display calculation 2

8/9/2019 Display Calculation 2

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Basics Of Computer Graphics

Screen R esolution

| Introduction | The Binary Notation | Color Depth | Screen Resolution |

'Screen resolution' is not a strange concept, yet there is a surprising ammount of peopleout there sitting by their computers not knowing anything about such basic concepts as'screen resolution', 'colordepth' and 'graphic memory-usage'. We will try to change thatnow! :)

Different screen resolutions and colordepth can be adjusted on your computer, but howyou do it depends on what computer you use (Windows95 /NT: Start:

Settings /ControlPanel /Display /Settings), (Amiga: System:Prefs /ScreenMode), (Mac:'TheApple' /ControlPanels /Monitors)There is a simple relationship between the ammount of graphics memory and themaximum resolution/colordepth you can use. So, if you know how much graphic-memoryyou have in your computer, you can also calculate which resolutions/colordepths youshould be able to display...

For non-professional users, the most common ammounts of graphic memory these daysare between 8 & 64 MB.The most common resolutions are 800*600 and 1024*768.

Example:A resolution of 800*600 means that the viewable size is divided into 800 picture elements(picture elements=pix_els) horizontally and 600 pixels vertically. The total ammount ofpixels in this case is: 800 times 600 = 480000How much memory this resolution requires depends on the colordepth.On the computers in use now, there are usually 3 alternatives when it comes tocolordepth. Those are: 8-bit (256 colors), 16-bit (65 thousands of colors, a.k.a. HighColor)and 24/32-bit (16,8 million colors, a.k.a. TrueColor).

Computer memory is 'measured' in 'bytes'. e.g. kilo bytes (kB), Mega bytes (MB); Onebyte= 8 bits.This is something that is valid for computers nowdays. In the past e.g., there have beencomputers where one 'byte' was only 4 bits .

Now, back to the 8-bit display...On an 8-bit display (the 8 bits describe the color depth; amount of colors) every pixeloccupies 1 byte in the computers graphic memory. In the example earlier we had a totalof 480000 pixels. This means that the total memory usage is: 480000 (ammount ofpixels) multiplied with 1 (1 byte occupies 1 pixel). The answere is still 480000.Simply speaking, a 800*600 display with 256 colors requires 480000 bytes of graphicmemory.Now, if you wanted to have more colors, like 16-bit instead, how much memory would

Page 1 of 6Basics Of Computer Graphics - Screen Resolution

7/21/2010http://www.danielsevo.com/bocg/bocg_screenres.htm


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2048*1536 16-bit 6,14 MB2048*1536 24-bit 9,21 MB2048*1536 32-bit 12,28 MB

What is 'Refresh rate'?

Most people (computer users) are familliar with the term 'refresh-rate'. It is quite simplythe rate at which your screen is being updated; refreshed.For a stable, flicker-free picture, at least 70 refreshes/second are recomended. (For every'refresh' the picure on your monitor is re-drawn) A refresh rate of 50 updates/secondgives you a more 'flickery' display, and less is worse.. (Bare in mind however that this only applies to the type of monitors that use 'Cathode ray Tube' (CRT) technology. Which is basically all monitors that are not flat)

The 'Refresh-rate'is measured in Hz (Herz); 1Hz=1 time/secIn ads for computer monitors you can sometimes see something called 'Horizontal SweepFrequency'. I'll explain what that is below...

RefreshRate tells you how often the screen is updated.Horizontal Sweep Frequency however, is the ammount of horizontal 'pixel-lines' themonitor can output/time unit... e.g. a resolution of 640 (width) * 480 (height) means thatthe screen consists of 480 horizontal lines that are 640 pixels wide each. The HorizontalSweep Frequency (measured in kHz=kiloHerz) tells you how many of these horizontallines that the monitor 'draws' every second. It is not your graphics card that does this job,but rather the monitor itself, so even if you have a very expensive graphics card in yourcomputer, it is still the monitor that sets the upper limit for the quality of your display.

Real life example: If you use a resolution of 800*600pixels, it means that you have 600 horizontal, 800pixels wide, lines... Let's say you want to have your refresh-rate at 76 Hz. (Which wouldgive you a nice flickerfree display).What it means to the monitor is that it must 'draw' 600 horizontal lines 76 times/sec!600*76= 4560045600 horizontal -800pixels wide- lines is what the monitor must manage to 'draw' eachsecond. 45600 Hz is the same as 45,6 kHz...To sum it up:45,6 kHz is the 'Horizontall Sweep Frequency' that your monitor must 'manage' if you areto display 800*600 at 76Hz!

That's it, wasn't very strange, was it?

Here below, you can see a table containing the most common resolutions, refresh-ratesthe required HSF's. (Horizontal...)So, e.g. if you are buying a monitor and you want to use 1600x1200 @ 85Hz, make sureit manages at least 102 kHz Horizontal Sweep frequency. (Keep in mind though that this only applies for CTR (Catode Ray Tube) monitors. If you're buying a flat TFT screen, this is not relevant.)

Resolution Refresh-rate H. Sweep Freq




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to understand that 'Megapixel' refers to a resolution which is measured in millions ofpixels. The resolution in question is obviously the resolution of the image that the digitalcamera can capture. Because there is no (chemical) film to capture the light as in oldercameras, a digital camera relies on the quality of the sensor to 'record' the information.I'm not going to describe in detail how digital camera sensors work, but let me just quicklysay that there are basically two types of sensors on the market today. The cheapercameras use CMOS ( Complementary Metal Oxide S emiconductor) sensors and the more

expensive cameras rely on CCD ( Charged Coupled Device) sensors. The CCD is moreexpensive but gives better quality in terms of light sensitivity and low graininess. CMOSsensors are getting better though, because there is a big market for cheap digitalcameras out there meaning a lot of research is being done in this area.If you have seen solar panels then you know that light can be converted into electrons(electricity). You might also know about light sensitive diodes. A typical example wouldprobably be a door that opens when you break a beam of light because a light sensorhas registered a change in the lighting conditions. Anyhow, that is the basic principle ofsensors used in digital cameras. They get hit by photons (light) and convert this energyinto electrical signals which end up as 1s and 0s which your computer can understand.Color information is typically obtained through use of red green and blue filters (the exactway of applying these filters varies a lot between different manufacturers and price

ranges of cameras. If you see a camera with a 3xCCD label, it means it has a dedicatedCCD for each of the 3 filters/colors (Red, Green, Blue) which usually means superiorcolors and quality.

Now let's get to the main point here. The quality and design of the sensor is the upperlimit of how much light information that can be gathered and then transformed into animage. So if your sensor would consist of a grid of 4 by 4 blocks, then the total resolutionof that image would be 16 pixels (4x4=16). Obviously, that wouldn't be very useful. As wehave already discussed, the term Megapixel describes the potential of the imagingsensor. If your camera is labeled '1 Megapixel' then the sensor in the camera is capableof capturing light information for 1 million pixels. If you have a more expensive camerayou might be able to capture e.g. 7 Megapixel images and so forth....

Below is a table that shows the screen resolutions (Screen Res) that correspond todifferent Megapixel ranges (Cam Res). It also shows the size of the image if you wouldchoose to print it out on a photo-quality printer (and if you print at 600dpi, just cut thatlength in half). Note however that the screen resolution is of a different format that atypical old style photo. A typical size of a photo might be 15x10cm meaning that the widthvs height ratio is 3/2 or 1.5 (15/10=1.5). A typical PC screen has the ratio of 4/3 or 1.33(e.g. 800/600=1.333). What all this means is that the screen resolution might notcorrespond exactly to say 1 Megapixel, but rather, it will be the highest possibleresolution that follows the PC standard of the 1.333 ratio and standard available PCresolutions. Typically, PC resolutions are evenly divisible by 16.

Cam Res Screen Res Print size (300dpi) Comment1 Mpix 1024 x 768 ~ 8.5 x 6.5 cm2 Mpix 1600 x 1200 ~ 13.5 x 10 cm3 Mpix 1920 x 1440 ~ 16 x 12 cm4 Mpix 2240 x 1680 ~ 19 x 14 cm Beyond most monitors5 Mpix 2560 x 1920 ~ 21.5 x 16 cm6 Mpix 2720 x 2040 ~ 23 x 17 cm




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7 Mpix 3040 x 2280 ~ 25.5 x 19.5 cm8 Mpix 3200 x 2400 ~ 27 x 20.5 cm9 Mpix 3360 x 2520 ~ 28.5 x 21.5 cm Roughly, A4 size

10 Mpix 3520 x 2640 ~ 30 x 22.5 cm12 Mpix 4000 x 3000 ~ 34 x 25.5 cm

15 Mpix 4480 x 3360* ~ 38 x 28.5 cm *probably (4320*3240 --> 14Mpix)20 Mpix 5120 x 3840 ~ 43.5 x 32.5 cm (A3)50 Mpix 8160 x 6120 ~ 69 x 52 cm Not yet available (>A2)

100 Mpix 11360 x 8520 ~ 96 x 72 cm Roughly 10x of normal PC screen1 Gpix > 36000 x 27000 > 300 x 230 cm One billion (Giga) pixels

As we can see from this table, you will need a 20 Megapixel camera to be able to print A4at 600 dpi with full print quality. Cameras exceeding 20Mpix are already available forprofessionals (or rich enthusiasts ;-).

Anyway, I hope someone finds these tables helpful.

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