The document discusses raster displays, focusing on key concepts like raster memory, frame buffers, and their attributes and operations. It explains storage structures like pixmaps and bitmaps, and the nature of frame buffers that drive displays with color information for each pixel. Additionally, it covers raster operations (raster-ops) that enable efficient manipulation of pixel data, which are essential for modern graphical applications and interfaces.

1. CSC 406
Applied Computer
Graphics
LECTURE 4:LECTURE 4:
Raster Displays - detailsRaster Displays - details

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Lecture 4:Lecture 4:
 Scope:
 Raster memory.
 Attributes.
 Raster Ops.
 Lecture Goals:
 To examine the memory concepts in raster
display.
 To understand the different attributes of raster
desplay.

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Raster Memory:Raster Memory:
 Pixmap:
 A pixmap is storage for a whole raster of pixel values.
 Usually a contiguous area of memory, comprising one row
(or column) of pixels after another.
 Bitmap:
 Technically a bitmap is a pixmap with 1 bit per pixel, i.e.
boolean colour values, e.g. for use in a black-and-white
display.
 But 'bitmap' is often misused to mean any pixmap - please
try to avoid this!

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Raster memory…
 Pixrect:
 A pixrect is any 'rectangular area' within a pixmap.
A pixrect thus typically refers to a series of equal-
sized fragments of the memory within a pixmap,
one for each row (or column) of pixels.

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Frame Buffer:
 Frame buffers are often special two-ported
memory devices ('video memory') with one
port for writing and another for concurrent
reading.
 Alternatively they can be part of the ordinary
fast RAM of a computer, which allows them
to be extensively reconfigured by software.

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Frame buffer…
 Defn: A frame buffer is a video output
device that drives a video display from a
memory buffer containing a complete frame
of data.
 The information in the buffer typically
consists of color values for every pixel (point
that can be displayed) on the screen.

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Frame buffer…
 Color values are commonly stored in:
 1-bit monochrome,
 4-bit palettized,
 8-bit palettized,
 16-bit highcolor and
 24-bit truecolor formats.
 An additional alpha channel is sometimes
used to retain information about pixel
transparency.

11. Lecture4 CSC 406 - Computer Graphics 11
Frame buffer…
 The total amount of the memory required to
drive the frame buffer depends on the
resolution of the output signal, and on the
color depth and palette size.
 Frame buffers differ significantly from the
vector graphics displays that were common
prior to the advent of the frame buffer.

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Frame buffer…
 With a vector display, only the vertices of the
graphics primitives are stored.
 The electron beam of the output display is then
commanded to move from vertex to vertex,
tracing an analog line across the area between
these points.

13. Lecture4 CSC 406 - Computer Graphics 13
Frame buffer…
 With a framebuffer, the electron beam (if the
display technology uses one) is commanded
to trace a left-to-right, top-to-bottom path
across the entire screen, the way a television
renders a broadcast signal.
 At the same time, the color information for each
point on the screen is pulled from the frame
buffer, creating a set of discrete picture elements
(pixels).

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Option1: Frame buffer is anywhere
in system memory
System Bus
CPU Video
Controller
System
Memory
Monitor
Frame buffer
Cartesian
Coordinates

15. Lecture4 CSC 406 - Computer Graphics 15
Option2: Permanent place for
frame buffer
System Bus
CPU Video
Controller
System
Memory
Monitor
Frame buffer
Cartesian
Coordinates
Frame
Buffer
•Direct
connection to
video controller

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Frame buffer…
 With respect to color displays, there are two
types of frame buffers:
 Direct color frame buffer
 Color lookup frame buffer

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Raster memory…
 In a bit-mapped display, the display processor
refreshes the screen 25 or more times per second, a
line at a time, from a pixmap termed its frame buffer.
 In each refresh cycle, each pixel's colour value is
'copied' from the frame buffer to the screen.
 Additional raster memory may exist 'alongside' that
for colour values.
 For example there may be an 'alpha channel'
(transparency values) a z-buffer (depth values for hidden
object removal), or an a-buffer (combining both ideas).

22. Lecture4 CSC 406 - Computer Graphics 22
Key Attributes of RasterKey Attributes of Raster
Displays:Displays:
 Major attributes that vary between different raster
displays include the following:
 'Colour':
 bi-level, greyscale, pseudo-colour, true colour:
 Refer to 'pixel values' in lecture3
 Size:
 usually measured on the diagonal: inches or degrees;
 Aspect ratio:
 now usually 5:4 or 4:3 (625-line TV: 4:3; HDTV: 5:3);

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Attributes…
 Resolution:
 e.g. 1024×1280 (pixels).
 Multiplying these numbers together we can say e.g. 'a 1.25
Mega-pixel display'.
 Avoid terms such as low/medium/high resolution which may
change over time.
 Pixel shape:
 now usually square;
 other rectangular shapes have been used.
 Brightness, sharpness, contrast:
 possibly varying significantly with respect to view angle.

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Attributes…
 Speed, interlacing:
 now usually 50 Hz or more and flicker-free to
most humans;
 Computational features, as discussed
next...

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Computational features:Computational features:
 Since the 1970s, raster display systems have
evolved to offer increasingly powerful
facilities, often packaged in optional graphics
accelerator boards or chips.
 These facilities have typically consisted of
hardware implementation or acceleration of
computations which would otherwise be
coded in software, such as:

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Computational features…
 Raster-ops: fast 2D raster-combining operations
explained next;
 2D scan conversion, i.e. creating raster images
required by 2D drawing primitives such as:
 2D lines, e.g. straight/circular/elliptical lines, maybe spline
curves (based on several points);
 2D coloured areas, e.g. polygons or just triangles, possibly
with colour interpolation;
 Text (often copied from rasterised fonts using raster-ops);

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Computational features…
 3D graphics acceleration - now often
including 3D scan conversion.
 It is useful for graphics software developers to be
aware of such features and how they can be
accessed,
and to have insight into their cost in terms of time
taken as a function of length or area.

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Raster Ops:Raster Ops:
 'Raster Ops' are logical operations affecting multiple
pixels in a pixmap (or raster frame buffer).
 Raster graphics terminals typically have special
hardware which executes Raster Ops very quickly.
 A raster-op assigns to a destination pixrect D a
logical function of the initial state D and an equal-
sized source pixrect S.
 This logical function is the same for each pixel of D and
each corresponding pixel of S.

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Raster Ops…
 All bits in a destination pixel are processed in
parallel.
 So each bit in a destination pixrect D is assigned the
specified logical function of its initial value and the value of
the corresponding bit in a congruent source pixrect S.
 Or S may be a bitmap; then the same source bit is applied
with each bit of a destination pixel.
 There are 16 possible 'logical functions' (boolean
operators) which may be used,
 See truth table on next slide:

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Raster Ops…
Source 001 1
Destination 01 01
0 0 (clear) 0000
1 and 0001
2 S and not D 0010
3 S (copy) 0011
4 D and not S 0100
5 D (no op) 0101
6 xor 0110
7 or 0111
8 nor 1000
9 equiv 1001
a not D (invert) 1010
b S or not D 1011
c not S 1100
d D or not S 1101
e nand 1110
f 1 (set) 1111

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Raster Ops…
 The functions commonly used are 0 (clear), 3
(copy), 6 (xor), a (invert) and f (set),
especially copy.
 Scrolling is generally done by repeated use of the copy
function such that the source and destination pixrects
are overlapping regions of the frame buffer.
 Another frequent use of the copy function is to save a
copy of part of a background image before drawing a
moving object over it, then copying back the saved
image and repeating this process for further positions
and states of the moving object.

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Raster Ops…
 The basic raster-op scheme is often
extended as follows:
 By the use of a clip mask to distinguish between
destination pixels the raster-op affects and
destination pixels which are unaffected.
 The clip mask is usually just a bitmap.
 Some raster-op hardware allows a clip mask bitmap to
be used in with independent source and destination
pixrects.
 In some cases a clip mask and a colour may be used
as an alternative to a source pixrect.

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Raster Ops…
 By the use of a plane mask to limit the
planes of a frame buffer that will be affected.
 A plane mask is a pixel value in which (usually)
1's specify affected planes and 0's specify
unaffected planes.
 Thus pixmaps with n-bit pixel values can be
treated as having n different 'bit-planes'.
 For example an 8-bit-pixel pixmap can be used to hold
two 4-bit-pixel images or four 2-bit-pixel images.

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Raster Ops…summary
 Among other things, raster-ops enabled draggable
icons, sprites (animated icons) and a whole
generation of computer games using 2D graphics
operations to achieve cheap-and-cheerful pseudo-
3D effects.
 Multiple window user interfaces make extensive use
of raster-ops.
 The X window system has long done this with particular
efficiency, both in using raster-ops in conjunction with
advanced repainting algorithms and in making raster-op
functionality accessible to applications programmers.

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Next Lecture…
 Scan conversion.
 Device independence/ normalization.