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The pixels received as input from the camera are in the YUV420sp format, which signifies how the data is packed into a linear array. The Wikipedia article on YUV , especially the section titled "Y'UV420p (and Y'V12 or YV12) to RGB888 conversion" , may be a useful reference; make sure that you thoroughly understand the structure of a single frame of data.

Open jni_part.cpp and complete the YUV2RGB() function. You will be able to see your results by selecting "Preview RGB" from the options menu.

To learn how to access individual pixel values, read the OpenCV documentation on basic operations with images . It is also important to know that you are working with images that use 8-bit unsigned pixel values.

To assist you in writing the conversion code, you may reference the YUV to RGB conversion code provided on Wikipedia. Remember, you will need to access the pixel values using the proper matrix syntax. See the OpenCV matrix documentation .

Make sure you understand every single line of the code. We are allowing you to use this (very helpful) resource instead of writing the code from scratch, but you need to fully understand how the conversion is taking place.

After you verify you are streaming color images, continue on to the next part to implement histogram equalization.

Part 2: histogram equalization implementation

In jni_part.cpp in the HistEQ() function, notice that the lines

Mat* pYUV=(Mat*)addrYuv;
Mat* pRGB=(Mat*)addrRgba;

set up pointers to the video input and output. One frame of video lies in the matrix pointed to by pYUV . Your assignment is to modify this matrix of pixels so that the left half of the image is histogram equalized, and the right half of the image remains unprocessed. Then you will need to convert the YUV format image to RGB format and save it in the matrix pointed to by pRGB . This HistEQ C function is called every time a new video frame is ready to be processed, allowing an entire video stream to be processed over time. The algorithm can be broken up into 4 steps:

  1. Compute the histogram of the Y channel
  2. Compute the CDF of the histogram
  3. Apply equalization to the Y channel
  4. Convert the equalized image to RGB

Unless you are already familiar with histogram equalization, reading the OpenCV histogram equalization tutorial should be helpful in understanding how this algorithm affects an image. OpenCV is an open source computer vision library that provides many useful functions for Android developers to use when creating applications that rely on image and video processing. You will use some of OpenCV's functionality in this lab.

OpenCV provides optimized functions for histogram equalization and color conversion. You are implementing your own in hopes that you will become accustomed to working with pixel values directly. For future projects, you will be encouraged to use the built-in functionalities, and focus on putting an entire system together.
Make sure you conceptually understand what you are about to implement (the above 4 steps). In a few minutes you will be diving in to a lot of details, so take a second to verify your understanding of the algorithm.

Step 1: compute histogram of y channel

Once you have received data in from the camera in YUV format, you will need to create a histogram of the values of the Y channel in that frame.

Your histogram can be represented simply as an array. What size will the array need to be? What type?

Step 2: compute cdf of histogram

Next, you must compute the cumulative distribution function (CDF) of your histogram. The CDF will be used in the next step to equalize the histogram.

Make sure to normalize your CDF so that the range is 0 to 255.

Step 3: apply histogram equalization

Take each Y channel value as an index into the CDF to obtain the equalized Y channel value. Read the OpenCV histogram equalization tutorial for more information on using the CDF as a remapping function.

Don't forget, you only want to equalize the left half of the image. The right half of the image must remain unmapped for comparison.

Step 4: convert from yuv to rgb

While the pixels coming in from the camera are in YUV format, the pixels going out to the tablet's display are in RGB format. You will need to convert your half-equalized YUV image into RGB format, and store the image in the matrix pointed to by pRGB .

When the application is launched on the Nexus 7, you must tap the ... near the bottom right of the screen, and select "Hist EQ". When working correctly, the right half of the video should display the unprocessed input, and the left half of the video should display the equalized video.

Extension: other tone mappings

Histogram equalization is one special case of tone mapping , which simulates higher dynamic range and results in more dramatic images. This section is completely optional, but if you are interested, explore and see what sort of "Instagram"-like effects you can achieve!

Questions & Answers

What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Ece 420 spring 2014. OpenStax CNX. Jan 18, 2014 Download for free at http://cnx.org/content/col11618/1.3
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