General glossary of image & printing

General Glossary of Image & Printing Terminology

image scanning & resolutions:
- image sizes
printing terminology
image formats
see also:

Determining desired image resolution for output to computer screen or video:
- Note if the image is to be used solely for viewing on screen, then no need for final image resolution to exceed the screen resolution = screen pixels width / screen inches width
  - ie. on 15" monitor with 28cm/11" displayable width then:
    - 800x600 video resol. = 800/11 = 73dpi screen resolution
    - 640x480 on same monitor has resolution of: 58dpi
    - 1024x800 on same monitor has resolution of 93dpi
- For Full Screen output:
  - scanning resolution = screen resolution width (pixels) / original image width (inches)
    - eg. 4"x3" image to 640x480 size then scan at 160dpi (640/4)
- For Actual Size output on screen:
  - res. = screen resol. width (pixels) / max. displayable screen width (inches)
    - eg. 4"x3" image to appear on 800x600pixel 11.75" wide screen at 4"x3" image then scan @ 68dpi (800/11.75")
- For Scanning 35mm slide/neg. (36x24mm) to size 100x67mm on 800x640 screen then scan at (100/36)*73dpi = 203dpi
Determining image resolution for output to printed media:
- desired scan res. = (final image ht/orig. image ht) x halftone freq (lpi) x 2
  - eg. 4"x5" image to output as 3"x3.75" on laser printer with 60lpi then scan at 90dpi ((3.75/5)x60x2)

Computer image file sizes (uncompressed):

= image height x image width x (scan res.)^2 x bytes/pixel

Image type	Bytes per pixel	Bits per pixel	No colors(2^bits)	Kbytes for 800x600 image
B&W	0.125	1	2	60
Indexed 16 color	0.5	4	16	240
Indexed 256 color	1	8	256	480
Grayscale	1	8	256	480
16bit	2	16	65,536	960
24bit "True color"	3	24	16,777,216	1440
30bit CYMK	3.75	30	1 x 10⁹	1800

Moire patterns:
- repetitive patterns on scanned images esp. if scanned from halftones
- minimise by either:
  - use continuous tone image source
  - scan at twice resolution needed, then resample to desired resolution
  - try using "Blur" or "Average" to soften image
  - if speckled then try using "Despeckle" function

Printing Terminology:

Print types:
- Continuous-tone art: original photographs
  - simulated by: dye sublimation printers
  - commercial CYMK / black ink printing
- Halftone screens: commercial printing using series of dots to make a cell:
  - "photographic halftone" - dots can vary infinitely in size
  - "digital halftone" dots fixed size
  - CMYK cyan/magenta/yellow/black - 4-color ink printing
  - screen or halftone frequency or line frequency:
  - # cells per inch = lines per inch (lpi)
    - as cells aligned in rows
  - screen or halftone angle:
    - orientation of the line of halftone cells:
      - B&W: usually = 45deg.
      - CYMK: black 45deg; M 75; Y 90; C 105 deg.
dot gain:
- used to compensate for spread of dot size due to absorbent paper or thin ink used.
printer resolution (dots per inch / dpi):
- dot matrix 180 dpi;
- laser printer 300dpi;
- image setters 1270 - 2540 dpi;
- dpi/lpi = #dots/cell
paper types:
- weight
- absorbency
- eg.:
  - newsprint: <= 85lpi standard
  - uncoated stock: <=120lpi
  - coated stock: <= 300lpi
Typesetting measurements:
- pica typsetting measurement approx. 1/6th of an inch = 12 points
- point typesetting measurement = 1/12th of pica ie. approx. 72pts per inch
- em typesetting measurement = width of point size being used
- en typesetting measurement = 1/2 em

Pixel "Picture Element"
- eg. VGA screen 640x480 pixels

Digital computer RGB format:
- 24bit color = 8bits per primary color (R/G/B) eg 640x480x24bits=921.6Kb;

Image compression:
- Bezier drawing:
  - similar to "connect-the-dots" drawing & can "pull" line to create curves;
- Huffman coding - compression technique
- Lempel-Ziv Welch (LZW) compression technique
- DCT compression -
  - processes 8x8pixel blocks
  - no good for dithered or quantised pictures as too many high frequencies;
- YUV color reduction - can reduce image file size by 30-60%
  - 4:1:1
  - 4:2:2 better

Image Formats:

Bitmap Images:
- BMP - MS Windows bitmap format
- PCX - mainly used by paint programs
- TGA - high quality bitmaps in 16, 24 & 32bit formats - ?needs TARGA video board;
- TIFF - various formats:
  - G, P, RVB, CMYK , YUV;
- GIF Compuserve compressed images for modem data transfers;
  - lossless - uses Lempel-Ziv Welch (LZW) compression;
- SCD - SCODL created by film recorders for slide making ? bitmap;
- RLE - run length encoding compressed bitmap file - only good for animation images;
  - - rows of adjacent identical pixels grouped; lossless;
- JPEG Joint Photographers Experts Group file format using superior compression;
  - various formats: JFF, JTF, JPG, CMP (Lead bitmaps));
  - lossless - uses only predictive & statistical techniques;
  - lossy divides picture into 8x8 pixel blocks & uses properties of DCT to reduce data
  - controlled by quality factor Q which is a % & controls quantification process prior to Huffman coding - the lower Q the poorer quality;
Vector Graphics ie "object-based" images:
- can be converted to bitmaps by a "rasteriser"
- WMF - MS Windows metafile
- CGM - Computer Graphics Metafile
- CDR - created by CorelDraw!
- WPG - Word Perfect 5 graphic
- PIC - 2 different formats:
  - Lotus graphic
    - created by various slide-making equipment;
- PIF - intermediate format used to enable translation by IBM mainframe to its GDF
- PLT plotter format for drawings based on HPGL (HP Graphics Language);
- PCT - Macintosh format - PICT 1 (B/W); PICT 2 (color);
- EPS - Encapsulated PostScript file format for PostScript printers
- GEM - ? vector; created by Graphics Environment Manager eg. Xerox Ventura Publisher