than in a parameter list for each primitive (too many parameters for that) • integer coordinates map directly to screen pixels on output device • immediate mode (no record kept of display commands) • no built-in functions for applying transforms to primitives • no built-in support for component hierarchy (no composite shapes) • Little more than assembly languages for display device 2D Graphics using OpenGL – 9/10/2019

  8. Problems with Early Graphics Platforms (1/3) Geometric Scalability • Integer coordinates mapped to display pixels affects apparent size of image: large on low-res display & small on high-res display • need to get away from thinking about pixels at all and instead think about samples which can then be rendered on arbitrary-resolution displays • Application needs flexible internal coordinate representation to match the natural coordinate system of the application domain, e.g., angstrom, lightyears,… • floating point is essential for precision • float to fixed conversion required; actually a general mapping 2D Graphics using OpenGL – 9/10/2019

  9. Problems with Early Graphics Platforms (2/3) Display updates • To perform operations on objects in scene, application must keep list of all primitives and their attributes (along with application-specific data) • Some updates are transitory “feedback animations,” only a display change • Consider our interior-design application: drag-and-drop to move an object • when user selects an object and drags to new location, object is highlighted and then follows cursor movement • interim movements do not relate to data changes in application model, purely visual changes to provide feedback in the “interaction loop” • application model only updated when user drops object (releases mouse button) • in immediate mode, application must re-specify entire scene each time cursor moves 2D Graphics using OpenGL – 9/10/2019

  10. Problems with Early Graphics Platforms (3/3) Interaction • Consider a simple clock example for an immediate mode graphics library: • User clicks minute hand, location must be mapped to relevant application object; called pick correlation • Developer responsible for pick correlation (usually some kind of "point-in-bounding box rectangle" test based on pick coordinates) • find top-most object at clicked location • may need to find entire composite object hierarchy from lowest-level primitive to highest level composite • e.g., triangle -> hand -> clock • Solution: retained mode can do pick correlation, as it has a representation of scene 2D Graphics using OpenGL – 9/10/2019

  11. Modern Graphics Platforms (1/2) • Device-independent floating point coordinate system • packages convert “application-space" to "device-space" coordinates • Specification of hierarchy • support building scenes as hierarchy of objects, using transforms (scale, rotate, translate) to place children into parents' coordinate systems • support manipulating composites as coherent objects • Smart Objects (Widgets, etc.) • graphic objects have innate behaviors and interaction responses • e.g., button that automatically highlights itself when cursor is over it 2D Graphics using OpenGL – 9/10/2019

  12. Modern Graphics Platforms (2/2) GUI Platforms: Layout Managers and Smart Controls • WPF (Microsoft) • Cocoa (Apple) • JavaFX (Oracle) • Qt (Qt Group) • SVG (Adobe) • GDI+ (Microsoft) • Quartz (Apple) • HTML5 canvas (W3C) • QuickDraw (Apple) • Orig. GDI (Microsoft) Retained Templates and Reusability Floating-point Coordinates Immediate Integer Pixel Coordinates 2D Graphics using OpenGL – 9/10/2019

  13. Immediate Mode Vs Retained Mode Immediate Mode (OpenGL, Vulkan, Microsoft’s DirectX, Apple’s Metal) • Driven from Application model: as always stores both geometric information and non-geometric information in Application Database • Graphics platform keeps no record of primitives that compose scene • Vulkan was originally the next version of OpenGL (code name OpenGL Next) but was eventually released as its own API. Both are maintained by the Khronos Group 2D Graphics using OpenGL – 9/10/2019

  14. Immediate Mode Vs Retained Mode Retained Mode (WPF, SVG, most game engines) • Application model in app and Display model in graphics platform • Display model contains information that defines geometry to be viewed • is a geometric subset of Application model (typically a scene graph) • Graphics platform keeps record of primitives that compose scene • Simple drawing application does not need Application model (e.g., clock example) • No right answer on which to use – context-dependent tradeoffs (see Chapter 16) • convenience vs. overhead of the library having to maintain and synch this data structure 2D Graphics using OpenGL – 9/10/2019

  15. In class Question 1 2D Graphics using OpenGL – 9/10/2019

  16. OpenGL (1/3) • Immediate-mode graphics API • No display model, application must directOpenGL to draw primitives • Implemented in C, also works in C++ • Bindings available for many other programming languages • Cross-platform • Also available on mobile (OpenGL ES) and in the browser (WebGL) • Different platforms provide ‘glue’ code for initializing OpenGL within the desktop manager (e.g. GLX, WGL) • Labs and projects for CS123 use Qt library to abstract this glue code away 2D Graphics using OpenGL – 9/10/2019

  17. OpenGL (2/3) • Created by Silicon Graphics Inc. (SGI, http://sgi.com) in 1992, now managed by the non-profit Khronos Group (http://khronos.org) • Originally aimed to allow any OpenGL program to run on a variety of graphics hardware devices • Invented when “fixed-function” hardware was the norm • Techniques were implemented in the hardware; OpenGL calls sent commands to the hardware to activate / configure different features • e.g., fixed-function API implemented linear algebra needed to move objects on screen 2D Graphics using OpenGL – 9/10/2019

  18. OpenGL (3/3) • Now supports programmable hardware – the common industry practice today • Modern graphics cards are miniature, highly parallel computers with many-core GPUs, on-board RAM, etc. • GPUs are a large collection of highly parallel high speed arithmetic units; several thousand cores! • GPUs run simple programs called “shaders”: take in vertices and other data and output a color value for an individual pixel. • GLSL, (O)GL Shader Language, is C-like language, controls arithmetic pipelines • Other shader languages: (DirectX) High-Level Shader Language, RenderMan Shading Language for offline rendering • Your final project (typically a team project) will involve writing your choice of shaders (learned in labs) • Only true for desktop; must use shaders exclusively to program with OpenGL ES 2.0+ or WebGL • We will use GLM (OpenGL Mathematics) to do our linear algebra instead of using the old Fixed-function API 2D Graphics using OpenGL – 9/10/2019

  19. Shaders • In future labs and your final project you will write your own shaders, but for now we will provide shaders for you • Various types of input to shaders • Attributes are the properties of a single vertex • Position, normal vector are examples of these • Uniforms are properties with a single value for multiple vertices (or an entire object) • Scale factor, rotation are examples of these • OpenGL has many built in types including vectors and matrices • Inputs are provided to a particular “location” within the shader • “Location” just an identifier used by shader to determine where value is stored • glGetAttribLocation returns location of particular attribute • glGetUniformLocation returns location of particular uniform • Labs will cover details on how to use these functions 2D Graphics using OpenGL – 9/10/2019

  20. Representing Shapes • 3D shapes are usually represented as a collection of vertices that make up triangles or quads • OpenGL uses triangles • Other methods include 3D voxels, polynomial spline curves and surfaces, etc. • We can use triangles to build arbitrary polygons, and approximate smooth shapes. A complex polygon made of triangle primitives An approximate circle made of triangle primitives 2D Graphics using OpenGL – 9/10/2019

  21. Coordinate Systems (1/3) • Cartesian coordinates in math, engineering • typically modeled as floating point • typically X increasing right, Y increasing up • Display (physical) coordinates • integer only • typically X increasing right, Y increasing down • 1 unit = 1 pixel • But we want to be insulated from physical display (pixel) coordinates • OpenGL is the intermediary 2D Graphics using OpenGL – 9/10/2019

  22. Coordinate Systems (2/3) • OpenGL Coordinates map from app to pixels in the window • Choose a convention • We will use a traditional Cartesian system • Units are based on the size of the window or screen • Visible area stretches to fill canvas – our OGL glue code provides a fixed-size square canvas • Stretches all content inside, so a square will stretch to a rectangle as window is expanded • Units correlate to percentage of window size, don’t correspond to physical units or pixels • Define coordinate system using the projection matrix. Supply it to shader as a uniform variable (the term projection matrix will become clear) • We use standard [-1, 1] coordinate system for 2D glm::mat4 projectionMat; // Our projection matrix is a 4x4 matrixprojectionMat = glm::ortho(-1, // X coordinate of left edge 1, // X coordinate of right edge -1, // Y coordinate of bottom edge 1, // Y coordinate of top edge 1, // Z coordinate of the “near” plane -1); // Z coordinate of the “far” plane 2D Graphics using OpenGL – 9/10/2019

  23. Coordinate Systems (3/3) • Two choices on how to think • Draw everything in OpenGL coordinate system • This can get very inconvenient! • Choose your own abstract coordinate system natural for your app (in nanometers, lightyears,…), then specify all app’s primitives to OpenGL using your coordinates. • Must also specify a transformation to map the application coordinates to OpenGL coordinates • “Transformation” usually means a change - scale, rotate, and/or translate – or a combination of changes Application Coordinates Display OGL Coordinates 2D Graphics using OpenGL – 9/10/2019

  24. In class Question 2 2D Graphics using OpenGL – 9/10/2019

  25. Winding Order (a little intrusion of 3D into our 2D world) • Order is important: vertices must be specified in counter-clockwise order relative to the viewer. Otherwise nothing shows up! • Winding order determines whether a triangle’s normal vector is facing in front or behind it. Triangles facing the wrong way will be invisible! • Counter-clockwise winding consistent with the “right-hand rule” GLfloat vertexData[] = { -.7, -.7, -.7, .7, .7, .7, .7, -.7, }; GLfloat vertexData[] = { -.7, -.7, .7, -.7, .7, .7, -.7, .7, }; N ✓ N X 2D Graphics using OpenGL – 9/10/2019

  26. Transformations (1/3) • Standard object transformations are scaling, rotation, and translation • These transformations can be represented by multiplying matrices • Don’t worry about specifics for now – the math is explained in viewing lectures • Use GLM to do linear algebra necessary for these transformations • Builds (hierarchical) models that constitute “the scene” (aka “the world”) • For now, we will only use the model matrix which is used to position objects in terms of the OpenGL coordinate system • For examples below assume our model matrix starts as an identity matrix • This means each object’s position will only be changed by the transformations we apply to it; our coordinate system is the OGL coordinate system • To create this identity matrix in code, initialize as: glm::mat4 model = glm::mat4(1.0); 2D Graphics using OpenGL – 9/10/2019

  27. Transformations (2/3) • Geometric Transformations in 2D (note the z-coordinate is 0) • Positive angles rotate counter-clockwise, here about the origin (i.e., Z-axis as vector) Rotate model = glm::rotate(-45, glm::vec3(0, 0, 1)) * model; model = glm::translate(.1, .1, 0) * model; Original Original Translate Original Scale 2D Graphics using OpenGL – 9/10/2019 model = glm::scale(2, 2, 1) * model;

  28. Transformations (3/3) Original Original model *= glm::rotate(-90, glm::vec3(0, 0, 1)); model *= glm::scale(2, 1, 1); model *= glm::rotate(-90, glm::vec3(0, 0, 1)); model *= glm::scale(2, 1, 1); 2D Graphics using OpenGL – 9/10/2019 Transformations can be composed (combined by multiplying matrices) but are NOT commutative, so proper order is vital

  29. Animation (1/3) • Rapidly displaying sequence of images to create an illusion of movement • Flipbook (https://www.youtube.com/watch?v=8k2h4c7uHWs) • Keyframe animation: specify keyframes, computer interpolates (e.g., ball bouncing) Image Credit: https://alexshawanimation.wordpress.com/2014/09/24/the-bouncing-ball/ Keyframe Animation 2D Graphics using OpenGL – 9/10/2019 Flipbook

  30. Animation (2/3) • Idea: Move objects incrementally every time we render • Example: Animating the hands of a clock • Given the number of seconds elapsed, how many degrees should we rotate the seconds hand? • need to convert from seconds to degrees • Idea: Use rotations around the clock as a common conversion factor • Seconds per revolution: 60 • Degrees per revolution: 360 • Every time we render, we need to recalculate the position of the hands 2D Graphics using OpenGL – 9/10/2019

  31. Animation (3/3) //Example in code float secondsElapsed = ...; // Time since last render float secondHandAngle = 0; Point pSecondHand; pSecondHand.y = center.y + (radius * sin(secondHandAngle)); pSecondHand.x = center.x + (radius * cos(secondHandAngle)); drawLine(center, pSecondHand); secondHandAngle += -1                   // Turn clockwise           * secondsElapsed// Δt           * DEGREES_PER_REVOLUTION   // Turn 360 degrees ...           / SECONDS_PER_REVOLUTION;  // ... every 60 seconds github.com/sprintr/opengl-examples/blob/master/OpenGL-Clock-Animated.cpp 2D Graphics using OpenGL – 9/10/2019

  32. Book Sections • Preface, Intro as useful background • Chapter 2 – while written in terms of Microsoft’s WPF, a retained-mode library, the concepts carry over to OGL. Useful to know about HTML/XML style syntax, given its prominence, but don’t worry about the syntactic details 2D Graphics using OpenGL – 9/10/2019

  33. OpenGL Basics Lab (1/2) • An intro to OpenGL data structures to represent vertex geometry held this week • Generate 2D graphics and learn the modern OpenGL pipeline • Fragment: a pixel-independent sample within a triangle with all its associated attributes, e.g., color, depth, texture coordinates, … the fragment shader provides a many-to-one mapping between fragments and pixels (e.g. for supersampling, see Image Processing 2) 2D Graphics using OpenGL – 9/10/2019

  34. OpenGL Basics Lab (2/2) • First lab available • It’s an important foundation • So start on Lab 1 if you haven't already (it pairs nicely with the OpenGL lectures)! • Reminder: you can get your labs checked off by a TA at hours as well, but please come to lab sections if possible! 2D Graphics using OpenGL – 9/10/2019

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