View Full Version : determine the lightest pixel?
06 June 2008, 06:52 PM
sounds simple, but i never found that. how can i get the absolute value of the highest level and lowest level pixel in an image? i can guestimate the rough thing using ctrl+alt+l to see the levels but i cannot see which pixel carries them and what is the exact value of each pixels brightness.
thanks a ton,
06 June 2008, 01:00 AM
try using the curves editor. If you open it up with an image open and hit the alt,crtl,shift,command key(one of those I can't remember exactly which one, you will have to expirement on that) it will bring up an eyedroper tool where you can go to the lightest/darkest value on the image, and it will give you a value within the curve editor diolog box so as you can pinpoint exactly the lightest or darkest pixil and its value.
Hope thats what your looking for
06 June 2008, 01:12 AM
bad info post edited
06 June 2008, 07:07 AM
thanks for answering.
as far as i understand it, the eyedroppers in the curves are for remapping, not for
measurement. also tried the key combinations you mentioned, but neither of the ones i commonly use worked...
i just found this, i think this mostly accurate:
basically, one uses the image adjust -> treshold, moves the slider to the right and left, so one gets the lightest and darkest areas encircled, and then, using the color sampler, with CAPS Lock on, one can shift click on these areas and the colour will be displayed in the info box of photoshop.
but still, it is where you click to measure.
i am still looking for the exact, absolute measurement of the lightest and darkest pixel of an image.
06 June 2008, 09:22 AM
sorry martin, I misunderstood what you were asking. I thought you were after something similar to colour channels technique that is in the link that you provided with the info palette giving the ummm info.
Its a good question and I would like to know the answer to it too, if you find the answer I hope you post it if someone else dosent give the answer on this post.
Hope I didn't waste to much of your time with my bum stear
06 June 2008, 12:10 PM
trambott thankyou for the levels vs curves explination many people say use curves but never say why now i get it and will make the switch
06 June 2008, 01:30 PM
No worries Kraal, pm me if you have any questions on how to operate the tool.
06 June 2008, 03:47 PM
"Hope I didn't waste to much of your time with my bum stear"
-> no reason to think that, you took the effort.
06 June 2008, 10:26 PM
I found a video the other day where someone was talking about using threshold on an adjustment layer to find the lightest and darkest pixels in an image.
Here's the link: http://www.totaltraining.com/prod/outlines/adobe/aftereffect7pro_adv.asp
It will take you to a demo of an After Effects tutorial, but when you clink the link to preview the first chapter, it takes you to a PS CS2 video instead.
Not sure if this is what you were looking for, but the guy does find the absolute lightest and darkest pixels in the image, and explains how to do it really easy. Although he does use the info pallette to get the color information.
06 June 2008, 01:24 PM
On a side note I would advise on not using the levels editor to manipulate an image. Use the curves editor everytime. It can do exactly the same as the levels editor and a hell of lot more.
The reason that I believe this is because the level editor destroys colour infomatioin, where as the curves editor just mearly changes the colour and it can be adjusted at a later time if needed. Sorry, I know this is off-topic, but what's this about Levels destroying colour information? Do you mean clipping? If so, that's applicable to both Levels as well as Curves because they will both "destroy" pixel information if used wrongly.
Also, are you implying that the Curves function is non-destructive? Because it's not. It's only non-destructive when used as an Adjustment Layer, but then Levels does that too.
Back on topic though, here's an imperfect way that I just came up with to hunt down the brightest and darkest pixel locations:
1. First check the Histogram. If the graph has spikes at the far left and far right, then your darkest and lightest pixels are already fully black and fully white and you can skip to step 4.
2. Desaturate the image (Ctrl+Shift+U)
3. Run an Auto Levels (Ctrl+Shift+L). This will make the darkest pixels fully black (0,0,0) and the lightest pixels fully white (255,255,255)
4. Run a Select -> Color Range and sample white to select the brightest pixels.
5. Run the Select Color Range again and sample black for the darkest pixels.
I'm sure there's a better way, but this works pretty well and doesn't take much time.
Edit: Read the posts about Threshold and that works too. Run the Auto Levels, then run Threshold and set the values to 1 for the darkest pixels and then to 255 for the ligthest pixels.
06 June 2008, 06:58 PM
I stand corrected, for some reason I thought that curves automatically worked like they do when used in an adjustment layer. Its been awhile since using them. I'll edit out my lies.
06 June 2008, 06:58 PM
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