I'd appreciate an education on artifacts...

Started Oct 18, 2013 | Discussions thread
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I'd appreciate an education on artifacts...

...especially as they are functions of upsampling.

Let me preface my post by saying the following is unsubstantiated opinion on my part.

I'm thinking that upsampling algorithms are a form of averaging. Let's take the following 2x2 grid of pixels with the following "values":

40 160

20 100

and upsample it into a 3x3 grid of pixels:

40 b1 160

a2 b2 c2

20 b3 100

It seems to me that the computed value for b1 would necessarily between 40 and 160, the computed value for a2 to be between 20 and 40, etc.

For example, in the most simple manner (bilinear upsampling), we would compute b1, a2, c2, and b3 as the averages of their endpoints. That is:

b1 = (40 + 160) / 2 = 100

a2 = (40 + 20) / 2 = 30

c2 = (160 + 100) / 2 = 130

b3 = (20 + 100) / 2 = 60

and then average all the pixels to get b2:

b2 = (40 + 160 + 20 + 100) / 4 = 80

However, as I said, this would be the most simple form of upsampling. That said, regardless of how the photo is upsampled, would not the interpolated points still fall between the endpoints?

Thus, I'm thinking that upsampling will not introduce artifacts, as I am unclear as to how interpolated values that lie between the endpoints would manifest themselves as artifacts. Rather, at worst, it would merely enlarge existing artifacts, and, most likely, smooth out the artifacts to be less of a problem.

Or, is my assumption that the interpolated values would always lie between the endpoints incorrect?  I'm thinking that at least some interpolated values would have to lie outside the interval bounded by the endpoints to create false detail and artifacts.

Am I just totally missing what artifacts and false detail are?


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