the mystery of the disappearing stretch

toryglen-boy (Duncan) · #1 12-11-2009, 09:52 AM

Guys

Looking for some more imaging guru's info.

When i take shots with the DSLR, and stack them, when i open them up in CS4, the histogram looks ok, and the image looks as bright as it did.

However, when i stack images from the QHY8 it looks great in DSS, but then when i open them up in CS4, the image looks almost black, like a 2 second sub with no information, and the histogram is like a needle, tall and thin, and doesnt cover the entire range, using curves etc to lighten it, doesnt help

what gives?

Using Maxim DL 4 and QHY8

Octane (Humayun) · #2 12-11-2009, 10:27 AM

Probably something to do with signed and unsigned 16-bit formats.

That is how the images appear in Photoshop after being saved as PSD's in IRIS.

All you have to do is coax the details out with subtle and numerous curves and levels iterations.

Regards,
Humayun

multiweb (Marc) · #3 12-11-2009, 10:29 AM

That's because you need to DDP and save it as a scaled 16bits file prior to opening it in PS. Make sure you don't clip the end of the histogram though. More is better than not enough. You can always further tighten in PS levels/curves.

toryglen-boy (Duncan) · #4 12-11-2009, 10:30 AM

Quote:

Originally Posted by Octane

Probably something to do with signed and unsigned 16-bit formats.

That is how the images appear in Photoshop after being saved as PSD's in IRIS.

All you have to do is coax the details out with subtle and numerous curves and levels iterations.

Regards,
Humayun

H, i was reading that reply, and all i was thinking was ...

WTF?

seriously, if the histogram is almost a straight line, in one point, then is there anything to coax out of it, at all ?

Octane (Humayun) · #5 12-11-2009, 11:04 AM

How big is the file? Email it to me and I'll have a play tonight.

Regards,
Humayun

toryglen-boy (Duncan) · #6 12-11-2009, 11:05 AM

Quote:

Originally Posted by Octane

How big is the file? Email it to me and I'll have a play tonight.

Regards,
Humayun

about 10mb, but i will crop it down.

top man, will be in touch !!

toryglen-boy (Duncan) · #7 12-11-2009, 11:06 AM

Quote:

Originally Posted by multiweb

That's because you need to DDP and save it as a scaled 16bits file prior to opening it in PS. Make sure you don't clip the end of the histogram though. More is better than not enough. You can always further tighten in PS levels/curves.

DDP?

there is no end of the histogram mate, there is a spike in the middle, and thats it !!

RB (Andrew) · #8 12-11-2009, 11:42 AM

Quote:

Originally Posted by toryglen-boy

seriously, if the histogram is almost a straight line, in one point, then is there anything to coax out of it, at all ?

Duncan can you post some screen shots of the native histogram?

It sounds normal and I'll explain why.

When we stack, most of the info gets compressed into a small section of the overall available histogram, especially since you are using the QHY8 since it has 16 bit depth.
Older DSLRs only have 12 bit per colour channel and the newer ones have 14 bit per colour channel.

The more bit depth an image has, the more the histogram is compressed after stacking.

The spike that you see actually should have info in there, it's just that it takes up only a small portion of the full histogram (since it's a 16 bit camera).

What we do is we use levels first to bring the end points of the histogram in so that the spike gets stretched and eventually that spike becomes a 'bell' curve.

From there, as you know, we do the other necessary curves and other adjustments but avoid any clipping.

I haven't used Maxim DL so I can only speak from my experience with CS3.

Hope that clears it up a little.

I'd be interested in seeing the histogram and also the file.
Unless there's a simple mistake somewhere along the process or a wrong setting, it sounds like what we get when we initially open a stacked image in PShop.

Octane (Humayun) · #9 12-11-2009, 11:51 AM

10 MB is fine.

Email it to hq AT hqphotography DOT com DOT au

Regards,
Humayun

Quote:

Originally Posted by toryglen-boy

about 10mb, but i will crop it down.

top man, will be in touch !!

toryglen-boy (Duncan) · #10 12-11-2009, 11:51 AM

Quote:

Originally Posted by RB

Duncan can you post some screen shots of the native histogram?

It sounds normal and I'll explain why.

When we stack, most of the info gets compressed into a small section of the overall available histogram, especially since you are using the QHY8 since it has 16 bit depth.
Older DSLRs only have 12 bit per colour channel and the newer ones have 14 bit per colour channel.

The more bit depth an image has, the more the histogram is compressed after stacking.

The spike that you see actually should have info in there, it's just that it takes up only a small portion of the full histogram (since it's a 16 bit camera).

What we do is we use levels first to bring the end points of the histogram in so that the spike gets stretched and eventually that spike becomes a 'bell' curve.

From there, as you know, we do the other necessary curves and other adjustments but avoid any clipping.

I haven't used Maxim DL so I can only speak from my experience with CS3.

Hope that clears it up a little.

thanks for that RB, it makes *some* sense to me. am at work just now, but will post something tonight

jase (Jason) · #11 12-11-2009, 12:37 PM

Indeed, the others have nailed it - good advice. You've got more bit depth than what photoshop is expecting hence the 16bit image will initially look black with perhaps a few bright stars presented as dim point sources. As Marc suggests, you can do a DDP stretch and log stretch to fill the bit space prior to opening it up in photoshop, but this is nothing some levels and curves can resolve if you don't want to do any prior stretching. Also, note that while with 16bit data, you've theoretically got 65k levels of brightness, photoshop summarises this from 0 black to 255 white (similar to an 8bit space).

toryglen-boy (Duncan) · #12 12-11-2009, 01:33 PM

Quote:

Originally Posted by jase

indeed, the others have nailed it - good advice. You've got more bit depth than what photoshop is expecting hence the 16bit image will initially look black with perhaps a few bright stars presented as dim point sources. As marc suggests, you can do a ddp stretch and log stretch to fill the bit space prior to opening it up in photoshop, but this is nothing some levels and curves can resolve if you don't want to do any prior stretching. Also, note that while with 16bit data, you've theoretically got 65k levels of brightness, photoshop summarises this from 0 black to 255 white (similar to an 8bit space).

ddp?

multiweb (Marc) · #13 12-11-2009, 02:03 PM

Quote:

Originally Posted by toryglen-boy

DDP?

there is no end of the histogram mate, there is a spike in the middle, and thats it !!

There is but you can't see it. Let's say you have a 16bits camera. The pixel intensity could go from let's say 0 to 65k. If you image something dim you'll get the sky glow at let's say 900, then your signal (faint neb) let's say from 1000 to 3000 then a gap then a few stars up to around 20k then a blank. That's it. So the range of your histogram is from 900 to 20k. Couple of spikes in the middle of the whole range from 0 to 65k. So you need to stretch your data and save it scaled. To do so you make the 900 closer to 0 and the 20k closer to 65k. DDP will do a non linear stretch. Once you're done you save as a TIFF 16bits or other the scaled data and you get your full histogram range in PS. Voila!

toryglen-boy (Duncan) · #14 12-11-2009, 02:10 PM

Quote:

Originally Posted by multiweb

There is but you can't see it. Let's say you have a 16bits camera. The pixel intensity could go from let's say 0 to 65k. If you image something dim you'll get the sky glow at let's say 900, then your signal (faint neb) let's say from 1000 to 3000 then a gap then a few stars up to around 20k then a blank. That's it. So the range of your histogram is from 900 to 20k. Couple of spikes in the middle of the whole range from 0 to 65k. So you need to stretch your data and save it scaled. To do so you make the 900 closer to 0 and the 20k closer to 65k. DDP will do a non linear stretch. Once you're done you save as a TIFF 16bits or other the scaled data and you get your full histogram range in PS. Voila!

Thanks fella, your a trooper for answering my inane drivel over all this, but again, whats DPP?

you shall have your reward in Heaven

jase (Jason) · #15 12-11-2009, 02:11 PM

Quote:

Originally Posted by toryglen-boy

ddp?

http://www.noao.edu/outreach/aop/glossary/ddp.html - basic explanation.

toryglen-boy (Duncan) · #16 13-11-2009, 12:46 PM

ok, so i have been reading up, and DPP comes back as Canon software? How and where can i do a DPP stretch? i am using Maxim DL, and CS4

guys, you are making headway, and i am starting to understand this.

jase (Jason) · #17 13-11-2009, 01:20 PM

In MaximDL, you access the DDP features via the Filter | Digital Development menu item. Auto check the background and mid-level to start then uncheck them. MaximDL will analyse the image and determine the appropriate values. Normally these are quite aggressive so you'll want to tweak accordingly. For the background level, what ever it determines, subtract around 100 to 200 ADU counts from the value to avoid any black clipping. Adjust the mid level based on the how you wish to display the image. My recommendation is to not be too aggressive with the mid-level just stretch the data to fill out the bit space, then bring it into photoshop. Give it a whirl!

toryglen-boy (Duncan) · #18 13-11-2009, 01:32 PM

Quote:

Originally Posted by jase

In MaximDL, you access the DDP features via the Filter | Digital Development menu item. Auto check the background and mid-level to start then uncheck them. MaximDL will analyse the image and determine the appropriate values. Normally these are quite aggressive so you'll want to tweak accordingly. For the background level, what ever it determines, subtract around 100 to 200 ADU counts from the value to avoid any black clipping. Adjust the mid level based on the how you wish to display the image. My recommendation is to not be too aggressive with the mid-level just stretch the data to fill out the bit space, then bring it into photoshop. Give it a whirl!

Jase, you're an absolute toff, and i salute you.

i will check it when i get home, although i dont remember seeing the menu you mention in maxim ... hmm !!

watch this space

TheDecepticon · #19 13-11-2009, 02:09 PM

Why don't you invest in some good literature?

You are obviously very inexperienced at processing, so a good information source is a great help.

Try "Zone Systems". This will teach you how to stretch your image after it is stacked and many other things as well.As the others have said, what is happening is normal, it is up to you to use your processing software to tease out the detail.

toryglen-boy (Duncan) · #20 13-11-2009, 02:19 PM

Quote:

Originally Posted by TheDecepticon

Why don't you invest in some good literature?

You are obviously very inexperienced at processing, so a good information source is a great help.

Try "Zone Systems". This will teach you how to stretch your image after it is stacked and many other things as well.As the others have said, what is happening is normal, it is up to you to use your processing software to tease out the detail.

yeah mate, i am aware i am asking some really dumb questions, been observing since i was 11, but only imaging in the last few months.

i like to think it evens itself out, when i can help people with stuff i know about.

usually IT issues, or guitars !!