DOUBLE BINNABLE RGBW
Modern mobile image sensors use binning to reduce resolution and improve SNR. There are two primary methods to increase SNR: charge domain binning and the use of white pixels in RGBW sensors. For example, Quad Bayer and Hexa-deca Bayer sensors utilize 4:1 charge domain binning, while RGBW sensors rely on white pixels. However, the popular RGBW-Kodak pattern has a limitation of 2:1 charge domain binning, which results in higher read noise.
Our Quad RGBW CFA pattern offers the best of both worlds by incorporating white pixels and 4:1 charge domain binning. This unique combination results in industry leading SNR. Additionally, our Quad RGBW CFA pattern can be binned once to produce half-resolution images or twice to produce quarter-resolution images, making it ideal for very high resolution image sensors.
Benefits
- Both 4:1 charge domain binning and white pixels for industry leading low light performance
- Double binnable, 1/2 and 1/4 resolution modes for very high megapixel sensors
- 2x2 homogeneous tiles for easy to manufacture CFA
- high quality demosaicking at all resolutions - superior to all binnable RGB CFAs available today
- 6 dB dynamic range advantage over RGB CFAs in full resolution and all binned modes
Comparison With Existing CFAs
| CFA | Bin Mode | Demosaic Quality | Low Light SNR Advantage | Bright Light SNR Advantage | Power Consumption | Frame Rate |
|---|---|---|---|---|---|---|
| Quad Bayer | Full Resolution | Poor | 0 dB (Reference) | 0 dB (Reference) | 100% (Reference) | 1x (Reference) |
| Binned Once | Good | 12 dB | 6 dB | 25% | 4x | |
| RGBW-Kodak | Full Resolution | Fair | 6 dB | 3 dB | 100% | 1x |
| Binned Once | Good | 15 dB | 9 dB | 50% | 2x | |
| Hexadeca Bayer | Full Resolution | Fair | 0 dB | 0 dB | 100% | 1x |
| Binned Once | Poor | 12 dB | 6 dB | 25% | 4x | |
| Binned Twice | Good | 18 dB | 12 dB | 6.25% | 16x | |
| Quad-RGBW (Ours) | Full Resolution | Good | 6 dB | 3 dB | 100% | 1x |
| Binned Once | Good | 18 dB | 9 dB | 25% | 4x | |
| Binned Twice | Good | 24 dB | 15 dB | 12.5% | 8x |
TECHNOLOGY
By combining several pixels into one, binning provides the benefits of a lower resolution image sensor, such as higher SNR, faster speed and lower power consumption. Binning can be performed in three different domains: charge, voltage, and digital.
Charge domain binning reduces readout noise by using shared floating diffusions, resulting in an SNR boost that exceeds what is achievable through voltage or digital domain binning or by reading out the full resolution image and downsizing it. However, charge domain binning is typically limited to a 4:1 ratio, as CMOS image sensors usually have four pixels sharing a floating diffusion and its downstream read circuit.
Prior Art: Quad Bayer
The Quad Bayer is currently the most widely used binnable color filter array (CFA). However, it can only bin down to half resolution, which is not be sufficient for high-resolution sensors.
Furthermore, demosaicking Quad Bayer is still challenging due to the mathematical degeneracies inherent in the CFA pattern. For a deeper understanding of these degeneracies, please refer to section 2.2 of our paper.
Prior Art: Hexadeca Bayer
Hexadeca Bayer is twice binnable, making it suitable for meeting the power and speed requirements of high-resolution sensors. However, even after one round of binning, demosaicking remains a challenge due to the mathematical degeneracies present in the resulting Quad Bayer pattern.
Moreover, large 4×4 tiles of the same color also create coarse-grained noise, which can be difficult to remove because it falls into the low-frequency part of the spectrum.
Prior Art: RGBW-Kodak
The RGBW-Kodak CFA includes W pixels for improved sensitivity, providing a 6 dB low-light SNR advantage over Quad and Hexadeca Bayer in full resolution mode. However, the full resolution mode is not typically used in low light, and the RGBW-Kodak can only be binned to a 2:1 ratio compared to Quad and Hexadeca Bayer, which can be binned to a 4:1 ratio in the charge domain. This results in a reduced low-light SNR advantage of 3 dB for RGBW-Kodak.
Additionally, RGBW-Kodak can only be binned once, making it less competitive in terms of speed and power consumption when compared to Quad and Hexadeca Bayer. Hexadeca Bayer, in particular, can be binned twice, achieving a 16x improvement in speed and power compared to RGBW-Kodak’s 2x binning capability.
Furthermore, RGBW-Kodak is also prone to challenging false color issues that have affected most, but not all, image signal processors.
Quad RGBW Binned Once
The Quad RGBW CFA produces the RGBW-IA CFA after the first round of binning, which is similar to the RGBW-Kodak CFA but without the latter’s false color issues. The false color problems of RGBW-Kodak are primarily due to its green pixels being arranged on widely separated diagonals. RGBW-IA addresses this problem by breaking up the green diagonals and redistributing the green pixels. For a detailed analysis of the RGBW-Kodak and RGBW-IA CFAs, please refer to our our paper.
Quad RGBW Binned Twice
After two rounds of binning, Quad RGBW produces an RGB mosaic and a W color plane. While demosaicking the resulting RGB mosaic can be challenging, the RGB mosaic-W difference image is easy to demosaic. By interpolating R-W, G-W, and B-W, the demosaicked RGB can be obtained by adding W. Additionally, the high SNR W color plane can be fused with the resulting RGB image to improve its SNR.