Graphics Concepts

CS 2302, Fall 2014

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Color Concepts

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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

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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.

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Human Eye Sensitivity

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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

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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

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RGB Model

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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

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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

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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

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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

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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

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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

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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.

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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

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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

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US Capitol

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Using Color in Android

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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.

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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

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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

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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

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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

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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)

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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

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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

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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!

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Drawing in Android

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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

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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

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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

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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

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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) Paint Paint.Style Path (will be discussed later)

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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

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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

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Start an Application

Start a new Android application In the same package with the main activity, create a new

class named ViewForGraphics

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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

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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

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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

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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!

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Drawing Text

Some terminology Baseline Ascent, descent Leading Font family

Drawing text Alignment