CS 2302, Fall 2014 Graphics Concepts
Color Concepts 11/17/2014 2
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 11/17/2014 3
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. 11/17/2014 4
Human Eye Sensitivity 11/17/2014 5
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 11/17/2014 6
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 11/17/2014 7
RGB Model 11/17/2014 8
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 11/17/2014 9
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 11/17/2014 10
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 11/17/2014 11
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 11/17/2014 12
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 11/17/2014 13
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 ● 11/17/2014 14
32-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. 11/17/2014 15
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 11/17/2014 16
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 11/17/2014 17
US Capitol 11/17/2014 18
Using Color in Android 11/17/2014 19
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. 11/17/2014 20
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: http://developer.android.com/reference/android/graphics/Color.html ● Predefined colors ● RGB methods ● HSV methods ● Lightening and darkening color 11/17/2014 21
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 11/17/2014 22
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 11/17/2014 23
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 11/17/2014 24
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) 11/17/2014 25
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 11/17/2014 26
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 11/17/2014 27
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! 11/17/2014 28
Drawing in Android 11/17/2014 29
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 11/17/2014 30
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 11/17/2014 31
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 11/17/2014 32
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 11/17/2014 33
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