Bitmapped Images

Bitmap Images

Today’s Objectives

• Identify characteristics of bitmap images

• Resolution, bit depth, color mode, pixels

• Determine the most appropriate color mode to edit images

• Calculate file size for various size images

• Explain relationship between bit depth, image size, and file size

• Determine appropriate image resolution for final output

• Resample images (up) and resize images for linking to page layout document

Raster Images & Resolutions

• Resolution refers to how well the image is resolved.– Full range of colors or tones

• Raster refers to images that contain dots or pixels in a vertical and horizontal row format– Synonymous with Bitmap, Pixel based

Bitmapped (Raster) Graphics

•Scanned or digital images

•Composed of individual dots or pixels–Defined by a rectangular grid

Bit Depth

• Refers to the number of values that can be stored in a pixel– 1 bit = 2 colors– 8 bit = 256 colors– 24 bit = 16,000,000 colors

Bitmap Mode– 1-bit image– Pixels are either black or white– Used for line art

Grayscale Mode– 8-bit image– Pixels can be any 1 of 256 tones– Used for continuous tone images for B&W

reproduction

RGB Mode

– 24-bit image using Red, Green, & Blue– Uses 3 (8-bit) channels of R,G,B light– TV and video monitors used RGB light– Pixels can any 1 of 256 shades of R,G, & B– Three channels combines to produce 24-bit color– RGB colors on screen brighter than CMYK

RGB Mode– Not all colors in RGB can

be reproduced using CYMK printing

– Should work on images in RGB until all editing completed

• Faster

• More editing options

CYMK Mode– Based on 4-color process printing– Uses 4 channels CYMK– The 4th (black) channel does not increase

number of colors– CYM = K (black)– CYMK has smaller gamut than RGB

RGB vs. CMYK

RGG CMYK

What Mode to Work In?

– Scan as RGB, – Most scanners/cameras capture as RGB It is

faster to work in RGB – Some options/filters only work in RGB– Perform most editing in RGB– Then convert and fine tune in CYMK– Trend towards RGB workflow

Index Mode– Uses 1 channel– Limited to max 256 colors (user defined)– Used primarily for web based images– Can vary the bit depth depending on

number of colors used– Web safe colors less than 256 colors

Bitmapped Graphics

Increase bit depth, increase file size arithmetically• 24-bit image; 3 times as large as a 8 bit image

– 1 bit = 2 colors

– 8 bit = 256 colors

– 24 bit = 16 million colors

Bitmapped (Raster) Graphics

• Increase bit depth, increase file size arithmetically– 24-bit image; 3 times as large as a 8 bit image

• Increase image dimension, increase the file size geometrically – Double image dimension, increase file size by a factor of 4

• To Determine File size– Resolution2 x width x height x bits / 8,192 = file size– Ex. 4 x 5 image

• 3002 x 4 x 5 x 1 / 8,192= 220K (1 bit)• 3002 x 4 x 5 x 8 / 8,192= 1,757 (8 bit)• 3002 x 4 x 5 x 24 / 8,192= 5,273K (24 bit)

Bitmapped Graphics (cont.)

• Image resolution– ppi = pixels per inch (dpi, lpi)

– Number of pixel displayed per unit length

– At 150 ppi a 1” x 1” = 22,500 pixels

– Once scanned, usually cannot improved resolution

DO NOT SCALE IMAGES IN PAGE LAYOUT APPLICATION!

Bitmapped Graphics (cont.)

• Image resolution– Need aprox 1.5-2 times image resolution compared to final

reproduction process

– Can resample down, (reduce number of pixels)

Printed images are made up of halftone dots to create illusion of tones

These are PIXELS, note the difference!

Bitmapped Graphics (cont.)

• Monitor resolution– Pixels are translated directly to monitor pixels

– Mac = 72 ppi

– PC = 96 ppi

– If resolution greater than monitor, image appears bigger on screen

Monitor Resolution

72 ppi

144 ppi