With this investigation I hope to bring a view into how various low-level image sampling and related processes, including pixel binning, pixel skipping and Bayer demosaic algorithms, affect the image quality particularly with respect to the appearance of aliasing and Moiré.

The focus here is DSLR video systems, which typically use quality compromising methods to reduce the computational load when  scaling down of the multi-MP sensors designed for still images. As the pixel count of those sensors increases the down-scaling becomes more demanding and computational heavy.

While newer DSLR cameras seem to have come a long way solving this problem, aliasing is still a main problem for many digital cameras, and it is not an easy tassk to find information on what underlying processes are. Having this information is crucial for the design of optical antialiasing filters (low pass filters)  matching the various  DSLR video cameras.

In this (rather long) article I will present simulation results from a range of different pixel binning, pixel skipping and demosaic schemes (btw. chroma subsampling no included) in order to see the effect on image quality and to see if it is possible to replicate the observed aliasing and Moiré of the Nikon d5100 best. This may also be used to evaluate the sampling method of other cameras, when using the same video test and compare with the simulation results. Read More…

Posted by: recordingsofnature | August 26, 2014

Effect of pixel fill factor on aliasing and Moiré

This is an investigations of how the pixel fill factor in the image sampling process influences aliasing and Moiré. It is here done by use of a sampling simulation (terated as simple mono chromatic – no Bayer filters etc.)  just to illustrate the fundamental aspects.

The low pass filtering is an essential part of any image sampling process, serving to eliminate any high frequency component that cannot be reproduced with the given sampling rate and resolution. Any frequency detail above half the sampling rate (Fs/2= the Nyquist rate) will be reproduced as mirrored and aliased frequency artifacts, only contributing with artifacts and noise to the sampled image.

 

Test chart with indication of locations of the harmonics of the Nyquist frequency

Test chart with indication of locations of the harmonics of the Nyquist frequency= Fs/2 of the sampled image. The original high resolution chart found here.

Read More…

Posted by: recordingsofnature | February 27, 2014

Simple test to evaluate DSLR video aliasing and Moiré performance

[Updated article...]

Aliasing and Moire issues are heavily discussed topics for DSLR HD video, however the performance evaluations remain very subjective and are typically based on random Youtube or Vimeo videos. Adding to this, camera manufactures hardly disclose any details about the related actual sensor level pixel processing.

This is an alternative method to determine /characterize the aliasing and Moire performance of a DSLR video. Basically, the method is a scanning for critical line densities which produce interferences/resonances in the image. Knowing this will give hints on what is going on at the low level sampling process, reveling the effective vertical and horizontal resolutions. This will also provide information on the requirements for an optical antialising filter, ie. the blur diameter needed, to eliminate the aliasing artifacts.

The test is demonstrated in the videos:

The Test

The test is basically 2 fine line patterns (stacks of 600 lines), one vertical and one horizontal. To carry out the test, simply record a video of the test patterns while zooming in and out.  In this way, the sensor is  scanned for line densities with critical interferences/resonance.  A subsequent analysis of the video will readily provide precise respective line density values. (Bear in mind that this line pattern is designed to be a worst case scenario for provoking Moire patterns)

The line patterns can be printed from this document:  600 lines – line width 0.5.pdf , -alternative line widths are available here: 600 lines – line width 0.2.pdf and  600 lines – line width 1.0.pdf

Read More…

Posted by: recordingsofnature | December 19, 2013

Field recording by bicycle – Copenhagen – 2nd December 2013

Being a quite experienced cyclist in Copenhagen, I like to share some of my experiences from the bicycle lane, – here through a few portable field recordings (audio+video). At the moment it’s interesting to follow the development of the cyclist culture in this city – as the number of cyclists grows and the formal regulation is very limited.

Østerbrogade morning December 2013

Østerbrogade morning December 2013

The following footages are captured from the bicycle using a new portable setup for my audio field recording equipment. First part of the footage is captured at 8:30 am, riding the bike from Copenhagen University Amager to Fisketorvet. Listen with headphones for best experience.

The audio track alone with regular 320 kbps mp3:

This second part is from around noon. Going from Stormbroen, Christiansborg to Østerbrogade through Kgs. Nytorv.

The audio track alone with regular 320 kbps mp3:

Read More…

Posted by: recordingsofnature | August 24, 2013

Morning recording in Copenhagen / Amager

This is a field recording from nearly the center of Copenhagen, 22nd August 2013 during the morning rush hour. The view shows a the green and beautiful areas from the old fortification system, however the sound image is completely dominated by the busy road just a few meters behind. There were also plenty of bicycles and runners in the area, but there is no doubt that the dominating sound in the city comes from cars.

Green areas in front of the microphone head, but the sound image is dominated by a busy road, just at few meters behind.

Green areas in front of the microphone head, but the sound image is dominated by a busy road, just at few meters behind.

Listen to the 24 minutes long field recording below, and also check out the 360 degrees panorama from the take on the link further below.

Download mp3 56MB

Follow the link on the image to see the 360 degrees panorama from the take.

Follow the link on the image to see the 360 degrees panorama from the take.

Follow the link on the image to see the 360 degrees panorama from the take. The microphones were pointing towards the bench by the lake, thus all the traffic noise comes from behind.

Read More…

Posted by: recordingsofnature | May 23, 2013

Sound of Rain – Nature recording Copenhagen May 2013

This is a nature recording from a rainy evening May 21st, 2013 in Østre Anlæg,- a park in central Copenhagen. During the last 2 weeks the leaves on any tree or bush have now unfolded. This is about 2-3 weeks later than last year. But now, what before was bare and gray is now heavily green and dense. And with the rain the experience is further enhanced by smells and sounds.

Recording setup in the rain, Østre Anlæg Copenhagen

Recording setup in the rain, Østre Anlæg Copenhagen

The recording was captured at 7-7.30 pm. The location is a central park in Copenhagen, part of the old fortification ring encircling the medieval city.

Frequent loud sounds of trains at the next door train track is the main remainder of this is in the middle of the city. The trains are producing some radical cuts through the otherwise relaxing sound scape.

With this recording an umbrella is placed above the recording setup to shelter the microphones and camera from the rain. The rain drops hitting the umbrella is what produces the loudest distinct rain drop sound.

Read More…

Posted by: recordingsofnature | February 28, 2013

Nature recording, Silent winter day February, Amager Fælled.

This is a nature recording from a gray February winter day at Amager fælled (24th Feb. 2013),  just a few kilometers from Copenhagen centre.  This cold, quiet and colorless recording should be seen in contrast to the green and luxuriant scenes, at the exact same location during mid summer, as can be seen in this previous recording from last year.

Amager fælled, February 2013-1

Gray winter scenes at Amager fælled, February 2013

The audio nature recording can be heard here (put on headphones for full experience)

Download Mp3 16:30 minutes

(Put on headphones to get the best experience!)

I think winter is very quiet, and the quietness sharpens your ears.  But the result is often, that the man-made city sounds becomes very pronounced.  So that means on the recording, the audible sounds are dominated and various city sounds, metro, planes, road noise, ambulances. While the nature itself are here  represented by noisy magpies.

Read More…

Posted by: recordingsofnature | December 4, 2012

CYL-Filter – Moiré and aliasing filter for HD DSLR video

Manufacturing is currently on hold, summer 2014. I hope to return with a version 2 of the filter method with highly improved means of estimating the optimal filter strengths for different image sensor and lens configurations.

This is an experimental moiré and aliasing filter (CYL-Filter) fitted into a standard filter frame (size 49, 52, 55, 58, 62, 67 and 72 mm). See more info about the this filter method in the earlier posts, here and here.  If interested, please contact me directly via the contact form at the end of this post, and I can give you more information about the filter.

CYL filter vA

Update May 2013

I have been working and elaborating to survey the possibilities with this  antialiasing filter method for DSLR videoing.  Here is the latest development status:

Read More…

Posted by: recordingsofnature | October 7, 2012

Autumn rain in the forest – field recording

Here is a nature recording from a rainy evening in Dyrehaven north of Copenhagen, October 5th 2012. The Autumn is a beautiful season in this old forest. The colors are getting quite intense both when the sun shines and when it rains!

Dense autumn rain in Dyrehaven

Dense autumn rain in Dyrehaven, October 2012

This summer I had planned to investigate the sound of rain.  I think rain can be a very  pleasant and relaxing, and also an interesting soundscape full of details and spatial sensation. Surprisingly, the summer this year turned out relatively dry. However, now it seems, it can’t hold back anymore.

The recording is made from a small shed with a good porch roof to shelter against the rain. What you hear is mainly rain dripping from the roof, while the surrounding forest produces a low whispering sound from the rain.

Download Mp3 – 7 minutes recording

The rain that day was pretty good for audio recording. Dense and nearly no wind. It had been on for the whole day, and intensified slightly during the recording session. In this way the soil and all surfaces were well soaked, giving the full rainy sound. I am starting to realize, that the sound from rain purely has to do with the sound of the things it falls on. In that way the sound is really very diverse. And that calls for further investigations.

Also see the this 16 minutes video footage from the same session, again using the same microphone setup attached to the camera:

More pictures below.

Read More…

Posted by: recordingsofnature | September 25, 2012

DSLR video Antialiasing filter – Sharpness vs. moiré

This article follows up on the antialiasing filter method described in the earlier post  Antialiasing filter for DSLR video, for reducing or eliminating moiré video artifacts typical for DSLR HD videoing, -quite visible on the Nikon d5100 DSLR camera. Now, I finally managed to do a more through-out test to map the antialiasing effect and sharpness for different filter and lens settings.

[Update 4. dec. 2012 - the filter mounted in a standard filter frame is now available - here]

Avoiding aliasing and moiré artifacts is all about avoiding a too sharp image to be formed on the image sensor.  The antialiasing filters must introduce a precise amount of blur, whereby resolution of the image on the image sensor corresponds to the resolution of the recorded video. This can also be described as a lowpass filter.

Example of the filter effect on moiré patterns

Example of the filter effect on moiré patterns from the video clips at the bottom of this article

The type of filters used here works by introducing small amounts of blur through astigmatism, but since this effect is highly depending on both the aperture and focal length, some sharpness tests were needed to provide an overview of the effects. The following tests are based on still images. To make the results more applicable, the amount of sharpness has been converted into a number corresponding to the maximum vertical pixel resolution for the 16:9 picture crop area.  This mean that values of about 1080 pixels should be a good start for a 1080p HD video. When the value is too high aliasing will occur. Too low, the image will start to blur. However let’s see the results.

Read More…

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