I think you’re on to something, but sort of accidentally. A couple replies to you are saying it’s not possible, but I think they’re making an assumption that is not correct in many cases.
The replies is saying it’s not possible because the layers are flattened before passed to the compression, thus the uncensored/unredacted data is not part of the input to the compression and therefore cannot have any impact on its output. This is true assuming you are starting with an uncompressed image.
Here’s a scenario where the uncensored/unredacted parts of the image could influence the image: someone takes a photo of their ID, credit card, etc. It’s saved in a lossy compressed format (e.g. JPEG), specifically not a lossless format. They open it in an image editing tool to 100% black out some portion, then save it again (doesn’t actually matter the format). I feel lile someone is going to think I’m misunderstanding if I don’t explain the different output scenarios.
First is the trivial case: amultilayer output with the uncensored/unredacted data as its own layer. In this case, its trivial to get the uncensored/unredacted data as it is simply present and visible of you use a tool that can show the individual layers, but the general assumption is that this is not the case – that the output is a single layer image, in which we have 2 scenarios.
Second case: lossy compressed original, lossless censored. Consider that this censored/redacted image is flattened and saved as a lossless format such as PNG. Certainly there will be no compression artifacts of the uncensored/redacted data both because it is lossless (no artifacts added by PNG) and that it was flatted prior to being passed to PNG. However, the uncensored/unredacted artifacts remain in the uncensored/unredacted portions of the image. These were introduced by the compression that was applied prior to the censoring (e.g. the JPEG compression that contained the pre censored image). I suspect this is actually a common case.
Third case: lossy compressed original, lossy compressed censored: same as second case, except now you have additional artifacts, in particular you bow have artifacts from the censored portion, and the artifacts of the previous lossy compression are also adding additional artifacts. This is probably more difficult, but the point is that the original uncensored/unredacted artifacts are still present.
I think you’re on to something, but sort of accidentally. A couple replies to you are saying it’s not possible, but I think they’re making an assumption that is not correct in many cases.
The replies is saying it’s not possible because the layers are flattened before passed to the compression, thus the uncensored/unredacted data is not part of the input to the compression and therefore cannot have any impact on its output. This is true assuming you are starting with an uncompressed image.
Here’s a scenario where the uncensored/unredacted parts of the image could influence the image: someone takes a photo of their ID, credit card, etc. It’s saved in a lossy compressed format (e.g. JPEG), specifically not a lossless format. They open it in an image editing tool to 100% black out some portion, then save it again (doesn’t actually matter the format). I feel lile someone is going to think I’m misunderstanding if I don’t explain the different output scenarios.
First is the trivial case: amultilayer output with the uncensored/unredacted data as its own layer. In this case, its trivial to get the uncensored/unredacted data as it is simply present and visible of you use a tool that can show the individual layers, but the general assumption is that this is not the case – that the output is a single layer image, in which we have 2 scenarios.
Second case: lossy compressed original, lossless censored. Consider that this censored/redacted image is flattened and saved as a lossless format such as PNG. Certainly there will be no compression artifacts of the uncensored/redacted data both because it is lossless (no artifacts added by PNG) and that it was flatted prior to being passed to PNG. However, the uncensored/unredacted artifacts remain in the uncensored/unredacted portions of the image. These were introduced by the compression that was applied prior to the censoring (e.g. the JPEG compression that contained the pre censored image). I suspect this is actually a common case.
Third case: lossy compressed original, lossy compressed censored: same as second case, except now you have additional artifacts, in particular you bow have artifacts from the censored portion, and the artifacts of the previous lossy compression are also adding additional artifacts. This is probably more difficult, but the point is that the original uncensored/unredacted artifacts are still present.