SAW gradients at various Gammas

[Randy Strome]

I started a new post here so as not to further hijack anyone elses post with matters that many may wish to avoid. For those who have interest, here are some photographs taken of a the waveform produced by the EX1 SAW Gradient as viewed on a TV logic monitor connected via HDSDI.

**apologies in advance for the perspective issues - these were hand shot**

The color temp was set to 3200 (color temp does effect the curve)

Gain at 0 on row 1 and 2.

Row 1, left to right: STD1,2,3,4

Row 2, left to right: Cine1,2,3,4

Row 3 left to right: STD3 Slope at +99, STD3 Slope at -99, STD3 Gain at -3, STD 3 Gain at +3

I hope this is helpful, and I would love to hear some comments on the issue of the mapping of such a wide ranche of brights to a flat value. This baffles me. Possibly the unadjusted SAW pattern includes an area of straight white just to the right of the gradient???

http://www.dvinfo.net/gallery/showimage.php?i=860&c=3

[Leonard Levy]

OK so what is a saw gradient anyway?

[Craig Seeman]

I would love to have a saw gradient as test chart to point camera at and tweak to see how various settings impact the gamma curve, etc.

Can you actually adjust PP settings and see how the curve changes?

You don't get to see how this is handled through the optical system though.

[Sebastien Thomas]

Just hope this will help you to understand :

http://www.lecentre.net/blog/pmw-ex1/gamma

Be carfull as pictures are with a +3 Gain. I hope to put picture at 0 gain tonight.

Also check the thread on hightlight clipping and picture profile receipe. They are full of information about that and how gamma curves change the picture look.

[Randy Strome]

Originally Posted by Craig Seeman

I would love to have a saw gradient as test chart to point camera at and tweak to see how various settings impact the gamma curve, etc.

Can you actually adjust PP settings and see how the curve changes?

Yes. Every change made in PP settings live updates the curve on the monitor, just as if you were pulling a curve in an editing program.

There are some very interesting thing to be learned here. For instance, black is not at zero on the waveform until PP "black" is set to -4. Also, it is not hard to "kink" the curves with combinations of settings, so operating without a visual reference might be ill advised.

[Randy Strome]

Originally Posted by Leonard Levy

OK so what is a saw gradient anyway?

This is brand new to me. I learned of its presence from Sebastien's post.

What makes sense to me (and what it looks like), is that the camera generates a constant black to white gradient (ie 0-255) that would mimic the sensor having recieved those values prior to processing. The camera settings are applied to this gradient before it shows on the LCD, HDSDI monitor, or SXS file. So, what you get when you view it on a wavefrom monitor is the actual curve that the camera is applying.

In short, these are your processing curves.

That is my understanding. Slam me if I am wrong. I want to know.

[Bill Ravens]

Great work, Randy!!!!
I'm off to look at SAW.

[Michael H. Stevens]

Originally Posted by Leonard Levy

OK so what is a saw gradient anyway?

Please respond someone? What is SAW?

[Greg Boston]

Originally Posted by Michael H. Stevens

Please respond someone? What is SAW?

Well it depends. If you are in the military, it stands for Squad Automatic Weapon.

If you were to view the SAW pattern in video on an oscilloscope, the black to white gradient would show as a SAWTOOTH type waveform.

-gb-

[Bill Ravens]

Well, after some initial runs thru HDRack, a few things are apparent to me.
1-I'm running BLACK way too low. This is confirmed by the noise problems I'm seeing in my shadows. The WFM shows BLACK should be at -3 to -4 for max dynamic range and no more.

2-Running at a gain of -3dB limits the max IRE to no greater than 90%. Running at a gain of 0dB allows max IRE to go up to 100% in the Std mode. Cine modes, of course, are restricted to max IRE of ~80%. So, if one is wanting max dynamic range, don't run at -3dB.

3-As someone said earlier, there is a definite hard knee at the top of the gamma curve. The knee can be tailored to smooth the hard corner transition.When operating with this hard knee, watch out for clipping, it can be disastrous. Tailoring the knee for a smooth transition to hard clip can have undesireable effects on the knee itself.

This is a way cool discovery. Mucho thanx to "prune" at lecentre and to Randy for posting.

[Sebastien Thomas]

Originally Posted by Randy Strome

This is brand new to me. I learned of its presence from Sebastien's post.

What makes sense to me (and what it looks like), is that the camera generates a constant black to white gradient (ie 0-255) that would mimic the sensor having recieved those values prior to processing. The camera settings are applied to this gradient before it shows on the LCD, HDSDI monitor, or SXS file. So, what you get when you view it on a wavefrom monitor is the actual curve that the camera is applying.

In short, these are your processing curves.

That is my understanding. Slam me if I am wrong. I want to know.

The SAW should work the same way the color bars do. It should effectively simulate what the sensor view.
I don't know for the EX1, but I worked a lot with Sony F900, and if you see the internal architecture diagram (it may exist for EX1 also ?) you will see that :
1) gamma curve is the first thing applied, with knee function
2) then detail, white..
2) matrix is the last, with color correction, as long as I remeber

Of course, before the gamma, you have the shading options, which compensate the levels directly out from the sensor.

What you learn here is that matrix don't change the way the camcorder get the image, only how it save it. this means it is equal as doing it in post. So try to get balanced colors while shooting, then grade in post.

you see, SAW can teach a lot :)

and yes, if you look at the parade on my screengrab, you'll see the 3 toothsaw, where the name comes from.

[Greg Boston]

Originally Posted by Sebastien Thomas

matrix don't change the way the camcorder get the image, only how it save it.

The custom matrix adjustments can be thought of as an artist's pallete with a RED, GREEN, BLUE choice. Matrix is the equivalent of the artist dipping their brush in one well, and mixing it into the other to alter the color output from that well.

Both analog and digital SAW waveforms are on the F350, but they are in the service menu. The XDCAM SD cameras offer the SAW pattern in the user menus.

-gb-

[Randy Strome]

Thanks to all of you for your thought and effort on this. Just to clarify, do you envision the unaltered input "curve" of the SAW gradient to be what I have represented by the escalating straight line in the link below? I have set it over the output curve for Cine2 just for reference.

Thanks again,
Randy

http://www.dvinfo.net/gallery/showimage.php?i=861&c=3

[Bill Ravens]

no.
a linear line isn't consistent with why the gamma exists. unlerss you plot on a log scale, which you're not.
gamma exists because the human eye doesn't see linearly. a gamma curve also represents the way a sensor(ccd or cmos) "sees" light. typically, a sensor sees the inverse of the way an eye sees. when the image is presented on a monitor, a "reverse" gamma is applied to make the way a sensor sees consistent with the way a human eye sees. there is no "linear" involved and really doesn't represent anything.

in mathematical terms, gamma is really an exponent. it represents a geometric value rather than a linear value. In very fundamental terms, there's 3 qualities a gamma curve has.
1-the clip at the foot
2-the clip at the head
3-the camber of the knee.(which can be related to the slope at the shadow, mid, and hi-lights), which is the geometric progression.

So, a sensor sees in the way you could envision a concave gamma curve would look. The eye sees in the opposite way, as a convex gamma curve. The "gamma" curve you apply is the correction to the sensor characteristics to match the human eye characteristic. Of course, playing one against the other allows certain "corrections" to the displayed image that enhances the image but doesn't represent the true inverse of what the camera sensor captured. In practice, a camera sensor is much more sensitive to changes in highlights than changes in shadows. Hence, more data is captured in the highlights than the shadows. Black stretch is an attempt to correct this non-linearity in the way a sensor sees light.

I suppose if you added one to the other, a true inverse added to the original gamma, would produce a linear line, but, it is rather meaningless except to denote a true inverse, or how much the vector sum varies from a true inverse vector sum.

[Randy Strome]

Thanks Bill for the thorough answer. It will take me some time to digest.

Maybe I can simplify my question to get at the root of why I am asking. Do you envision the digitally produced test image (prior to any curves, etc being applied) to have an prolonged area of straight white just to the right of the gradient itself, or is the flat spot on each of the curves actually representing a portion of the initial gradient itself now mapped to a single value?

Thanks as always