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kellybellis
kellybellis
Known Participant
July 12, 2018
Answered

How to edit 4-band imagery using layers?

  • July 12, 2018
  • 2 replies
  • 3438 views

RE: PS CS6 64-bit

This discussion is regarding geospatial 4-band imagery where the alpha channel (4th band) is near infrared.

During work session, all 4 channels are active with only RGB visible.

Edits made to the flattened (i.e., no layers, background), such as cloning operations appear to include the active, but not visible 4th band. However, there arises the issue of when using layers, edits are made and then the document is flattened, only the RGB channels reflect the edits with the 4th band unchanged.

How can one edit all 4 bands using layers?

This topic has been closed for replies.
Correct answer c.pfaffenbichler

Might

• Image > Mode > Multichannel

• Image > Mode > CMYK

not allow using the fourth Channel in Layers and then creating a copy and using

• Image > Mode > Multichannel

• Image > Mode > RGB

get the result back to a flattened RGB + Channel?

Edit: The layered CMYK image would naturally look hugely different than intended, though, but the resulting RGB should get back to that …

2 replies

kellybellis
kellybellisAuthor
Known Participant
July 13, 2018

I've spent the past week making errors like the one above because I was using layers and masks to fix problems with seam mismatches and drooping bridges before flattening and saving, all the time thinking that everything looked good. I've yet to find a clearly written narrative of the correct procedure. My instructors aren't Photoshop experts by a long shot and had only one means (cloning flattened source pixels to another flattened target document) of achieving the needed fixes, but cloning not-as-bad pixels in lieu of worse pixels becomes particularly disagreeable when the projections are impacted by changes in the terrain surface. But at least their cloning method was somehow able to adequately handle the 4th band/ channel.  Transforming 8-bit RGB to CMYK (via Multichannel) has been mentioned and it might work, however, there's uneasiness about 1) how lossy things become going from RGB to CMYK and back to RGB, and 2) how or if color space plays a significant role during such color transformations.

c.pfaffenbichler
Community Expert
c.pfaffenbichlerCommunity Expert
Community Expert
July 13, 2018

1) how lossy things become going from RGB to CMYK and back to RGB,

Not RGB to CMYK, but RGB to Multichannel to CMYK, and not CMYK to RGB, but CMYK to Multichannel to RGB.

and 2) how or if color space plays a significant role during such color transformations.

If the Multichannel steps are honored there is no color transformation, though if the RGB image does not have the RGB Working Space you may have to assign its original Profile again at the end. 

Edit: The transformation I did in CMYK (rotate by 180˚) is naturally meaningless and by rotating 180˚ back in RGB the effect is negated, but it should illustrate that the Channels themselves have not been changed by the round-trip.

    kellybellis
    kellybellisAuthor
    Known Participant
    July 13, 2018

    1) Good distinction, thank you for emphasizing this.

    2) I'm not clear as to the means of making that determination

    From the Information, Advanced tab:

    Mylenium
    Mylenium
    Legend
    July 13, 2018

    Propagate your channel data to actual layer data using Image --> Calculations and Apply Image and use blending modes to create your overlays, then simply take the reverse route to create new channels.

    Mylenium

      kellybellis
      kellybellisAuthor
      Known Participant
      July 13, 2018

      I think I understand a little bit:

      1) create a new layer of editable data from/ including the Alpha 1 channel (near infrared); however,

      I don't understand: a) which channel data you intend to perform the calculation on; e.g., R,G, B, and Alpha1, or only the 4th band (alpha1); nor b) which blending method; e.g. add, multiply, normal, etc., you're saying should be used in the calculation. In any case, the possible results don't include New Layer; i.e., only New Document, New Channel, and Selection.

      I'm totally guessing in the above example of taking the Alpha 1 channel from the original flattened image and Blending: Normal. In this case, I added a new layer, Layer 1, before performing the calculation as it seemed required, but I'm unsure about this also.

      If this is even close to the correct work flow, then the resultant new channel, Alpha 2, is to be used for step 2), Apply Image that actually gets the channel data onto Layer 1.

       

      Once again, I'm unclear which blending mode you intended to be used in the Apply Image procedure; e.g., Normal.

      I realize, I'm only half way following through your sentence, but also realize I may/ probably have already messed up; please correct my understanding. E.g., is it necessary that Layer 1 be created?

      kellybellis
      kellybellisAuthor
      Known Participant
      July 13, 2018

      At what point is Black returned to Alpha 1?

      Its name is now »Black«, but the content is the same as the original fourth Channel or its edited content depending on what you did in the CMYK file.

      It's not clear what you mean by 'not allowing the fourth channel in Layers' as it is the 4th channel that has failed to be edited in the first place.

      It was just a question along the line of: »Might X not enable one to do Y?«, but it may have been unnecessarily confusing.


      Actually, the content has changed from it's original reddish cast to grayscale and was why I asked if maybe the bit about 'not allowing' could have explained this.

      Aside from that issue, the color fidelity appears to have been fully maintained throughout the procedure of going from and to RGB.

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