Megapixels

When buying a new digital camera, vendors are trying to convince you to buy a camera with the most Megapixels(the highest resolution). But is this always necessary?

Do people even notice the difference between images with different resolutions?
David Pogues, a writer for the New York Times, conducted multiple tests on this subject.[1]

These tests showed that only 6% of the people could distinguish different resolutions of the same image printed on a 16 x 24″ ( 40x 60 cm)paper.
He chose this size because according to him most amateur photographers wouldn’t often print images larger than that size.
The resolutions shown were 5, 8 and 13 Megapixels.

So do you think that, for amateur use, digital cameras with high resolution are useful?

[1]http://www.nytimes.com/2007/02/08/technology/08pogue.html?pagewanted=1
http://www.kenrockwell.com/tech/mpmyth.htm

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Why do we use vibration test

Vibration tests are normally used in modal analysis, a process in which we describe a structure in terms of its dynamic properties like natural frequency, damping and mode shapes. To introduce modal analysis clearly, let’s consider a simple example. Imagine there is a freely supported flat plate, and we apply a force that varies according to sinusoidal pattern. So the plate will start to vibrate, and we measure the response of the plate with an accelerometer attached to one corner of the plate.

Now if we keep the amplitude of the exciting force the same, but change its frequency, we would notice that the amplitude of the response will also change. This is actually reasonable: the response is amplified more when the excitation frequency is closer to the natural frequency of the structure, and the response will be the maximum when the excitation frequency is the same as the natural frequency.

 

To do some further analysis, I would like to introduce a function which represents the relationship between input and output signal: frequency response function or FRF. To get the FRF of a system, we measure both the applied force and the response of the structure at the same time. Then the measured data is transferred from the time domain to the frequency domain using Fourier Transform algorithm. If we calculate the FRF of the flat plate, we will notice that the peaks in the FRF are at the natural frequencies of the system.

 

So we can see using the vibration tests on one structure we can get the FRF, and then we can find the natural frequencies according to the peaks of FRF.

Then why do we need the natural frequencies? And what’s its relation with THA? First, the natural frequencies are related to the stiffness of the structure: the higher the stiffness is, the higher the natural frequencies are. Second, the stiffness of the structure is related to the fixation between the implant and the bone: the better the fixation is, the higher the stiffness is. So we can use the vibration tests to get the natural frequencies of the implant-bone structure, then we can use the value of the natural frequencies to evaluate the fixation of the implant during THA.

Source: Peter Avitabile, Experimental Modal Analysis, 2001

do we really need higher resolutions?

Nowadays, when people buy tv’s, they always want to have the biggest resolution possible for their viewing pleasure.
But is a higher resolution always better? Is the difference between 1080p and 720p always noticeable?

A higher resolutions takes care of the ability to deliver more detail to the picture and make it sharper.
There are a few factors that take part in making the viewer able to distinguish between image resolutions:
the resolution of the screen, the size of the screen, and the viewing distance. To be able to detect differences between resolutions, the screen must be large enough and the viewer must sit close enough.

The human eye (for people with perfect vision) can resolve 1/60 of a degree of an arc. With this knowledge, it is possible to estimate when the differences between resolutions will become apparent. On the following picture you can see when a certain resolution becomes noticeably better than a lower one.

For example, we take a 50 inch screen (127 cm).
The benefits of 720p vs 480p only start to become noticeable at viewing distances closer than 14,6 feet (4,45m) and become fully apparent at 9.8 feet (3m). For the same screen the benefits of 1080p vs 720p start to become noticeable at distances closer than 9,8 feet (3m) and are fully apparent at 6,5 feet (2m).

So we can conclude that for smaller screens, there isn’t much difference between 1080p and 720p, but for bigger screens like in the cinema it’s a massive improvement.

Greetings,

Alexander & Tom

[1] http://carltonbale.com/1080p-does-matter

Why we all don’t use a Cross-platform IDE.

Every developer has a favourite platform, let your developer work on his a favourite platform and you will get a happy developer. It’s one of several advantages of using a cross platform development environment.  But developing application on different platforms, for different platforms may introduce some problems.

It can lead to the “Works For Me”-Syndrome.  A developer assumes it will work on other platforms if it works on his platform. And as there are applications who can be compiled for multiple platforms without changing one line of code, in most cases some things need to be changed and tested. This means you need the resources to be able to test  it on all of these platforms. Which cost money and time.

Also if you have the intention to release you program on only one platform you probably find a better platform platform dependent IDE than a platform independent one. This takes us to a next topic, what makes a good IDE?

Bavo

[1]http://www.tortuga.com.au/products/ozibug/www/cross_platform_devel.html
[2]http://www.uwplatt.edu/csse/courses/prev/s05/csse411/SE411_05_Cross%20Platform%20Development.ppt

Knee implant, total knee arthroplasty and post-surgery vibration test

We have discussed the hip implant, THA and its related vibration assessing technique. So, now, what about the knee joint? Is it different from the hip? What is the difference? And what are the similarities that we can take advantage of in surgery of bad knee joints?

Principally, TKA, a surgical procedure to replace the weight-bearing surfaces of the knee joint with a prosthesis, is more or less the same as THA. Old people who suffer from diseases such as osteoarthritis or rheumatoid arthritis will come across the problem of pain in their knee joint, as well as a reduction of range of motion. In the USA, more than 230,000 TKA’s are performed every year. Of all these surgeries, approximately 5 to 10% of the prostheses need to be replaced within 10 to 15 years due to the worn-out connections between implant and bone. Either loosening or infection can cause bad connections.

Two photos are displayed to give a first impression of the hip implant and knee implant.

                      

There are several methods currently used in detect the worn-out of medical implants, including physical examination, radiography(most widely used), laboratory tests, and vibration techniques (latest). Physical examination is a relative subjective and inconclusive assessing method, and laboratory tests mainly focus on the diagnosis of infection in the bone.

Radiography is the most classic way of loosening diagnosis. It is used to evaluate prosthesis alignment, fixation,gross polyethylene wear and quality of periprosthetic bone. But it is subjective, inconclusive and less sensitive( e.g., a sensitivity of 83% has been reported for the tibial component and a specificity of 72%).

Vibration analysis has been used to determine bone mechanical properties, to monitor fracture healing, and to assess the stability of  dental implants. It is also used to assess the initial stability of the femoral THA component, intra-operatively. Two differences exist between a TKA and a THA. Firstly, knee joint is not accessible, so a small incision should be cut to use vibration test post-operatively. Secondly, in THA, vibration technique is used to  as a comparison between two succeeding insertion stages during surgery as a comparative method, while in TKA it is used in an absolute way.

References:

Baré J, MacDonald S, Bourne R. Preoperative evaluations in revision total knee arthroplasty. Clin Orthop Rel Res, Vol 446, p40-46, 2006.

Marx A, Saxler G, Landgraeber S, . Comparison of subtraction arthrography, radionuclide arthrography and conventional plain radiography to assess loosening of total knee arthroplasty. Biomedizinische Technik, Vol 50, No 5, p 143-147, 2005.

Pastrav L, Monitoring of the fixation of orthopaedic implants by vibration analysis, promoters G. Van der Perre, R. Van Audekercke, 28 October 2010.

Pastrav L, Jaecques S, Mulier M, Van der Perre G. Detection of the insertion end point of cementless hip prostheses using the comparison between successive frequency response functions. J Applied Biomat & Biomech, Vol 6, No 1, p 23-29, 2008.

http://bonesmart.org/joint-replacement-surgery/hip/