The Sony "Star Eater" v2
What is Sony "Star Eater v2"?
The original Sony "Star Eater" algorithm is described here: Sony Star Eater.
Originally it affected only Bulb mode exposures but then Sony released a firmware update to the A7RII and A7SII cameras that introduced "Star Eater" to all exposures of 4 sec and longer.
Following a wave of complaints, Sony later released a new firmware update for the A7RII and A7SII cameras that made some changes to the algorithm to make it less destructive.
Which firmware versions use Sony "Star Eater v2"?
Firmware v4.0 for the Sony A7RII applies "Star Eater v2" to all exposures of 4 sec and longer. It is believed that firmware v3.0 for the Sony A7SII does the same thing.
Analysis has shown that the new algorithm is less destructive but the improvement is made only for the green pixels.
The blue and red pixels are affected just as badly as previously. The effect is that more stars survive than previously but these "new" survivors are left with an overall green colour.
How does "Star Eater v2" differ?
Since the red and blue channels behave exactly as previously, here is a diagram showing the green pixels only:
The formula for "Star Eater v2"
For any particular pixel V, the original algorithm examined the values in the 8 neighbours V1-V8.
The new algorithm instead uses the 4 immediate diagonal neighbours V9-V12 and the 4 orthogonal neighbours V5-V8. The algorithm can be implemented in pseudocode as follows:
If V > Max(V5,V6,V7,V8,V9,V10,V11,V12) Then V = Max(V5,V6,V7,V8,V9,V10,V11,V12)
If V < Min(V5,V6,V7,V8,V9,V10,V11,V12) Then V = Min(V5,V6,V7,V8,V9,V10,V11,V12)
A hot pixel or a single pixel star will be removed just as previously.
The effects of "Star Eater v2"
For a star fully contained within a 2x2 block of pixels only the green pixels will survive, as the following before and after diagrams demonstrate:
A star that spreads to a 3x3 block is slightly more complex. If the central pixel is red or blue then the centre of the star is punched out, as the following before and after diagrams demonstrate:
If the central bright pixel of a 3x3 block is green then its value is reduced to one of its diagonal immediate neighbours, instead of being deleted:
For a star that spreads to a 4x4 block the green pixels in the bright core will survive and the red/blue pixels will be dimmed, as the following before and after diagrams demonstrate:
The following is a simulated demonstration of the effects of star eater and of the updated version of star eater - FW4 (A7RII firmware v4.0)
The unfiltered image is a crop of an image taken with a Sony A7S on a Takahashi Epsilon ED180 telescope (focal length 500mm at f/2.8)
The reverse engineered algorithms have then been applied to that data:
The new algorithm is an improvement to the old algorithm and more stars survive the spatial filtering. However, these new survivors are predominately green in colour and diagonally oriented.