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Raster (Bitmap) Graphic File Formats & Standards
Contents
• Raster (Bitmap) Images
• Digital Or Printed Images
• Resolution
• Colour Depth
• Alpha Channel
• Palettes
• Antialiasing
• Compression
• Colour Models
• RGB Colour Model
• CMYK Colour Model
• LAB Colour Model
• Raster Graphics Output Options
• References
Raster (Bitmap) Images
• Made up of a grid, or raster of small squares, called pixels.
• An image using this method is drawn with a group of pixels to
create the appearance of the object.
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http://www.sthelens.oxon.sch.uk/ICT/Graphics/Graphics/Raster_Example1.jpg
Raster (Bitmap) Images
• To edit, you work with groups of pixels.
• Work best with photographic or paint style images.
• Are device dependent, that is, they have a defined size
and are not easily scaled up.
• Up scaling can cause the “jaggies” (ragged edges).
• To avoid this, get correct image size first or use special
anti-aliasing (smoothing) software or algorithms (maths
sums) to enlarge.
• Common paint/imaging software are Photoshop,
Paint.NET, The Gimp.
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Digital Or Printed Images
• Most photos are called continuous tone images as there
is an illusion of constant transition of colour or shades of
greys.
• Pixel based images work the same way as each pixel
can be coloured independently to create a smooth,
continuous transition of colour.
• Non digital printing also creates this illusion using half
tone dots. These are rows of small dots that create the
appearance of different colour or grey shades.
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Resolution
• Resolution: Refers to the unit of measurement to determine the following.• Image Resolution, the size of an image file in pixels, called
pixels per inch (PPI) or dots per inch (DPI)
• Monitor Resolution, the way an image is displayed on your monitor called dots per inch (DPI)
• Output Device Resolution, the quality of a final printed image, measured as either dots per inch (DPI) or lines per inch (LPI).
Resolution
• Dots per inch or DPI: The dots a medium can display. In a
computer monitor a dot is a pixel.
Maths Idea - one inch = 25.4 millimetres.
• For example:
• A 640 x 480 pixel image created at 75dpi (ppi) would have a size
of 8.53”x 6.4” or 217mm x 163mm.
• An 10” x 8” photo image that is to be printed at 150dpi (ppi) would
need an image resolution of 1500 x 1200 pixels in your image
editing application.
• A 1 inch (2.5cm) image at 72dpi (ppi) contains 5,184 pixels. That
is multiplying 72 pixels x 72 pixels with a file size of 6Kb. If it was a
150dpi (ppi) image, it would contain 22,500 pixels. That is,
multiplying 150 pixels x 150 pixels.
Resolution
• Aspect Ratio: The relationship between heightand width in an image, for example, 1.33:1 for avery basic computer screen. Film/video can bebetween this value and 2.35:1 depending on filmtype or projection system.
• A few resolution & aspect ratios.• 1920 x 1080, 1.78:1 – Common wide screen monitor & HD resolution
• 2048 x 1536, 1.33:1 - Moderate print resolution.
• 4096 x 3072, 1.33:1 - High print resolution
• 5656 x 4240, 1.33:1 – 24 megapixel camera photo resolution
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Colour Depth
• Colour Depth: A measure of how many colours each pixel in an
image is capable of displaying.
• For example, in an 8 bit image each pixel is represented by 8 bits
or 1 byte of data and allow the pixel to be any one of 256
colours.
• Other colour depth options are• 24 bit: A maximum of 16,777,216 colour possibilities which is almost the full range
of colours our eyes can detect. In this model there are 8 bits each of red, green &
blue. Each of these 3 channels can display 256 levels of brightness.
• 48 bit: Known as deep colour. It expands the colour range possibilities from
millions to billions. It gives a vividness and colour accuracy that also gets rid any
on-screen colour banding, for tonal transitions that are very smooth and changes
of colour that are very subtle.
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Alpha Channel
• Alpha Channel: An optional 8 bits of image data thatprovides information about transparency.
• It is added onto RGB data that defines an image to turn a 24bit image into a 32 bit image.
• The alpha channel provides 256 levels of intensity. This isused to vary the transparency of the image against abackground.
• An alpha channel functions as a mask or matte to allowparts of a image to be seen while the rest is replaced with abackground image. 0 (black) is fully transparent & 256(white) is opaque in the alpha channel
• It is used for composition of complex 3D renders,compositing of scenes in video post production, inPhotoshop as layer masks or in digital video editing foroverlaying video.
Alpha Channel
Alpha Channel Example
(a) Foreground image (FG) image with opaque and translucent objects.
(b) Alpha channel from FG image.
(c) Background (BG) image.
(d) Composite of FG image over BG image using alpha channel to control transparency.
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Palettes
• Palette: A set of colours used or available for use in animage. Generally limited to 256 colours or less, that is 8bit colour images.
• Palettes are often used in interactive design to optimiseGIF & PNG format images to get the smallest file sizeyet correctly display image colour.
• 24 bit images can be reduced to an 8 bit images bygenerating an optimised palette that creates the bestcombination of colours or they can be dithered.
• Dithering: The process of adjusting adjacent pixels ofdifferent colours to give the illusion of a third colour in apalette. This simulates the display of colours that arenot present in the current colour palette.
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Antialiasing
• Jaggies or stairstepping: The effect of strong edges or lines
that aren’t horizontal or vertical displayed in pixel based or raster
graphics.
• Anti-aliasing: The method of reducing this jaggedness by filling
in pixels with colours midway between the edge or line and the
background colour.
• Most programs allow anti-aliasing to be adjusted or to turn it on
or off.
Antialiasing Example
(a) Aliased line showing stair stepping
effect.
(b) Antialiasing inserts middle tones
that blend the line into the background
to reduce the “jaggies”
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Compression
Compression: Reducing data size in image files. Two
general types, lossy and lossless.
• Lossless: Reduces file size without effecting image
quality by encoding colour and location of repeated
data.
• Lossy: Reduces file size by changing data or throwing
some of it away. Image degradation is offset by
generating custom palettes or by using small amounts
of compression.
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Colour Models
• A colour model is a way for displaying & measuring
colour.
• The human eye understands colour according to the
wavelength of the light it gets. With the full colour
spectrum (range) it is white and with no light it is black.
• The common colour models in Computer Graphics are
RGB (red, green, blue) and CMYK (cyan, magenta,
yellow, black. There is a third one called LAB
(luminance, ‘a’ chromatic component, ‘b’ chromatic
component).
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RGB Colour Model
• The RGB colour model is known as an additive colour model
where equal amounts of red, green & blue at full intensity
produce white.
• A large percentage of the natural spectrum (range) of colour can
be replicated by the mixing of red, green & blue.
• As a general rule you work with RGB images because;
• RGB files are smaller than other colour models
• Monitors & screens display RGB colour best
• It has a larger colour spectrum (range)
than CMYK.
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CMYK Colour Model
• The CMYK colour model represents the four inks used to print images on a traditional printing press. Because all printing inks contain some impurities, the C (cyan) , M (magenta) & Y (yellow) inks actually produce a muddy brown and must be combined with black (K) ink to produce a true or “rich” black. (K is used to avoid confusion with Blue in RGB.)
• To print an image four printing plates are produced, one for each colour. These are called colour separations.
• The combined separations form a complete image called a composite .
• It is called a subtractive colour model as combining all the colours equally produces black.
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LAB Colour Model
• The LAB (L*a*b) colour model is designed to be
device independent. That is, to create consistent
colour regardless of the device (monitor, printer,
computer, or scanner) which is used to create or
output the image.
• L*a*b color model consists of:
• a luminance or lightness component (L) and
• two chromatic (colour) components: the “a” component
(from green to red) and the “b” component (from blue to
yellow).
LAB Colour Model
L*a*b* model: • A. Luminance =100
(white)
• B. Green to red
component
• C. Blue to yellow
component
• D. Luminance = 0
(black)
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Colour Gamut
• Colour Gamut: The range of colours that a colour system can effectively display or print. The spectrum (range) of colours seen by the human eye is wider than the gamut (colour range) available in any colour model.
• L*a*b has the largest gamut, encompassing all colours in the RGB and CMYK gamuts. RGB gamut contains a subset of these colours that can be viewed on a computer, television or screen (which emits red, green, and blue light). Some colours, such as pure cyan (light blue) or pure yellow, can't be displayed accurately using RGB.
• The CMYK gamut is even smaller, consisting only of colours that can be printed using process-colour inks. To overcome this problem special mixed inks such as “Pantone” colours are used. In printing these are known as “spot” colours.
Colour Gamut
Color gamuts:
• A. LAB color
gamut
• B. RGB color
gamut
• C. CMYK color
gamut
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Raster Graphics Output Options
• Screen Based File Formats:• GIF (Graphics Interchange Format): An 8 bit (256 colour) lossless
compression format developed by CompuServe/Unisys.
• Best suited for solid colour based images & widely used, particularly for
web based still and animated images. The Gif89a version also supports
interlacing and transparency.
• JPG, JPEG (Joint Photographic Experts Group): Developed as a
lossy compression way to radically reduce the file size of photographic
images by cutting out minor differences in pixel colours. Achieves one
of the highest compression rates but breaks down if pushed too far or if
saved more than once.
• Best suited for full colour 24 bit (16.7million colour) and gradient style
images.
Raster Graphics Output Options
• Screen Based File Formats:
• PNG (Portable Network Graphics): Developed by the Joint
Photographers Group as an alternative to the GIF format and, like GIF, is
used for displaying images on the web and other online/screen based
services. Preserves all colour information and alpha channels in an image
and uses a lossless compression scheme to reduce file size. Now
supported in all web browsers but used to need a specific browser plug-in
for older browsers.
• Best used for solid colour type images in its 8 bit format and for
continuous tone type images in its 24 bit format.
Raster Graphics Output Options
• Other Common File Formats:
• PCT, PICT (PICTure): Native Macintosh still image format. Can contain
both bitmap & vector information.
• TGA (TarGA): Still image format developed by Truevision. Available in a
wide range of images from 8 bit to 32 bit depth and can be compressed
using lossless compression or uncompressed. Was orginally designed for
capturing video & outputting computer graphics or digital video to tape.
• BMP (BitMaP): Microsoft developed as the native format for icons &
images in the Windows environment. Available in a wide range of image
types from 8 bit to 32 bit and can be compressed or uncompressed.
Raster Graphics Output Options
• Other Common File Formats:
• TIF, TIFF (Tagged Image File Format): Still image format developed
by Aldus/Microsoft specifically for desktop publishing and print
markets. It can use lossless compression and has slightly different
implementations on PCs or Macs and available in many different
versions. Most programs will now accept either the PC or Mac
implementation.
• Open EXR: Image format developed by Industrial Light & Magic (ILM)
for use in Computer Graphic applications and is used in all their movie
productions and now widely adopted by other animation and feature
film production houses. It supports 16 bits per channel which allows for
a dynamic range of over 30 stops of exposure. It also supports both
lossless and lossy compression.
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Raster Graphics Output Options
• Other Common File Formats:
• RAW: Camera raw image file formats are like digital negatives. They
are created by a variety of digital cameras and contain all the
information the camera has about the image. The image information is
directly captured from the camera sensor without filters and
adjustments applied by the camera. This lets photographers interpret
the image data rather than letting the camera make the adjustments
and conversions. They are similar to TIFF files. A camera raw image
file does not throw away any image information to generate a file--it's
lossless--but camera raw image files have the advantage of being
smaller than uncompressed TIFF files. Only camera raw images
contain the actual data captured by the sensor without any in-camera
processing.
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Bibliography
• Adobe help files.
• Computer Arts magazine.
• http://www.abccables.com/info-deep-color.html
• http://www.gemaga.com/2007/12/28/taking-old-
games-into-the-hd-era
• http://www.openexr.com
• http://en.wikipedia.org/wiki/OpenEXR