Canon Line Offset Noise

Started Oct 20, 2009 | Discussions thread
xxD Contributing Member • Posts: 726
my 7D ISO100 black frames don't look that bad

Just thought I'd add that my 7D ISO100 black frames don't look as bad as that top portion.

xxD wrote:

Alright. I can see what you've done and it makes sense and seems to be a straightforward data manipulation.

But that's a black frame and you get to make a lot of assumptions that make the correction easy because it's a black frame.

What complications are added to your algorithm if it's an photo with unpredictable subject and background detail?

John Sheehy wrote:

In another thread, I mentioned the ease at which Canon can remove artifacts from RAW data. In this post I will demonstrate how simple the offset banding in Canon RAW data is, and how easily it is removed.

The following composite image is the upper left corner of a 7D ISO 100 RAW blackframe. The image is 1000*700 pixels, composed of two versions of a single 1000*350 pixel crop. The top is the RAW data as is, with color interpolation (simple demosaic). The first ~150 columns of pixels is masked in the camera, and sees no light. The top ~32 rows is the same, but something strange is done to that data. I tried using the 27 rows at the top to make a mask to subtract from the RAW image, and the data was not very effective. Too few samples, and some artifacts not seen elsewhere in the RAW. If Canon meant for these to be masked pixels, they really messed it up.

So, for the bottom half, being that the image is a blackframe, and is effectively just like a fully masked image, I made a mask from the bottom 100 rows of pixels (less than 3% of the image height). I took the average in each 100 pixel column, and subtracted it from every pixel in the line. As you can see, even though I took the data used to subtract the banding from the bottom of the image, and my crops are from the top, 3400 pixels away, the pattern cancelled almost perfectly, proving that most of the banding runs at an exact offset, consistently from one end of the image to the other, and is very easily removed.

Now maybe some of you can understand why it is so frustrating to those of us who understand the simplicity of these problems, that Canon leaves this garbage in their RAW files.

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