Chapter03

Ch 3: Digital Imaging Fundamentals

Photoshop CS6 EssentialsBy Scott Onstott


Benefits of More Megapixels• Larger DSLR sensors capture millions

more pixels (megapixels) compared to point-and-shoot cameras

• More megapixels means you have more detail and less noise

• You can digitally zoom in to large images and the details remain crisp

• More megapixels also means more data, larger file sizes, and more RAM required


Pixel Aspect Ratio• Pixels can have non-square aspect

ratios• Use square pixels for print and for the

Web• Video clips often use non-square

pixels• You can change the pixel aspect ratio

using the View menu and in the File > New dialog box


Aliasing and Anti-Aliasing• Aliasing is the stair-stepping artifact

that occurs in grids of pixels• Lines are not aliased when you draw

them horizontally, vertically, or at 45 degree angles

• Off-angled lines exhibit aliasing which can be reduced by dithering the edge (blue line at right) with intermediate values

• Anti-aliased edges appear blurry compared to aliased edges but the stair-stepping is removed


Primary Colors• Red, Green, and Blue are the primary

additive colors• Cyan, Magenta, and Yellow are the

primaries of subtractive colors• CMY are secondary colors in the RGB

system• RGB are secondary colors in the CMY

system• Black (abbreviated as K) is added to the

subtractive system, making CMYK the 4-colors used in offset printing


Color Channels• Color is split into primary components such as Red, Green, and Blue• Primary color components are stored in grayscale image channels• Color is reconstituted by illuminating red, green, and blue sub-pixels in

your monitor with varying intensities according to the luminance values in the corresponding color channels

Photo courtesy of iStockphoto, ©Neustockimages, Image #7007990


Alpha Channels• Alphas are additional channels• Alpha 1 is used to store

transparency information • When the Alpha 1 channel is

displayed against the RGB channels a red rubylith overlay appears to indicate areas of transparency

• Not all formats support alpha channels

• TIF and TGA do support alphas

Photo courtesy of iStockphoto, ©Neustockimages, Image #7007990


Understanding Lab Color• L* a* b* stands for Lightness, a, and b

channels• Detail is carried in the Lightness

channel• Color is carried using a combination of

a and b channels• Lab color is a convenient mode that

isolates image detail (can be used for sharpening or grayscale conversion)

Photo courtesy of iStockphoto, ©Terry J Alcorn, Image #14990625


Grayscale vs. Bitmap Modes• Grayscale images have a range of

tonal values in a single channel (256 gradations in 8-bit)

• Bitmap images also employ a single channel

• Bitmap images are the most primitive images having only black and white pixels without any intermediate shades of gray

• Bitmap images can be dithered or processed with a halftone screen

Photo courtesy of iStockphoto, ©Terry J Alcorn, Image #14990625


Understanding Histograms• Histograms help you see how well the

dynamic range is being utilized• Histograms are made of black vertical bars

representing how many pixels are in the shadows, mid-tones, and highlights

• Blank areas at the edges of a histogram show there are values (blackest blacks or whitest whites) that aren’t being used

• Gaps or spikes in a histogram show that information has been “destroyed” by being pushed beyond certain mathematical limits

Art & Photos

Chapter03