Digital Media Dr. Jim Rowan ITEC 2110 Chapter 3

Roll call

Computer Graphics... Arguably a very different viewing media –Forced to look carefully at the processes that move stuff from the real world to the computer... AND BACK! –Usually consumed on a fairly low resolution monitor 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... magazine (gloss)

Computer Graphics Complex enough to be arguably an enabling technology not just a single thing WWW –fostered the shift from text based media –has begun to develop its own visual vocabulary Ways to model computer graphics –bitmapped images –vector graphics –each with their advantages and disadvantages

But first: How to display a bunch of data? This is a field of study all by itself that includes computer graphics, cognitive science and psychology For example: Different graphing forms –pie charts –bar charts –use color for delineation?

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 Externally an application must project this internal model onto a bitmapped display –This process is called “rendering” An application must also project this internal model onto a storage model to be able to store and recall it later

Two approaches to graphic modeling Bitmapped graphics –grandfathered name... more like pixel mapped graphics Vector graphics –more like object graphics because you describe objects using vectors (formulas) a line is defined by slope, length & position a circle is radius & position a square is length of side & position

Bitmapped graphics... 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 and different resolution –will probably require clipping and scaling to move from logical to physical pixels

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 specification instead of a single bit

for 1080 more bit

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?

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

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 graphic vector graphic

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

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 converted to bitmapped (rasterized)... this process already exists since you 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 this 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

Questions?