Graphics Concepts CS 2302, Fall 2014

11/17/20142 Color Concepts

11/17/20143 Color Color is a “visual perceptual property” Color is something that we perceive, it is a product of our visual system: eyes and brain Color is just one property of what we perceive

11/17/20144 Color Spectrum Color can also be taken to mean a physical characteristic of certain electromagnetic phenomena The image below is a picture of the spectrum. This mainly includes those wave-lengths of electromagnetic emissions that are seen with the human eye The numbers are the wave-length of the light. Nominally, this is the distance from one peak of an electromagnetic wave to the next peak. Shorter wavelength is higher frequency.

11/17/20145 Human Eye Sensitivity

11/17/20146 Human Eye Sensitivity Electromagnetic waves in the visible spectrum cause cells in the retina of the eye to react Different cell types respond to different wavelengths of light Roughly speaking, different types of cone cells respond to red, blue and green light Rod cells are more sensitive. They are what we use to see when light levels are low. However, rod cells do not provide color pereception

11/17/20147 RGB Model Typically what we perceive as a particular color is a mixture of many wavelengths of 'pure color' This complex mixture of wavelengths would be hard to simulate It turns out that we can 'fool' the eye by mixing red and blue and green in various intensities This is the RGB model of color

11/17/20148 RGB Model

11/17/20149 RGB Model Various colors can be described as a mixture of Red and Blue and Green in varying intensities Let 0 be the absence of a component and 1 be the most intense possible Then R = 1, G = 0, B = 0 is the most intense red that can be represented R = 1, G = 1, B = 0 is the most intense yellow R = 1, G = 1, B =1 is the most intense white R = 0, G = 0, B = 0 is black R =.5, G =.5, B =.5 is gray

11/17/ RGB Model In usual practice, the intensity of each component is represented by an integer in the range from 0 to 255, inclusive This means each component can be represented by 8 bits A full color can be represented in 24 bits or 3 bytes Most people cannot tell the differences in colors when changing a single value in a mixture

11/17/ RGB Model and Hardware The RGB model can be implemented in hardware by covering a screen with small dots that can glow either red or blue or green By varying the intensity at which these dots glow, different colors can be simulated RGB has

11/17/ RGB Limitations Darkening a medium orange to a dark orange requires an unintuitive change in R and G and B The RGB representation of a color may look quite different on different displays

11/17/ HSL and HSV Models There are other ways to describe colors The Hue-Saturation-Lightness and Hue-Saturation-Value describe colors in way less tied to hardware (both silicon and carbon based) Hue represents a point on the spectrum Saturation represents how deep the color is, ranging from gray to pastel to intensely tinted Lightness and Value measure the overall brightness of a color There are algorithms for transforming between RGB, HSL and HSV

11/17/ Mixing Graphical Components Very often in graphical images, two components will overlap. How are the colors of the two images mixed? One rule is to use the ‘z axis’: the component that is nearest to us will obscure the other In some cases, the colors will be mixed in some way. We perceive that as the front component being partially or fully transparent The alpha channel specifies complete transparency at 0 and complete opacity at some maximum value, depending on the system

11/17/ bit color representation By using the range 0 to 255 for alpha, a fourth byte can be used to specify colors This is called ARGB Each channel (red, green, blue, alpha) is represented by a value from 0 to 255. This means each channel takes one byte or 8 bits. So, all four take up 32 bits Using 32 bits works well with typical hardware.

11/17/ Pixels Images are displayed on modern systems using rectangular arrays of dots. Each dot can be independently set to whatever colors the device is capable of displaying Each dot is called a picture element or, for short, a pixel The number of rows and columns in a display is called the resolution Resolution is typically stated as rows x columns. Another important measure is the density of pixels

11/17/ Aspect Ratio The ratio of the number of rows to the number of columns is called the aspect ratio and is often quoted as a ratio My current display is 1920 rows by 1080 columns. This is an aspect ratio of 1920:1080 which reduces to 16:9 Aspect ratios sometimes can explain visual anomalies. Folks will tend to be very aware if you modify the aspect ratio of a picture

11/17/ US Capitol

11/17/ Using Color in Android

11/17/ Warning! If you have worked with Java desktop GUI applications (AWT or Swing) you will have used the Color class in the regular Java library. The Color class provided for Android is completely different.

11/17/ Class android.graphics.Color The Android Color class is not intended to be instantiated, it is used only to hold several methods useful in manipulating colors Reference: Predefined colors RGB methods HSV methods Lightening and darkening color

11/17/ A Demonstration Project We'll use a simple project to demonstrate using color in Android Start a new Android project The layout will be two TextView's arranged vertically The two views will be configured to take up the usable space The weight will be used to fill vertically but still share Erase any text in the two views Given the views helpful id's

11/17/ Setting Color Directly Set the color of one view by setting the background property directly Create a color entry in styles.xml (in the values folder) and use that for the other view

11/17/ Setting Color By Program We will look at several ways that a program can alter the color of components Since the different ways conflict with each other, each example will replace the previous one The different examples will be implemented as methods that are called by the onCreate method

11/17/ Using Color Class Resources Set the color of one TextView using a predefined color Set the color of the other TextView using RGB Goldenrod is a standard color name with RGB equal to (218, 165, 32)

11/17/ More Color Class Resources Get the red component of the top view and use that to set the bottom component to just the red part of the top view

11/17/ More Color Class Resources Set the bottom view to a darkened version of the top view This requires converting the RGB to HSV Modify the V, lowering it Convert HSV back to RGB Set the color of the bottom view If there is time: modify the saturation

11/17/ Use a Color Resource We saw how to use a color resource in the user interface editor Color resources can be used from the program as well Define a color resource named test_color Get a Resources object using getResources Call the getColor method on the resources object, providing the color id as a parameter: R.color.test_color Use the color number returned. Don’t use the color id itself, that will not give helpful results!

11/17/ Drawing in Android

11/17/ Android Alternatives Android provides several ways to create graphic images Drawable objects Defined by program code Defined in resource files Can be manipulated and animated 2D Canvas More in a moment, this is the approach we will take 3D Canvas

11/17/ Constructive Graphics Android supports building graphic images from simple components This is sometimes called vector graphics, recalling very early graphics display systems The organization of the visible interface and underlying code is very similar to other systems Event driven drawing Constructive graphics

11/17/ Drawing Surface Any View can be used as a drawing surface Reference Every View has an associated Canvas object that provides various drawing methods Canvas reference Even buttons and other widgets can be drawn on, though that may conflict with the drawing done for their basic look View has a method onDraw() that is called by the system when any drawing needs to be done

11/17/ Why Event Driven Drawing? In older systems, especially before Windows and Linux, drawing was carried out directly and immediately by programming commands In Android and other GUI systems, drawing is delayed until the system asks for it by calling a method This is because there are many situations in which the drawing may be needed, based on external circumstances The application is starting The application is being uncovered The application is being un-minimized

11/17/ Supporting Classes There are numerous supporting classes needed to carry out effective drawing. We mention just a few for now (the links lead to reference pages) Color (already discussed) Color Paint Paint.Style Path (will be discussed later) Path

11/17/ Class Paint Paint objects are used to specify many characteristics of drawing Paint can be used to specify The color used to outline and/or fill shapes The style of drawing a shape: filled or outlined Width of lines and curves How lines and curves will be 'joined' and 'capped' Characteristics of text such as font family, font size, and weight

11/17/ A Widget to Draw On Since drawing is carried out when the system calls the onDraw method in View, we must create a new class that Extends View Overrides onDraw The overriding method will carry out the drawing The onDraw method takes one parameter, a Canvas that can be used to create the graphics we wish

11/17/ Start an Application Start a new Android application In the same package with the main activity, create a new class named ViewForGraphics

11/17/ View Details View does not have a default constructor, so we must provide at least one constructor that uses 'super' It is convenient to use the constructor that takes two parameters public ViewForGraphics(Context context, AttributeSet attrs) This will allow the new class to be used like any other widget in the GUI editor We won't use that constructor in code When overriding onDraw, call the super method to make sure any standard actions are taken

11/17/ Creating the Interface In the GUI editor, remove the standard TextView Find the new 'widget' at the bottom of the palette Drag the new widget into the editing area Set the widget to fill the space completely Running the app at this point will show nothing

11/17/ Creating Paint Any drawing needs paint A default Paint object will work, but is monochrome Define a color Define the style for filling or not Define the stroke width

11/17/ Drawing a Figure Start with a Paint object Use one of the drawing methods from Canvas Rectangle, Circle and Oval are easy to use The example we will do is Draw a rectangle outline Draw a filled oval using the same coordinates The oval touches the sides of the rectangle Drawing a circle uses the center and radius, so beware!

11/17/ Drawing Text Some terminology Baseline Ascent, descent Leading Font family Drawing text Alignment