Réparation d’image en post-production
This post shows how to remove hot pixels in video with minimal artifacts using DaVinci Resolve and the Dead Pixel Fixer OFX plugin included in the Studio version of the software. Splitter/Combiner and Layer Mixer nodes are used to retain image detail without degrading it.
As always, a full list of the articles in this blog can be found HERE.
I recently spotted a lingering red spot on the rushes of my a7s2 in the bottom right edge of the image. Oddly, this spot only shows up in 4K24p Full frame mode, but not in the 1080p24 Full frame setting. It’s strange as the sensor readout areas are the same and the oversampling of the 1080p mode suggests that as many pixels are read as in the 4K mode, so the defect should show up in both, but it’s not.
I first did some research and tried remapping the sensor (an automatic process that takes place when the device is turned off every month to remove hot and dead pixels from the image, which can be forced by advancing the device internal date by 2 months in the menu). Unfortunately, the area remains visible and the body would therefore have to be sent for repair. I don’t know if there are several hot pixels close to each other and the remapping cannot take them into account, or if the problem comes from elsewhere.
My a7s2 being long out of warranty, I chose to see if it was possible to mitigate the problem in post-production to avoid the cost of a sensor repair.
Video tutorial
I am using DaVinci Resolve Studio version 17.4.4. Here is a quick video tutorial demonstrating the process. Text explanations are available below.
Detailed tutorial
I first used the Dead Pixel Fixer OFX plugin at the end of my color grade, but soon found it to be very limited as is. The area to be corrected is relatively large, covering several rows and lines of pixels. The plugin appears to use a form of content-aware fill based on areas adjacent to the window drawn over the defect. The results may be passable for an area with no motion or detail, but as soon as hot pixels pass through detail or contrasted areas, the effect becomes very noticeable and disturbing.
Test Caption 1
Test Caption 2
Before: Without Plugin | After: With Plugin
So I looked at the R, G and B channels independently and saw that the hot pixels only really appear on the R channel. Some artefacts are present on the other two layers but not in a very noticeable way. I used a Splitter/Combiner node to separate the three channels and applied the plugin only to the R channel. The result is already much better, but since the plugin reconstructs details based on adjacent areas and that the hot pixels “disappear” in the dark areas, we see red color blotches appear in the dark details.
Before: Plugin on all channels | After: Plugin on R Channel
To reduce this artifact, I simply added a parallel node to that of the OFX plugin, then transformed it into a Layer Mixer node. I then changed the compositing mode to Darken, in order to keep only the darkest value in the R channel between the original image and the one reconstructed by the plugin. This way, hot pixels are erased (being brighter than the original image), but dark areas are no longer colored. In the parallel node, I just placed a round window largely covering the hotspot area, so that the image processing is less cumbersome and only done in the affected area.
Before: Plugin on R Channel | After: Plugin on R Channel + Darken
Hot pixels are erased with minimal impact on image quality and detail.