Dr. Jim Rowan ITEC 2110 Chapter 3 Digital Media Dr. Jim Rowan ITEC 2110 Chapter 3

About Chapter 3 This is an overview of the next three chapters Vector graphics Bitmapped graphics Color This material is covered in 5 lectures Vector graphics part 1 Color part 1 Color part 2 Vector graphics part 2: 3D

Computer Graphics... A very different viewing media than print Usually consumed on a fairly low resolution monitor Forcing us to look carefully at the processes that move stuff from the real world to the computer... AND BACK! Graphic images work very differently on a screen than when in print can be seen with lights out will be viewed from different resolution monitors viewing angles are different reflections off screen

Computer Graphics Complex… arguably graphics are so important that they are an enabling technology in themselves Computer graphics on the WWW fostered the shift from print based media has begun to develop its own visual vocabulary Inside the computer there’s a numeric model of a real-world phenomenon Two ways to model computer graphics (images) numeric model #1: bitmapped images numeric model #2: vector graphics each with their advantages and disadvantages

But first: How to display data? This is a field of study all by itself that includes computer graphics, cognitive science and psychology The way data is displayed affects how people interpret the data how color is used the numeric scales used Different graphing forms emphasize different aspects of the numbers pie charts bar charts line graphs

Designing information display How to lie with statistics Edward Tufte, Yale University Visual Display of Quantitative Information Envisioning Information Visual Explanations

Computer Display types Now... all are rectangular arrays of pixels Not always that way Early graphics (1976) used a “steerable” electron gun, not raster graphics Since then... we have moved away from electron gun

Internal and External graphics models Internally an application keeps a numeric model Inside a computer it’s all numbers To display the internal model so we can see it, an application must project this internal model onto a bitmapped display The internal model, the numbers, are in the computer This process of projecting this model onto a computer display is called “rendering”

Two approaches to graphic modeling Why two approaches? drastic filesize differences each was good for its type of image Bitmapped graphics grandfathered name... more like pixel mapped graphics Vector graphics It’s more like object graphics because you describe objects using vectors (formulas)

With bitmapped graphics... There are logical and physical pixels images are modeled internally as an array of pixel values... the logical pixels physical pixels are the actual dots on screen Moving from logical and physical pixels called rendering may be different size, shape and different resolution will probably require clipping and scaling to move from logical to physical pixels for example…

00011000000011110000010110100000111100000001100000 A true bitmapped image is black and white Each logical pixel is represented by a single bit

When color came along it borrowed the idea... except that each logical pixel became a 3 byte RGB color specification instead of a single bit

1111 1111 . 1111 1111 . 1111 1111 0000 0000 . 0000 0000 . 1111 1111 1111 1111 . 0000 0000 . 0000 0000 ... for 1080 more bits... 1111 1111 . 1111 1111 . 1111 255 2550

72 bits in the color table 100 bits in the pixel map 172 bits total Question: With 2 bits encoding the color, if we expanded the color table, how many colors could be represented? 255 00 01 10

Vector graphics Internal model is very different than bitmapped graphics Images are described as mathematical equations Rendering is very different must translate EQUATIONS to physical pixels not simply clip or scale must compute the array of physical pixels from the equations

bitmapped graphic vector graphic Here are two images, blue squares Both are displayed at 72 pixels per inch Both are 1024 X 1024 pixels in size Each with 3 byte (24 bit, millions of colors) color encoding Which would be bigger? Why?

bitmapped graphic vector graphic Here are two images, blue squares Both are displayed at 72 pixels per inch Both are 318 X 318 pixels in size Each with 3 byte (24 bit, millions of colors) color encoding Which would be bigger? Why?

Bitmapped/Vector Graphics Bitmapped image size is set by affected by size, resolution and color resolution not affected by contents Vector graphics size is set by affected by the contents of the image the more complex, the larger the file gets size of the file is not affected by resolution

Bitmapped/Vector Graphics Selection of objects in the image vector is easy, objects are described by mathematical equations bitmapped, no objects, just pixels Special effect (like blur) differences bitmapped, easy vector, first convert to bitmapped, then blur

Bitmapped/Vector Graphics Scaling and Resize Vector? Simple... change formula Changes can be made BEFORE pixel values are calculated Bitmapped? Complicated... frequently results in artifacts Why is bitmapped complicated?

Original image: 10 x 5 Now make it twice as big [Draw on image]

Original image: 10 x 5 [Draw on image] Now make it twice as big What happens if there are two colors next to one another? Strictly duplicate? jagged edges Interpolate them?

Original image: 10 x 5 To make it 50% larger... What do you do? Do you make it 15 x 7? or 15 x 8? 1 pixel => 1? 2? There is no such thing as 1.5 pixels...

Bitmapped <==> Vector Vector can be easily converted to bitmapped... this process already exists since you must RENDER vectors to display them. Bitmapped to vector is complicated Vector is based on shapes… but bitmapped does not define any shapes Software must identify edges and find the shapes.

Image layers Bitmapped and vector graphics use layers as an organizational device In bitmapped graphics layers are used like digital tracing paper to isolate objects in the image colors can be separated and manipulated individually

Show Image: testPageImage.tiff 1574 pixels wide 1987 pixels tall RGB encoded No compression No table How Big?

Show Image: testPageImage.tiff Inspect it with mac cmd-I Open image with hexFiend How big is it? What is in it? Mostly 00... why?

HMMMMMmmm… We’ll talk more about this size issue in the chapter dedicated to bitmapped graphics We will also consider compression techniques other than the table method

And now for something completely different And now for something completely different! FLASHBACK To audio-related calculations!

Size and download speed calculations How big is the audio file? 44,100 samples per second 16 bit quantization stereo 3m16 seconds in length How long to load to youTube over highest speed dial up connection? adsl? How long to download from youTube over

Size and download speed calculations How big is the audio file? 44,100 samples per second 8 bit quantization stereo 3m16 seconds in length How long to load to youTube over highest speed dial up connection? adsl? How long to download from youTube over

Size and download speed calculations How big is the audio file? 8,000 samples per second 8 bit quantization stereo 3m16 seconds in length How long to load to youTube over highest speed dial up connection? adsl? How long to download from youTube over

Questions?