 Research
 Open Access
Analysis and processing of pixel binning for color image sensor
 Xiaodan Jin^{1} and
 Keigo Hirakawa^{1}Email author
https://doi.org/10.1186/168761802012125
© Jin and Hirakawa; licensee Springer. 2012
 Received: 11 October 2011
 Accepted: 29 May 2012
 Published: 21 June 2012
Abstract
Pixel binning refers to the concept of combining the electrical charges of neighboring pixels together to form a superpixel. The main benefit of this technique is that the combined charges would overcome the read noise at the sacrifice of spatial resolution. Binning in color image sensors results in superpixel Bayer pattern data, and subsequent demosaicking yields the final, lower resolution, less noisy image. It is common knowledge among the practitioners and camera manufacturers, however, that binning introduces severe artifacts. The indepth analysis in this article proves that these artifacts are far worse than the ones stemming from loss of resolution or demosaicking, and therefore it cannot be eliminated simply by increasing the sensor resolution. By accurately characterizing the sensor data that has been binned, we propose a postcapture binning data processing solution that succeeds in suppressing noise and preserving image details. We verify experimentally that the proposed method outperforms the existing alternatives by a substantial margin.
Keywords
 Aliasing
 Neighboring Pixel
 High Dynamic Range Imaging
 Sensor Resolution
 Color Filter Array
1 Introduction
Recent progress on digital camera technology has had extraordinary impact on numerous electronic industries, including mobile phones, security, vehicle, bioengineering, and computer vision systems. In many applications, sensor resolution has exceeded the optical resolution, meaning that the additional hardware complexity to increase pixel density would not necessarily result in large image quality gains. The significant improvement in sensor sensitivity has allowed cameras to operate in lighting conditions that were unthinkable with film cameras.
Despite increased sensitivity, however, noise remains a serious problem in modern image sensors. Available technologies for reducing noise in hardware include backside illuminated architecture [1, 2], color filters with higher transmittance [3, 4], and pixel binning [5–7]. Processing techniques at our disposal include image denoising [8–10], joint denoising and demosaicking [11–14], image deblurring [15, 16] (long shutter to compensate for light), and singleshot high dynamic range imaging [17].
The remainder of this article is organized as follows. We begin by briefly reviewing CFA sampling and demosaicking in Section 2. Section 3. provides a rigorous analysis of binning. A novel binningaware demosaicking technique is developed in Section 4. We experimentally verify its effectiveness in 5. before making concluding remarks in Section 6.
2 Background
2.1 CFA sampling
where $\mathit{c}:{\mathbb{Z}}^{2}\to {[0,1]}^{3}$ denotes the translucency of CFA at location n. The advantage to the representation is that the difference images x_{ α }=x_{ r }−x_{ g } and x_{ β }=x_{ b }−x_{ g } enjoy rapid spectral decay and can serve as a proxy for chrominance. On the other hand, the “baseband” green image x_{ g } can be taken to approximate luminance. As our eventual image recovery task will be to approximate the true color image triple x(n) from acquired sensor data y(n), note that recovering either representations ({x_{ r }x_{ g }x_{ b }}or {x_{ g }x_{ α }x_{ β }}) are equivalent. Moreover, the representation of (1) allows us to recast the purecolor sampling structure in terms of sampling structures c_{ α } and c_{ β } associated with the difference channels x_{ α }and x_{ β }. For more extensive investigation on the bandlimitedness assumptions of {x_{ g }x_{ α }x_{ β }}, see [18–20].
2.2 Demosaicking
Most demosaicking algorithms described in the literature make use (either implicitly or explicitly) of correlation structure in the spatial frequency domain, often in the form of local sparsity or directional filtering [14, 19, 21–23]. As noted in our earlier discussion, the set of carrier frequencies induced by c_{ α } and c_{ β } include Π,0]^{ T } and [0,Π^{ T }, locations that are particularly susceptible to aliasing by horizontal and vertical edges. Figures 3b,c indicates these scenarios, respectively; it may be seen that in contrast to the radially symmetric baseband spectrum of Figure 3a, chrominance–luminance aliasing occurs along one of either the horizontal or vertical axes. However, successful reconstruction can still occur if a noncorrupted copy of this chrominance information is recovered, thereby explaining the popularity of (nonlinear) directional filtering steps [19, 21–23]. We can, therefore, view the CFA design problem as one of spatialfrequency multiplexing, and the CFA demosaicking problem as one of demultiplexing to recover subcarriers, with spectral aliasing given the interpretation of “cross talk” [19].
where d(·,·) is some distance metric and εis a tolerance parameter.
3 Analysis of binning
Let us rigorously analyze the effects that binning has on the acquired sensor data. We begin in Section 3.1 with a brief review of the signaltonoise ratio (SNR) gains that binning is expected to improve [24]—the main motivation behind binning. An indepth analysis in Section 3.2 will prove that a combination of binning and demosaicking results in a loss of resolution that is far worse than commonly believed. Section 3.3 offers an alternative perspective that paves a path towards recovering artifactfree images.
3.1 Signal measurement uncertainty
where t is the exposure time, Q is the quantum efficiency constant, D is the dark current constant, and N is the read noise power.
3.2 Binning “sampling”
Due to the fact that binning combines M electric charges of neighboring pixels, each pixel cannot be shared by more than one superpixel. Moreover, the charges can be combined by summation only (i.e. no fractional combinations). As such, the options for binning schemes are fairly limited. Furthermore, the superpixels produced by pixel binning in color image sensors form a Bayer pattern that requires the additional step of demosaicking to recover the full color low resolution image. We will show that superpixel Bayer pattern suffers from many problems that the pixellevel Bayer pattern does not, leading to the conclusion that combining pixel binning and demosaicking is the wrong approach.
Consider Kodak PIXELUX, the most widely used binning scheme illustrated in Figure 1a,c [7]. It combines four neighboring pixel values together to form one superpixel. This process of combining neighboring pixels to form a single superpixel is equivalent to applying a convolution operator followed by downsampling:

filtering: let h_{bin}denote the filter coefficients$\begin{array}{l}{h}_{\text{bin}}\left(\mathit{n}\right)=\Delta \left(\mathit{n}\left(\genfrac{}{}{0ex}{}{1}{1}\right)\right)+\Delta \left(\mathit{n}\left(\genfrac{}{}{0ex}{}{1}{1}\right)\right)\\ \phantom{\rule{5.5em}{0ex}}+\Delta \left(\mathit{n}\left(\genfrac{}{}{0ex}{}{1}{1}\right)\right)+\Delta \left(\mathit{n}\left(\genfrac{}{}{0ex}{}{1}{1}\right)\right),\phantom{\rule{2em}{0ex}}\hfill \end{array}$(8)where Δ(·)denotes the Kronecker delta function. Then the charge summation in PIXELUX is$\begin{array}{l}{y}_{\text{bin}}\left(\mathit{n}\right)=y\left(\mathit{n}\right)\star {h}_{\text{bin}}\left(\mathit{n}\right).\end{array}$

downsampling: to yield the superpixel Bayer pattern data s, do$\begin{array}{c}s\left(2\mathit{n}\right)={y}_{\text{bin}}\left(4\mathit{n}\right)\\ s\left(2\mathit{n}+\left(\genfrac{}{}{0ex}{}{0}{1}\right)\right)={y}_{\text{bin}}\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{0}{1}\right)\right)\\ s\left(2\mathit{n}+\left(\genfrac{}{}{0ex}{}{1}{0}\right)\right)={y}_{\text{bin}}\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{1}{0}\right)\right)\\ s\left(2\mathit{n}+\left(\genfrac{}{}{0ex}{}{1}{1}\right)\right)={y}_{\text{bin}}\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{1}{1}\right)\right).\end{array}$(9)
The main advantage of binning in (9) over (2) is that the signal strength of the baseband X_{ g } and the chrominance components X_{ α }and X_{ β }are boosted by four times—consistent with the SNR analysis in the previous section. As evidenced by Figure 5a, the Fourier support of (9) closely resembles the Bayer pattern of Figure 3a. Superpixel Bayer pattern data in (10) is far from an ideal Bayer pattern representation of the true image x(n) we hope to recover from s(n), however. One distortion we see is the unwanted filtering term $\sum _{\mathit{\theta}\in {\mathbb{Z}}_{2}^{2}}{e}^{j{\mathit{\omega}}^{T}\mathit{\theta}/2}$ that degrades the baseband luminance/green signal X_{ g }(ω). Another complication is that the antialiasing is only partially effective, allowing aliasing to corrupt the baseband X_{ g }(ω) near $\mathit{\omega}=\pm {[0,\frac{\Pi}{4}]}^{T},\pm {[\frac{\Pi}{4},0]}^{T},\pm {[\frac{\Pi}{4},\frac{\Pi}{4}]}^{T},\pm {[\frac{\Pi}{4},\frac{\Pi}{4}]}^{T}$.
Contrary to the popular belief that Kodak PIXELUX binning results in 2×2reduction in resolution, the main conclusion we draw from (9) is that the “Nyquist rate” of this binning scheme is Π/4due to high risk of aliasing—implying that the actual resolution loss is 4×4, far worse than the presumed 2×2. Even if this Nyquist rate did not cause problems (e.g. increase sensor resolution), s does not escape the unwanted filtering term in (9)—this cannot be eliminated simply by increasing sensor resolution. Hence when a demosaicking algorithm is applied to the superpixel Bayer pattern data s, what is expected is a filtered and aliased image that we have already seen in Figure 2.
3.2 Binning “subsampling”

filtering: The charge summation in PIXELUX is$\begin{array}{l}{y}_{\text{bin}}\left(\mathit{n}\right)=y\left(\mathit{n}\right)\star {h}_{\text{bin}}\left(\mathit{n}\right).\end{array}$

subsampling: to yield the binning subsampling data t, do$\begin{array}{c}t\left(4\mathit{n}\right)={y}_{\text{bin}}\left(4\mathit{n}\right)\\ t\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{0}{1}\right)\right)={y}_{\text{bin}}\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{0}{1}\right)\right)\\ t\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{1}{0}\right)\right)={y}_{\text{bin}}\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{1}{0}\right)\right)\\ t\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{1}{1}\right)\right)={y}_{\text{bin}}\left(4\mathit{n}+\left(\genfrac{}{}{0ex}{}{1}{1}\right)\right)\\ t\left(\mathit{n}\right)=0\text{otherwise.}\end{array}$(11)
4 Binningaware demosaicking
Motivated by the analysis of pixel binning subsampling in (12), we now present a novel binningaware demosaicking aimed at recovering fullcolor image xwithout introducing binning artifacts. We accomplish this in three stages.
Step 1: Chrominance estimation
where (·)^{ † } denotes a pseudo inverse matrix and h_{0} is a lowpass filter whose passbands matches the support of X_{ α }and X_{ β }. The reconstruction of vertically oriented image feature (denoted ${\widehat{x}}_{\alpha ,v},{\widehat{x}}_{\beta ,v}$) is same as (13) but with 90° rotation.
Step 2: Luminance filtering
As illustrated in Figure 8c,d, the modulation by f(n)not only shifts the spectrums, but also creates additional aliasing copies. Hence, the filter h_{2}is needed to attenuate them. The same procedure can be used to find the green image ${\widehat{x}}_{g,v}$ based on ${\widehat{x}}_{\alpha ,v}$ and ${\widehat{x}}_{\beta ,v}$.
Step 3: Directional selection
Once ${\widehat{\mathit{x}}}_{h}=\{{\widehat{x}}_{g,h},{\widehat{x}}_{\alpha ,h},{\widehat{x}}_{\beta ,h}\}$ and ${\widehat{\mathit{x}}}_{v}=\{{\widehat{x}}_{g,v},{\widehat{x}}_{\alpha ,v},{\widehat{x}}_{\beta ,v}\}$ are found, they must be combined to yield the final estimate, ${\widehat{\mathit{x}}}_{t}=\{{\widehat{x}}_{g},{\widehat{x}}_{\alpha},{\widehat{x}}_{\beta}\}$ via the convex combination (3). As already mentioned, the directional selection variable τ has received considerable attention in research and many techniques are available. However, these studies often lack analysis under noise—although binning reduces noise considerably, most directional selection variables are nevertheless sensitive to random perturbations.
where ${\widehat{\mathit{x}}}_{\tau}$ and ${u}_{\widehat{x}}$ are as defined in (3) and (4), respectively. Contrast this to the original AHD formulation which selected either ${\widehat{\mathit{x}}}_{h}$ or ${\widehat{\mathit{x}}}_{v}$ (i.e. τ∈{0,1} instead of τ∈[0,1]) as the final output ${\widehat{\mathit{x}}}_{t}$. The modified strategy of (16) behaves similarly to the original AHD near the edges of an image, but encourages averaging in the flat regions of the image. It was found empirically to be far more robust to directional selection under noise.
5 Experimental validation
5.1 Setup
Reconstruction performance in PSNR with various noise levels
Noise (σ_{ n })  Color  LRCFA  HRCFA  HRCFA + binning  

[19]  [19] + DS  PO + [19]  K + [19]  K + Proposed  
0.000  R  48.1787  51.4671  45.2485  45.3275  47.8061 
G  51.8147  54.7110  46.5344  47.1429  48.6259  
B  46.5116  50.9532  43.6715  44.5752  45.5689  
0.005  R  47.1788  46.5149  44.6596  44.7572  46.8994 
G  50.1223  48.5953  46.3633  41.5903  47.6607  
B  45.6221  45.2237  43.1426  43.9331  44.6091  
0.010  R  45.4430  42.1931  43.4914  43.5929  45.2686 
G  47.6553  43.9223  44.5383  44.9318  45.9540  
B  44.0420  40.7293  42.0733  42.7001  42.9732  
0.015  R  43.7677  39.1858  42.2368  42.3349  43.6632 
G  45.5264  40.7919  4.18183  43.4857  44.3119  
B  42.4968  37.6716  40.9172  41.4211  41.3987  
0.020  R  42.2672  36.9118  41.0434  41.1293  42.2202 
G  43.7388  38.4621  41.9128  42.1503  42.8488  
B  41.0955  35.3795  39.8064  40,2189  39.9910  
0.025  R  40.9430  35.0944  39.9375  40.0160  40.9391 
G  42.2200  36.6133  40.7530  40.9454  41.5568  
B  39.8450  33.5524  38.7712  39.1164  38.7417  
0.030  R  39.7693  33.5816  38.9267  38.9959  39.7997 
G  40.9107  35.0803.9327  39.7008  39.8595  40.4102  
B  38.7310  32.0235.3426  37.8168  38.1076  37.6279 
The linear images used in this simulation study are a part of the collection of [25, 26], examples of which are shown in Figure 9. Numerical scores in Table 1 and Figure 13 were obtained by averaging performance over 84 images. Noise is simulated by adding pseudorandom white Gaussian noise to the CFA data y(n), the superpixel CFA data s(n) and p(n), and the lower resolution CFA data y^{ ′ }(n). In the experiments, the 12 bit image data in [25, 26] were renormalized to ranges 0–1—meaning noise standard deviation σ_{ n }=0.01 correspond to standard deviation of 40.96 in a 12 bit camera processing pipeline, etc. Considering the noise models in (57), one may ask if such a simplified noise model is appropriate. As evidenced by the analysis in (7), however, the difference between SNR and SNR_{sum}is M (the number of pixels combined together); and the difference between SNR_{sum} and SNR_{bin} is the read noise power N. Hence the SNR gains in binning is attributed only to the signalindependent portion of the noise, and not on the signal dependent portion. Furthermore, the read noise dominates in the low light regime. Hence simulated additive white Gaussian noise suffices for experimental verification. The binning subsample signal t(n) represents the same data as s(n) and is computed by upsampling s(n)(insert zeros where necessary).
5.2 Results
Example outputs from four different methods (${\widehat{\mathit{x}}}_{{y}^{\prime}},{\widehat{\mathit{x}}}_{y},{\widehat{\mathit{x}}}_{s},{\widehat{\mathit{x}}}_{p},{\widehat{\mathit{x}}}_{t}$) are shown in Figures 10, 11 and 12. As expected, demosaicking applied to a full resolution CFA (${\widehat{\mathit{x}}}_{y}$) has a noisy appearance due to low SNR of individual pixels. However, edges and image features are clearly defined even after downsampling thanks to the full resolution description. Demosaicking applied to superpixel CFAs (${\widehat{\mathit{x}}}_{s},{\widehat{\mathit{x}}}_{p}$), on the other hand, yields the opposite qualities—the noise is significantly reduced owing to high SNR of binning, but the image suffers from severe artifacts stemming from aliasing in (10). More specifically, the aliasing in Kodak PIXELUX binning manifests itself as a pixelization artifact, while PhaseOne binning results in zippering artifacts. However, one may argue that the aliasing artifacts in ${\widehat{\mathit{x}}}_{p}$ become less bothersome at the highest level of noise because the zippering and noise become less distinguishable. By contrast, the proposed binningaware demosaicking method (${\widehat{\mathit{x}}}_{t}$) succeeds in suppressing noise while preserving the image features. Of particular interest is the comparison between ${\widehat{\mathit{x}}}_{s}$ and ${\widehat{\mathit{x}}}_{t}$, since they both use Kodak PIXELUX binning but the proposed method yield drastically improved outcomes. Overall, the proposed method has better visual quality than ${\widehat{\mathit{x}}}_{s}$ and ${\widehat{\mathit{x}}}_{p}$ for σ_{ n }<0.03; but proposed has a slightly noisier appearance at the highest level of noise (σ_{ n }=0.03). Finally, the output from the low resolution camera ${\widehat{\mathit{x}}}_{{y}^{\prime}}$ is both robust to noise and aliasing. This is expected, as lower resolution CFA data y^{ ′ }(n) does not share the problems that superpixel CFAs p(n),s(n),t(n) have. However, ${\widehat{\mathit{x}}}_{y}$ has superior reconstruction over ${\widehat{\mathit{x}}}_{{y}^{\prime}}$ without noise (σ_{ n }=0). Figure 12 shows an example where none of the reconstruction methods produced a satisfactory output (except for ${\widehat{\mathit{x}}}_{y}$ under no noise).
The performance is evaluated also in terms of peak SNR, using the downsampled version of the ideal lowpassed (antialiased) image x^{ ′ }in (18) as their reference. The results are summarized in Table 1. When there is no noise (σ_{ n }=0), ordinary demosaicking reconstruction ${\widehat{\mathit{x}}}_{y}$ and lower resolution sensor ${\widehat{\mathit{x}}}_{{y}^{\prime}}$ yields the best results, as expected. However, the proposed ${\widehat{\mathit{x}}}_{t}$ is a very close third, yielding comparably satisfactory results. Binning result ${\widehat{\mathit{x}}}_{s}$ is worst by far due to binning artifacts.
When noise is taken into consideration, the quality of ${\widehat{\mathit{x}}}_{y}$ suffers greatly as expected. Even with noise variance as little as σ_{ n }=0.005, the performance of ${\widehat{\mathit{x}}}_{y}$ deteriorates significantly, while performance of ${\widehat{\mathit{x}}}_{s}$, ${\widehat{\mathit{x}}}_{p}$, ${\widehat{\mathit{x}}}_{t}$, and ${\widehat{\mathit{x}}}_{{y}^{\prime}}$ in terms of PSNR are far less sensitive to noise. With moderate noise levels (σ_{ n }<0.03) the proposed binningaware demosaicking clearly outperforms the artifactplagued demosaicking of superpixels. With the largest noise level considered (σ_{ n }=0.03), PSNR performances of ${\widehat{\mathit{x}}}_{s}$, ${\widehat{\mathit{x}}}_{p}$, and ${\widehat{\mathit{x}}}_{t}$ are closer to each other because deteriorations in output images are dominated by noise (rather than by artifacts).
The analysis in Figures 10, 11, 12, 13 and Table 1 sheds a light on the decadesold debate about resolution versus noise. On one hand, the lower resolution sensor delivers consistent performance under noise (${\widehat{\mathit{x}}}_{{y}^{\prime}}$). However, Figure 11 shows that under no noise, extra sensor resolution is still desirable. Consider Figure 13. The comparison between green (low resolution) and red (high resolution) curves is consitent with the image quality of Figures 10 and 11. With the availability of pixel binning, we would compare the green curve with the “max function” over the red and blue (binning) curves in Figure 13. Hence one can think of binning as a way to narrow the gap between the red and green curves in noise, without making sacrifices to the advantages of higher spatial resolution.
6 Conclusion
In this article, we proved via a rigorous analysis of binning sampling that Kodak PIXELUX binning scheme results in 4×4reduction in image resolution—contrary to the popular belief that binning of four pixels should result in 2×2reduction in resolution. We proposed a binningaware demosaicking algorithm based on the Fourier analysis of binning subsampling to combine unaliased copies of the Fourier spectra together via the demodulation. The resultant method succeeds in reconstructing the color image with only 2×2 resolution loss—or increasing the resolution by 2×2over the traditional approach of applying demosaicking to superpixels. The binningaware demosaicking also succeeds in suppressing noise and preserving image details. We verified experimentally that the binningaware demosaicking outperforms the alternatives.
Appendix 1: Proof of Fourier Representation of binning subsampling
where we used the fact that H_{bin}(0)=4.
The Fourier support of T(ω) is illustrated in Figure 6b. Note that the summation over λsuggests that binning subsampling will result in 16 modulations. However, $\sum _{\mathit{\theta}\in {\mathbb{Z}}_{2}^{2}}{e}^{j\left\{{\mathit{\lambda}}^{T}\mathit{\theta}\right\}}$ is 0 for many values of λ, as shown in Figure 7. As a result, there are only nine actual modulations.
Appendix 2: Proof of Fourier representation of binning sampling
7 Endnote
^{a}Filter h_{bin} is a combination of highpass and lowpass. However, binning takes advantage of the fact that the sensor resolution exceeds optical resolution, meaning h_{bin} is taken to be a lowpass/antialiasing filter on x_{ g }.
Declarations
Acknowledgement
This work was funded in part by Texas Instruments.
Authors’ Affiliations
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