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calculations beginner (selection beginner)

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  #1  
Old 09-25-2009, 07:05 PM
hansolo hansolo is offline
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calculations beginner (selection beginner)

i aplogise for the cross posting and i would delete last post if i new how too
as it was very confusing

hi i posted a similar problem but my explanation was babble and confusing so i hope this can explain my problem better, this is image copied from the web. would lto select the tower by using caculations. as seen calculations can select areas which are of a similar colour .

calculations to my knowlege uses to channels either the red or the green and blue channel to make a selection.

on this iimage, the sky is blue, green so would i use the blue channel please if any can offer any advice thank u
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  #2  
Old 09-25-2009, 11:08 PM
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mistermonday mistermonday is offline
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Re: calculations begginer (selection begginer)

Hansolo, there are dozen tools/ways to make select selections. You have some very well defined edges in this image and Calculations would not be my 1st choice. Is there any particular reason you are using the Calculations command.
The Calc command combines two channels to produce a 3rd new channel. That new channel is usually designed to have greater contrast than either of the two that generated it. That increase contrast channel can be turned into a mask from which a selection can be made but often the channel needs additional work to turn it into a usable mask. For images with targetable edges, colors, tones, or patterns, many of the other tools work more quickly than the Calculations command.
However, the Calculations command is very powerful and can be a very useful tool in ones workflow so I would encourage you to experiment with it.
Regards, Murray
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Old 09-26-2009, 05:46 AM
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Der_W Der_W is offline
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Re: calculations begginer (selection begginer)

For this image I'd probably not even use the calculations command but the red channel, but if you really want to use it, you can try the (inverted) blue channel added to the red channel with an offset of -160 and a scale of 1.
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Old 09-26-2009, 09:19 AM
hansolo hansolo is offline
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Re: calculations begginer (selection begginer)

yep there is a reason im trying to practice with objects which are easier to select but my goal is to have a better understanding of calculations so i can select softer edges like hair and fur, as when i use the pen tool and bur the object edges work fine on large objects, but not when selecting human hair it doesnt work ,

so i thought this would be good place to start .
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Old 09-26-2009, 09:53 AM
hansolo hansolo is offline
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Re: calculations begginer (selection begginer)

what happens when i invert the blue channel does it make a selection of eveything in the image doesnt have the colour blue

say as an exmple say the

sky blue

grass green

if i inverted the blue channel would the green grass be selected

sorry if my qeustion sounds silly,.
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Old 09-26-2009, 10:01 AM
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Der_W Der_W is offline
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Re: calculations begginer (selection begginer)

Just take a look at the individual channels. Inverting the blue channel does simply mean, that the density of the blue channel gets selected. If your sky is completely blue (0/0/255) and your grass is completely green (0/255/0), inverting the blue channel would cause a selection of the grass. The modes are exactly (plus Add & Subtract) the blend modes for your layers, so you can use them just like this.
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Old 09-26-2009, 03:11 PM
hansolo hansolo is offline
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Re: calculations beginner (selection beginner)

ok thanks alot that doe make sense

so i am guessing that rgb (red green blue) 255 is the top amount of saturated colour


say for example the

channel 1 is red
channel 2 is blue ( add)

in the blending mode there is only one drop down box
add and subtract are only available on the second channel , so how do i know which
channel im adding or subtracting from.

cheers things making a little more sense . thank you for you patience
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Old 09-26-2009, 05:01 PM
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Re: calculations beginner (selection beginner)

Having a 100% saturated color means that in any case at least one and a maximum of two of the RGB values will be 255 (simply try it out with the color picker, choose a Saturation ("S") of 100% and scroll down the hue wheel).
Your channel two is always the one that's going to react as the base channel. Look at it like it was the layers palette, where channel two is on the bottom and you're throwing channel one on top of it with any blend mode you want (again you can try it out by looking at the channels and blending two different channels in "Normal" mode with 100% opacity, the result will be channel one).
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  #9  
Old 09-27-2009, 09:59 AM
hansolo hansolo is offline
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Re: calculations beginner (selection beginner)

right just to clarify or hopefully repeat what u have said

thanks again channel one is the top channel and channel two is the bottom channel , so if i have 100 percent opacity in the selection box the top channel will only be used , right that does make sense, as an example if i use a blending mode overlay both channels will be used right ok that also makes sense thanks very much.

so if i use the blending mode add
does it add the bottom channel( extra channel from the bottom channel )or from the top
channel

cheers Nice1
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