Edge ghosting with XL2

[Barry Goyette]

It looks like a plain old edge sharpening artifact to me...(and one that I have seen in virtually every frame of DV from every canon camera I own.) Used for its intended purpose (NTSC video), this type of artifact is not a bad thing...it make the footage look better (sharper) on the lower resolution output of a typical television. However, when one stands too close to the computer, as I often do, these things do get annoying. These are the same type of artifacts that are seen in photoshop when you use USM with a high radius (which looks bad on screen, but often can make an image printed on newsprint with a coarse screen look amazingly good).

Try lowering your the sharpness setting on the camera. If that doesn't help, then its what AJ said (which went right over my head!!!)

Barry

[Frank Aalbers]

I already tried sharpness to the lowest setting on the camera. Still the same thing.

Frank

[Frank Aalbers]

Hi AJ ! The example you posted can be easily fixed using horizontal chroma blur. It did NOT work on the edge ghosting I had though.

Frank

[Lauri Kettunen]

<<<-- Originally posted by A. J. deLange : because DCT artifacts are usually blocky resulting in a jagged appearance like the artifacts in this frame grab: -->>>

A.J., For some reason can't reach the site you gave, but yes, I understand your point., and will check the frame as soon as the connection is available. In the mean time --as it a pleasure to talk with somebody who is up to the technical details-- if you look at the figure of the DCT basis functions given in e.g. http://www.cs.cf.ac.uk/Dave/Multimedia/node231.html, do you refer to those functions which are indeed like tiles. Basically, what I had in mind was that in addition to those tiles there are also other kind of basis functions.

[Frank Aalbers]

Hi AJ !

This is your USM grab turned through chroma blur. As you can see it fixed 80% of the problem. I'm sure if I would run this FX on the original footage it will be almost totally gone.

http://home.comcast.net/~chalbers/chblur.jpg

Frank

[A. J. deLange]

This last round of comments strengthens my suspicions (but does not prove!) that it's luminance leakage.

Lauri - yes, the "basis functions" illustrated in the site you referenced are exactly what I have in mind. The images our cameras capture are broken into square groups of pixels and then each of the 64 basis functions are overlayed and the "dot product" computed (actually that's the way JPEG works - there is another twist in video having to do with the two fields). The dot products are then quantized with the dot products from the upper left hand corner being assigned the most bits and the ones from the lower right being assingned the least. If the picture is busy (like my pinwheels) then the lower right corner dot products are significant but as there is a limited bit rate available they must be quantized coarsely or discarded altogether. When the image is reconstructed fine detail is lost as a consequence of this so that the tile involved is more regular in tone than the original. In the extreme case it would be of uniform tone with that tone being the average over the original (this would be the case if only the upper left corner dot product were retained). In these cases the edges of the tiles become perceptible (especially if you are looking for them) and that's why the DCT artifacting looks "edgy".

As to the image not being available: the "server" here is an old laptop kluged up to get around my ISPs schemes to make serving impossible from home accounts. Please be patient and try again (my router hung during the night.)

[Lauri Kettunen]

<<<-- Originally posted by A. J. deLange : This last round of comments strengthens my suspicions (but does not prove!) that it's luminance leakage. -->>>

A.J. --Say one creates a frame with Photoshop which has only two colors, say the left side is light yellow and the right side is its opposite/complementary color in RGB space. Then, one exports this kind of stationary footage to DV-tape and captures it back. Now, the question is, will the border between the two colors be precise/sharp, and if not, what will that prove in your view? (You see my point, I'm thinking of some simple test which would reveal the origin of Frank's problem.)

The preliminary test idea explained above basically converts the compression issue to a 1D-problem simplifying things. What I still can't quite get in your explanation is, how are you going to express the abrupt jump on the border between the colors (in 1D) once you have suppressed the basis functions related to the higher frequencies? Of course, if there were a basis function with a jump, then everything would be easy, but aren't all the basis functions cosine terms, see http://dynamo.ecn.purdue.edu/~ace/jpeg-tut/jpgdct1.html.

[A. J. deLange]

Lauri,

If they are yellow and blue I'm not sure what the border would look like but if they are magenta and green there will be a luminance error (dark space) between them. To see this turn on the color bars on your camera and record a few cm of tape. Now play back and capture or just play back to a monitor. You will notice a dark region between the magenta and green color bars. This is caused by the luminance leakage which I suspect is what's behind the phenomenon we are trying to understand. Neither the color bar pattern nor Frank's frame have much high frequency content so there is no need for the DCT algorithms to compress very much, high frequency basis function dot products will only be large in blocks (tiles) that span the boundary between bars, the algorithm will not have to throw out bits and so no DCT artifacts should be seen. The only bar pair that sould show the dark band is the magenta/green pair. As the other bar pairs have equally sharp transitions (and equally strong high frequency components) but do not show any error I call that proof that the magenta/green error is not from the DCT. Further, Poynton clearly states that it is from luminance leakage. This seems to me to be similar to the situation in Franks frame but it could be something else. I just don't think it's DCT related. Cheers, A.J.

[Lauri Kettunen]

A.J., Ok, I see now what you mean.

[Adam Bowman]

I'm surprised someone has brought up the haloing effects of the sharpening processing in the Xl2, because they're so very fine!

Compared to the XL1 I've been used to they're almost non-existent.

I have a technique which may help reduce the halo artifacts you've highlighted, you can read about it here: http://www.bargus.org/articles.html

But I like I said before, the halos are very fine on footage from the XL2 and it's probably not worth the effort (it may also introduce more noise than is acceptable).

Adam Bowman

[Frank Aalbers]

<<<-- Originally posted by Adam Bowman : I'm surprised someone has brought up the haloing effects of the sharpening processing in the Xl2, because they're so very fine!

Adam Bowman -->>>

But if it's a sharpening effect in XL2 , how come it's still there when I change the XL2 sharpeness to minimum. It even doesn't change a bit !

Frank

[Adam Bowman]

That's strange, maybe something odd about the sharpening method in-camera.

It would be maybe helpful if you posted say, some shots of a resolution chart or something that highlights the halos well, with varying degrees of in-camera sharpening. Just so we can see exactly what the the sharpening setting seems to change.

Adam Bowman

[Andre De Clercq]

What we see in Frank's pic is just the good(?) old edge enhancement, not at all a DCT related issue. Some cams only reduce vertical edge enhancement (vertical filtering) when lowering the "sharpness" setting and keep horizontal enhacement constant.

[A. J. deLange]

If it were sharpening it would look like this: http://www.wetnewf.org/ghostshp.jpg i.e. you'd see ghosting on either side of any edge, not just the left side of edges with particular color changes.

[Andre De Clercq]

Luma sharpening filters (if well designed) are meant to enhance transients in a symmetrical way indeed. However, if an object has luminance steps (amount and steepness...the second derivative of the luma change is often used in luma sharpening algorithms) containing different derivative values in the horizontal direction one will get different edge sharpening levels (pre and overshoot). The left side in the Frank's pic is the shaded part and gets the most contouring. F.Y.I. color transient enhancers (CTI...) are another story, and based on totally different processing principles