Clarification Regarding Gain Array and Center Luminance Calculation in Infinite ISP.

[GoutamiNaragund]

Dear Infinite-ISP Developers,

I am currently working on a project involving the Infinite ISP, and I am looking into the Auto Exposure (AE) and Digital Gain(DG) module as part of my research. I have a few questions regarding the implementation, and I would greatly appreciate it if you could provide some clarification.

1. Gain Array:

I would like to understand how the gain_array is obtained and used in the digital gain (DG) algorithm. Could you provide some insight into the methodology or the specific formula you used to compute the gain values in the array?

2. Center Luminance:

I also noticed that the AE algorithm uses a center luminance value (e.g., 105 in the configuration). Could you explain how this value is determined or calculated in the context of the ISP? Is it a fixed constant, or is it dynamically calculated based on the input image or scene conditions?

Your insights on these topics would be extremely helpful to my understanding of the system. If you have any references or documentation that might further clarify these points, I would be grateful for them.

Thank you for your time and assistance. I look forward to your response.

[bakhtawar-10xe]

Thank you for your interest in the Infinite ISP and your questions about the Auto Exposure (AE) and Digital Gain (DG) modules. I'm happy to provide some clarification.

1. Gain Array:

• Regarding the gain_array in the Digital Gain (DG) module: The gain_array is a sequence of digital gain values. In the Infinite ISP, it's implemented as an array to provide flexibility in how gain is applied.

• While it can be a simple linear progression (e.g., 1, 2, 3, ...), the array structure allows for non-linear gain adjustments. This can be useful for specific image processing needs. For example, the provided code uses 1, 2, 4, 6, 8.

• The specific values in the gain_array are typically determined during the ISP tuning process. The optimal values can depend on the sensor characteristics, the desired image output, and the overall AE strategy.

• In essence, the array provides a lookup table of gain values that the DG module can select from, often based on feedback from the AE module.

2. Center Luminance:

• Regarding the center luminance value in the AE algorithm: The AE algorithm in Infinite-ISP uses a target luminance value, and in the example, a value of 105 is used.

• The AE algorithm implemented in Infinite-ISP uses the third moment (skewness) about middle gray (128) to evaluate exposure.

• Using middle grey 128 for skewness equilibrium resulted in overexposed results, so the value of 105 was found empirically. We iterated through various scenes and brightness errors, the we found that 105 was the value around which skewness equilibrium gave suitable brightness to the image.

• In a more general sense, a target luminance value represents the desired "average" brightness of the image. It's the value the AE algorithm aims to achieve.

• Currently average value of center luminance is fixed and working well as basic AE algorithm. But it would be better if it is calculated dynamically.