lecture4 raster details in computer graphics(Computer graphics tutorials)

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

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Lecture4 CSC 406 - Computer Graphics 3
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.

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!

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.

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.

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.

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.

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.

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.

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).

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

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

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

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).

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);

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.

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

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:

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);

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.

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.

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:

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

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.

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.

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.

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.

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

lecture4 raster details in computer graphics(Computer graphics tutorials)

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