How to remove hot pixels with a dark frame?

Question

I took some photos at night and I didn't have long exposure noise reduction on. My photo has quite a few hot pixels, here's a 100% crop of a part of the sky:

Since long exposure noise reduction is sometimes called "dark frame subtraction", I naively thought I could just subtract a dark frame from the image by loading it in another layer in Photoshop (or Affinity Photo in my case) and setting its blend mode to "Subtract". But of course that just converts the hot pixels into black pixels:

So I guess I somehow have to overwrite the pixels' channel values with those of the adjacent pixels, proportional to the value in the dark frame. What is a good software/procedure to achieve that?

Answer 1

Ideally, dark frame subtraction should be done with raw images before demosaicing. Then the resulting black spot is 1 pixel, and after demosaicing it will typically be invisible in the result due to the interpolation during processing.

You seem to have used converted (jpeg?) files, in which the stuck (hot) pixels have already been smeared over the neighbouring pixels, and the subtraction shows the expected black.

Two ways to handle this: fill in the black spots, or decrease the opacity of the subtraction layer until the holes become invisible. The latter has the advantage that it acts on all the hot pixels in one go.

Answer 2

As @remco wrote, your procedure was probably done on JPGs, whether camera does "long-exposure noise reduction" on RAW data. Basically, in JPGs single pixel "hotness" leaks into adjacent pixels, which makes subtraction difficult.

What you can try, is to use your dark image as a reference, and use clone tool to fill in the hot pixels with nearby pixels from original image. Dark image will tell you what are real stars, and what are the hot pixels. It's ,manual, so works only for low-throughput processing, I guess.

Answer 3

This can be rectified in Adobe Photoshop, lightroom or any other similar software.

In Photoshop these spots can be removed by using multiple ways. I can give 2 ways. 1. Healing brush 2. Content aware tool

I too had a similar issue with my old camera. But when I shot the pics in raw format and loaded the file in Photoshop, the hot pixels were removed automatically.