An Android App for Photo Brightening

I published an Android app used for photo brightening named Brighter Photo. Using a low light photo as the input, the app can post process it and make it look brighter. It is used only for post processing but not as a camera. The app is available at Google Play store and the link is here

Figure below shows what the app can do. The left half is a photo taken at night and the right half is the same photo after processed by Brighter Photo app.

To use the app, the GUI has three buttons: Load, Save and Setting. "Load" button can load photo from Android gallery. After loading, post processing immediately starts. Post processing takes 10-30 seconds depending on the image size. It runs slower than some of the similar apps but produces better results. "Save" button allows to save the results. Note that to enable saving, the storage permission needs to be enabled in Android setting for this app.

This Youtube video shares how to use the app.

Hope that this introduction is useful and you can enjoy using this app. Any feedback is welcomed.

A Simple Example of Image Processing Using Java

In this post, we share a simple example of image processing using Java. This example benefits from Java tutorials available from Internet.

This example has three steps: 1) read the input image; 2) image processing; 3) display the processed image.

For step 1, an image is read into image buffer. 

    File file = new File("Lenna.png");
    BufferedImage image = null;

  // Read image
    try
    {
        image = ImageIO.read(file);
    }
    catch (IOException e)
    {
        e.printStackTrace();
    }
    System.out.println("done");

Step 2 has a simple operation of image processing. It does 2D convolution filtering for the image using filt3x3 function in the ConvolutionMatrix class.

    // 2D convolution for the image
    double[][] config = {{1,2,1}, {0,0,0}, {-1,-2,-1}};
    ConvolutionMatrix imageConv = new ConvolutionMatrix(3);
    imageConv.applyConfig(config);
    image2 = imageConv.filt3x3(image, imageConv);

Step 3 displays the modified image

    // Display the modified image
    ImageIcon icon=new ImageIcon(image);
    JFrame frame=new JFrame();
    frame.setLayout(new FlowLayout());
    frame.setSize(image.getWidth(),image.getHeight()); //Window.setSize(int width, int height)
    JLabel lbl=new JLabel();
    lbl.setIcon(icon);
    frame.add(lbl);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

Java-based image processing is used in many places. Hopefully some people can benefit from this tutorial. The code can be found from here

LibRaw Post Processing Pipeline Translated Into Python

LibRaw is a widely used open source library for raw image conversion. In many people's opinion, the most valuable part of LibRaw library is that it can handle various types of raw image formats, which is not commonly available to other libraries. But people are also interested in its post processing pipeline, which converts a interleaved RGB dot map to a colorful picture by using procedures such as white balance adjustment, demosaicing, gamma mapping, etc. People want to understand LibRaw's post processing pipeline but the code can be hard to deciphered. That is why I wrote a Python script which appears to match with LibRaw's post processing pipeline quite well. The hope is that by reading this Python script, it is easier for folks to understand what actually happens in LibRaw.

I should admit that some corners have been cut in this Python script. For example, the output of the script is only half the size of the input, which means no need for me to write a full-fledged demosaicing which matches the size of the input. However, what I found over the time is that quite often, the most puzzling part of post processing to people include me is how inputs from different color channels are scaled and mixed together. The scaling and mixing parts are covered in this Python script.

Here is the approach we take when writing this script: the input of this script is a raw .dng image. The image is taken using Samsung Galaxy S9 phone. The decoding before post processing is done using Python rawpy library. Python rawpy library is LibRaw wrapped in Python. After decoding, post processing is done using two parallel paths. The first path is to use postprocess() function provided by rawpy. Under the hood, it calls dcraw_process() from dcraw library written by Dave Coffin. The first path is used for reference. The second path is my own pipeline written in Python. The second path takes rawpy decoding outputs as inputs including raw image and some parameters. At the end of the script, we compare the reference and reconstructed image by my code and find them to be almost the same.

Now let me introduce the post processing code I wrote. It includes three main steps: 1) scale the input by white balance, 2) mix R/G/B by using color matrix, 3) Gamma scaling

In step 1, raw image containing RGB info is scaled by white balance (wb_scale). Since Bayer pattern is used for raw image input, there are two green channels (color4[:,:,1] and color4[:,:,3]). The white balance scale comes from the parameter of camera_whitebalance in rawpy decoding output. One can use another parameter of daylight_whitebalance but it makes the final output image yellowish. Since the next step of pipeline only needs three inputs, there is minor step called mix green which mixes two green channels as: color4[:,:,1] = (color4[:,:,1] + color4[:,:,3])/2.

color4[:,:,0] = raw_py.raw_image[0::2,1::2]*wb_scale[0]
color4[:,:,1] = raw_py.raw_image[0::2,0::2]*wb_scale[1]
color4[:,:,2] = raw_py.raw_image[1::2,0::2]*wb_scale[2]
color4[:,:,3] = raw_py.raw_image[1::2,1::2]*wb_scale[3]

In step 2, color_matrix is applied. I believe the purpose is convert camera sensor input to standard RGB image. color_matrix is also among rawpy decoding outputs. While white balance changes from photo to photo, color_matrix is camera's inherent property and it stays the same with the same camera setting.

pic_temp[:,:,0] = color_matrix[0,0]*color4[:,:,0] + color_matrix[0,1] * color4[:,:,1] + color_matrix[0,2] * color4[:,:,2]
pic_temp[:,:,1] = color_matrix[1,0]*color4[:,:,0] + color_matrix[1,1] * color4[:,:,1] + color_matrix[1,2] * color4[:,:,2]
pic_temp[:,:,2] = color_matrix[2,0]*color4[:,:,0] + color_matrix[2,1] * color4[:,:,1] + color_matrix[2,2] * color4[:,:,2]

Step 3 is Gamma mapping. Gamma mapping is a legacy from CRT era to compensate the nonlinear distortion of CRT monitor. But it outlives CRT. The parameter used in Gamma curve is Gamma(0.45, 4.5).

pic_temp = gamma_curve[pic_temp]

Finally we compare the results between reference image by rawpy's postprocess() function and reconstructed image by our code. It turns out that max delta between them is 2 output 255. Thus we claim that these two images are almost the same. The code/image can be found here.

Image White Balancing with Python

Below is a photo I took in the viewing deck of Taipei 101. Taipei 101 has a glass wall which is green color. Thus, all photos taken inside the building through the wall has a green cast including this one.

To remove the cast of green color, we use the technique of image white balancing. First, we need a reference object with known color. Fortunately this photo has clouds (pointed by the arrow). Clouds are good references since they are white. By averaging the pixels of cloud (mean(img_array[400,500:550,:])), the mean of R/G/B is [201.5, 254.9, 253.9]. Since white color has R/G/B channels roughly equal, to make it white, 52 needs to be added to R channel of the whole image. Like DC cancellation in communication, white balancing removes the bias of the signal.

Code below shows how to do white balancing

filename = 'IMG_9254.JPG'
img = image.load_img(filename)
img_array = np.array(image.img_to_array(img),dtype=np.uint8)
img_shape=img_array.shape
plt.figure()
plt.imshow(img_array)

img_array2 = img_array
img_array2 = np.array(img_array2, dtype=np.uint16)
img_array2[:,:,0] = img_array2[:,:,0] + 52 # white balancing
img_array2 = np.clip(img_array2, 0, 255)

The newly generated image after white balancing, img_array2,  is shown below. The color does look more natural. In case that you want to repeat this, code/image can be found here.

Note on Implementation of “Fast Noise Variance Estimation” by J. Immerkær

Noise variance estimation is a fundamental task of image processing. Among various approaches of noise variance estimate, fast noise variance estimation method proposed by J. Immerkær in [1] stands out because of its good balance between complexity and accuracy.

Based on J. Immerkær's method, to estimate the standard deviation of image noise \(\sigma\), we can use equations below:

\(F=
\begin{bmatrix}
1 & -2 & 1\\
-2 & 4 & -2\\
1 & -2 & 1\\
\end{bmatrix}
\)

\(\sigma=\frac{\sqrt{\pi/2}}{6(W-2)(H-2)}\sum_{I}|I(x,y)*F|\)

\(F\) is the high-pass filter. To estimate noise variance, filter \(F\) will be applied on the whole image \(I(x,y)\) with convolution. The sum of absolute value of convolution results is normalized by width of height of the image, which is \((W-2)(H-2)\). "-2" counts for the boundary effect. The summation is also scaled by \(\frac{\sqrt{\pi/2}}{6}\), which is explained next.

Assume a 3x3 block of \(I(x,y)\) with each element of \(x_{i}\) to be a Gaussian random variable with mean \(\mu\) and standard deviation \(\sigma\)
\(I=
\begin{bmatrix}
x_{1} & x_{2} & x_{3}\\
x_{4} & x_{5} & x_{6}\\
x_{7} & x_{8} & x_{9}\\
\end{bmatrix}
\)
Then \(|I*F|=|x_{1}-2*x_{2}+x_{3}-2*x_{4}+4*x_{5}-2*x_{6}+x_{7}-2*x_{8}+x_{9}|\)
Since \(E(|I*F|^2)=36\sigma^2\), \(|I*F|=6|s|\) where s is Gaussian random variable with mean 0 and standard deviation \(\sigma\). We can derive the distribution of \(y=|s|\) and it is \(\frac{2}{\sqrt{2\pi}\sigma}e^{-\frac{y^2}{2\sigma^2}}\)  for \(y>=0\) and 0 for \(y < 0\). <0 .="" br=""> 

Since \(E(|s|)=\sqrt{\frac{2}{\pi}}\sigma\), it explains why the summation is also scaled by \(\frac{\sqrt{\pi/2}}{6}\).


[1] J. Immerkær, “Fast Noise Variance Estimation”, Computer Vision and Image Understanding, Vol. 64, No. 2, pp. 300-302, Sep. 1996

Google Cloud Service GPU Profiling: K80/T4 vs GTX1060

Recently I tried training neural network using Google cloud service. The user experience is in general good. The 300$ free credit also helps. To support deep learning task, one can create virtual machines in Google cloud service which contain both CPU and GPU. There are a few options of GPU ranging from K80 to V100 depending on how heavy the computation task is and user's affordability. I compared the performance of two GPUs: K80 and T4 with my local machine. Here are the results:

The benchmark used is a Keras example based on MNIST data set. The metric for profiling is the duration used for one epoch of training. The table below shows the profiling results:

GPU	CPU	Time
GTX1060 6GB GDDR5	Intel 6-Core i5, 16G RAM	35s
Tesla K80, 12GB GDDR5	4vCPU, 26G RAM	8s
Tesla T4, 16GB GDD6	4vCPU, 26G RAM	4s

GTX1060 is the GPU used in my local machine. The profiling results show that for this MNIST benchmark, the time used by K80 is about one fourth and the time used by T4 is about one ninth of that of my local machine. Note that when CUDA/Tensorflow libraries are not set up correctly, the computation might be done in CPU and the computation time will be increased drastically. For example, the duration of training one epoch will increase from 8s to 84s in case the computation is done in CPU not GPU. Thus when computation time is unexpectedly long in Google cloud service, please confirm that the computation is performed indeed in GPU.

Signal Processing Magic (6) -- Half Band Filter

Half-band filter is a FIR filter often used in decimation by 2 filtering operation. Benefit of this filter is that half of the filter coefficients are 0's, which reduces the complexity of filtering operation. The name of "half-band" comes from that the filter's cut-off frequency is roughly fs/4, thus the filter's pass band is around half of the total.

To design a half-band filter, it starts from a sinc function as \[s[n]=\frac{sin(\pi*n/2)}{\pi*n/2},n=-N,-(N-1),...,N\]
We know that when N approaches infinite, freq response of this sinc function is rectangle ranging from -fs/4 to fs/4. But in real world, N is always a limited number, and when that occurs, the filter does not look great. Ripples occur in pass band and rejection in stop band is often not good enough either.

How to improve this? Windowing method comes for rescue. As introduced before, windowing method enhances the pass band (main lobe) and suppresses the stop band (side lobes). Assuming window is w[n], after adding window, the new filter becomes
\[h[n]=s[n]*w[n]\]
Since half of \(s[n]\)'s coefficients are 0's, half of \(h[n]\)'s coefficients are 0's too.

Matlab script to generate a 21-tap half-band filter with Hamming window is shown below:

N = 10;
x = [-10 : 10]/2;
s = sinc(x)
w = hamming(2*N+1);
h = s.*w.';

figure;
subplot(2,1,1);stem(s);title('Sinc');
subplot(2,1,2);stem(h);title('Half-band');

[H, F] = freqz(s, 1, 'whole', 2^15);
[Hf, Ff] = freqz(h, 1, 'whole', 2^15);

figure; hold on;
plot(F(1:end/2)/pi/2, 20*log10(abs(H(1:end/2))));
plot(Ff(1:end/2)/pi/2, 20*log10(abs(Hf(1:end/2))));
grid;
ylabel('Magnitude (dB)'); xlabel('Freq (fs)');
legend('Sinc','Half-band');

Filter coefficients for Sinc and half-band filter

Freq response for Sinc and half-band filter