lecture4 raster details in computer graphics(computer graphics tutorials)
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CSC 406Applied Computer
Graphics
LECTURE 4:LECTURE 4:
Raster Displays - detailsRaster Displays - details
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Lecture4 CSC 406 - Computer Graphics 2
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.
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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.
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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.
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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 anywherein system memory
System Bus
CPU Video Controller
System Memory
Monitor
Frame bufferCartesian
Coordinates
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Option2: Permanent place forframe buffer
System Bus
CPU Video Controller
System Memory Monitor
Frame bufferCartesian
Coordinates
FrameBuffer
•Direct connection tovideo controller
Lecture4 CSC 406 - Computer Graphics 16
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).
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Key Attributes of Raster Key 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 1Destination 01 01
0 0 (clear) 00001 and 00012 S and not D 00103 S (copy) 00114 D and not S 01005 D (no op) 01016 xor 01107 or 01118 nor 10009 equiv 1001a not D (invert) 1010b S or not D 1011c not S 1100d D or not S 1101e nand 1110f 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.
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