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  1. #31
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    Thanks for the feedback guys - so glad you like the site. PelleTm, the idea of a glossary is great, I've started work on one, and I'm also planning some additional diagrams. I'll post an alert here as I add them.

    If any of you come across anything that seems confusing on the site, or needing more explanation, or just plain wrong, PLEASE feel free to post your questions here.


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  3. #32
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    There is something I observed today and you may found this completely obvious but I got intrigued for a while. It was late afternoon, really dark and wet from the rain. There were some red lights from the cars that were reflected by the sidewalk. However the edges of these reflections were affected by dark blue light and went into purple while the core of the red light didn't "sink" into the sidewalk. I found photo which shows very similar thing.

    Attachment 312875

    So to me the centre light rays that straight into the eye are more dense, but on the edge there is more diffused light and lack of red is complimented with blue. Those two colours then mix in the eye and as a result I see purple . Is it good thinking?
    Last edited by Farvus; February 27th, 2008 at 04:18 PM.

  4. #33
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    Quote Originally Posted by Farvus View Post
    So to me the centre light rays that straight into the eye are more dense, but on the edge there is more diffused light and lack of red is complimented with blue. Those two colours then mix in the eye and as a result I see purple . Is it good thinking?
    This is 'mixing paint thinking' not 'physics of light thinking' Farvus.....I think.
    My guess is that the longer wavelengths are not absorbed as much at these angles from your eye and are therefore the ones you see 'haloing' the edges. But this is just a guess and probably just as wrong. The fact we see it in the photo means it is not a subjective physical effect produced by the retina at least.
    From Gegarin's point of view
    http://www.chrisbennettartist.co.uk/

  5. #34
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    Hi Briggsy

    I have a question about Photoshop & light, but first- thanks a million for putting the site together. A truly huge undertaking both in the research for and the creation of the site. It's an amazing resource that I'm already referencing constantly to answer questions.

    My question is maybe slightly off-topic, but I thought I'd post it anyway. I noticed on the page "Effects of Colored Illumination" that the "Multiply" blend mode in Photoshop suggests the effect of colored illumination. Do any of the other blend modes have similar suggestions of the physics of light?

    Specifically- for me a sort of Photoshop holy grail would be a way to have a base layer with the local colors for a given object, with a layer on top with a single color for shadows, and another layer with a single color for diffuse light, such that the local colors on the base layer could be changed and the "shading" (light & dark) would remain consistent visually. In other words, you could change the local colors on the base layer, and the relative shift in dark & light produced by the two upper layers would remain the same regardless of the chroma and value of the colors on the base level. I understand this is somewhat dubious in terms of the physics, and might be impossible given what Photoshop has to offer, but it would be very useful for me in a lot of situations.

    I've done a fair bit of experimenting to no avail. "Multiply" works acceptably well for the shadow layer as long as the shadow color is almost totally neutral- something I find undesirable as in some situations I like to shift the color of the shadows slightly (towards blue or brown usually). I've yet to find anything that works well for the light layer across a wide range of base local colors. "Screen" seems to come closest, but the relative shift in lightness is much greater for darker base colors, so that the base color can't be changed freely.

    In general though, I'm also just curious how blending modes relate to the physics of light...

    Tim

  6. #35
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    Without being there I can really only guess too, but if you had an uneven wet surface, then each reflection would be centred on the point where a smooth surface would be at just the right angle to bounce light from the light source to your eye. At that point, most microfacets on your uneven surface might reflect light from some point within the area of light source, but moving out from that point, fewer and fewer microfacets would do so, and more and more would reflect the dimmer light from the sky around the light source.

    Wherever these two components were not visibly distinguishable, the resulting stimulus would be the result of additive mixing. I'm not quite sure from your description whether the sky was actually bluish, or was a dull grey that appeared bluish by contrast with the red lights. Mixing with bluish light could easily make a purple, but mixing with dim white light could also account for a more subtle shift towards pink.

    The term "mixing in the eye" is widely used for situations like this, but may give the impression that the process is more subjective than it really is. What we are really talking about is mixing of physical stimuli that for one reason or another can not be distinguished separately. Many if not all of these effects can be photographed.

    P.S. Tim

    Your question came up as I was about to post this answer. Could you post an example of your best attempt so far, and maybe email me the psd file (djcbriggs at the dreaded gmail.com)? Glad you like the site!

    David

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  8. #36
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    Tim, I'm not totally sure if this is what you were after, but you can use Adjustments on the colour layers to vary both the colour of the object and the colour of the ambient illumination in any way you want.


    The psd file is here:

    http://djcbriggs.googlepages.com/dose-3.psd

  9. #37
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    briggsy@ashtons - Thanks for reply. I just wasn't entirely sure it it was something about light or that it didn't loose it's hue but mixed with surrounding colors in the eye. You explained it more thoroughly than me. The sky was rather cool gray but everything around appeared a blue like in late afternoon. I couldn't say right now if it was contrast to red lights or more general phenomenon.

    Today I realised that color brilliance mentioned in Loomis book is something a bit different than saturation and it all suddenly makes sense with all these colors in the shadow. Going back to basics definately helps .

  10. #38
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    Blending modes in Photoshop

    To answer dose's main question (as best I can at the moment), the three blending modes that I've found to more or less emulate effects of light are multiply, screen and normal.

    In multiply mode the relative brightnesses of the R,G and B components of the two layers are multiplied together, e.g. if the relative brightnesses of the two components are both 0.5 (normalized to 1.0), the resulting brightness will be 0.25. Multiply mode accurately emulates the effects of subtractive mixing, including the effect of coloured illumination. The only proviso here is that if you employ fully saturated colours, you will be emulating light of greater colour purity than comes from most light sources, or is reflected/transmitted from ANY actual coloured surfaces or filters respectively, so you may get some unrealistic results.

    In screen mode, the differences between each component and one are multiplied together and then subtracted from one, e.g. if the relative brightnesses of the two components are both 0.5, the resulting brightness will be 0.75. The results of mixing in screen mode often resemble those of additive mixing qualitatively, though generally not quantitatively. Because of the nonlinear response of our visual system to light energy, a light that has twice the energy of another light will look brighter by a factor of, not two, but between the cube root and the square root of two, i.e. around 1.37. However, adding two identical lights together in screen mode results in a light that is brighter by a factor of nearly two for dim lights, reducing progressively to one (i.e. no increase) for bright lights. I assume that some sort of flattening of response of this sort is inevitable, given that brightness has a finite range in RGB space, unlike its open-ended range in the real world. In any case, mixing of bright lights in screen mode can give quite different results to additive mixing. For example, yellow and magenta at maximum brightness mix to give pure white, not the reddish white that would result from additive mixing (and which you in fact get if you mix them at 50% brightness).

    I used layers in screen mode to emulate additive mixing in many of the diagrams and interactive animations on the Dimensions of Colour site. For this animation demonstrating additive mixing, you can see the individual layers that I used to make it (against a black background) if you slide any two of the three sliders to the left:

    http://www.huevaluechroma.com/pics/4-5.swf

    Finally, layers in normal mode (at low opacity) can be used to emulate the effects of atmospheric fog or turbid water.

    I should add that I make no claims to be any sort of Photoshop expert - the little I know I've picked up by having to make the illustrations for the site. If anyone with some real experience has anything to add, please jump in and help out.

    When I was first trying to find out about the various blending modes I found that a lot of what I read was not very specific on exactly (i.e. mathematically) what each blending mode does. I found this link to be helpful:
    http://www.simpelfilter.de/en/grundlagen/mixmods.html

  11. #39
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    I grabbed a bit of time over the Easter break to reorganize a few pages (although the re- part could be a euphemism), fix some typos (and add others), and add a few more diagrams (on CMY and CMYK, and demonstrating advancing and receding colours, below).

    By the way, dose says the stuff I just posted was a big help but he's a bit too busy to drop by and say thank you.

  12. #40
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    Here's another new diagram explaining an efficient way of painting a complex multicoloured surface turning out of a light source. Explanation here.

  13. #41
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    Just stopping in to say that I think Briggsy should win a prize for all his work...

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  15. #42
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    Hey Dose, glad you made it back! I'm not looking for a prize, but it would be great to get a bit more in the way of specific feedback, especially on my stuff about digital painting/Photoshop. Otherwise, since I'm entirely self-trained and relatively inexperienced at the latter, it's hard for me to know if what I'm posting on those subjects is actually helpful, or just common knowledge.

  16. #43
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    David solved some issues of chroma for me. The chart of chroma reduction is an excellent concept. Thank you.

  17. #44
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    O...M...G... I'm guessing there aren't too many people walking around who can claim to have solved some issues of chroma for Graydon Parrish. Thank you! (Looks like I won my prize after all...)

  18. #45
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    Quote Originally Posted by briggsy@ashtons View Post
    I'm not looking for a prize...
    That's probably why you deserve one!

    Did you get my PM in response to yours? I wrote a bit of feedback in there. I can post it here if you think it would be valuable to the discussion.

    I've been giving some thought lately to the placement of the specular highlight. I was taught correctly that the highlight is not at the point of the form closest to the light (which, as you say, is often taught incorrectly). However, I was also taught to largely ignore specular highlights, as for the most part they undermine the illusion of form on a 2D surface. There are, however, times when it useful to put them in when it enhances the feeling of form or conveys critical info about a surface's material. Any thoughts on this?

    Tim

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