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Levels Brightness Sliders: how do you use them?

893 Views 5 Replies Latest reply: Oct 30, 2011 8:09 PM by Kirby Krieger RSS
Kirby Krieger Level 6 Level 6 (11,550 points)
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Oct 29, 2011 12:34 PM

Hi.  I use the Levels Brick on almost every Image I process in Aperture.  I have long known that I can change the what Aperture calls the "Brightness Levels" by using the Brightness Levels Sliders (the inverted triangular handles along the top edge of the Levels Brick Histogram).  (What they do is easier to see than to describe -- a couple slider-slides will show you if you're not familiar.  Here is the User Manual's page on the Levels Brick.)

 

My first question is simple:  do you use these, and if so, under what conditions for what goal?

 

Yesterday I discovered that holding down a modifier key changes the function when sliding the Brightness Levels Sliders.  I am trying to understand how this works, and how it could be useful.

 

Holding down "{Command}" while sliding the Brightness Levels Slider(s) resets the Levels settings and allows one to reposition the half-tone and quarter-tone controls.  (Again, immediately apparent in use -- not worth the words it would take to describe.)  My second question then is, What is this for; when would it be used?

 

Holding down "{Option}" while sliding the Brightness Levels Slider(s) changes not just the Brightness Levels Sliders' positions, but also the Black, the White, the Gray, and the Quarter-tone Levels Sliders.  (Which begs the question: what are these to be called as a family?  I would have called them "Luminance Levels Sliders", but having separate Sliders on the same control for "Luminance" and "Brightness" confounds me.)  What is this for?

 

If I have images that are mostly dark, or mostly light, I have found it useful to shift the half- and quarter-tone controls towards the hump in the histogram.  (Which makes me think that this reposition of the default start positions for the controls should be automated: instead of fixed by equal separation of the luminance gamut, they should be repositioned in congruence with the Image's luminance histogram.  (If I understand this correctly, they are currently positioned accorded to the median points of a whole gamut.  I am suggesting auto-positioning them according to the mean(s) of the selected Image.)

 

My last question is hard.  As with _every_ control in Aperture, I want to know (and strongly feel the right to know) what operation is actually being done (at the pixel level of luminance/channel).  And in particular, what is the difference between the Black, White, Gray, and Quarter-tone Levels Sliders and the Brightness Levels Sliders.  An explication would be both elucidating and massively appreciated. 

 

Thanks. 

MacBook Pro 13, Mac OS X (10.6.8), 8 G / 500 G internal / 5 TB external / NEC 2490 / ColorMunki Pho
  • JohnTheAppleFan Level 3 Level 3 (690 points)

    You have so many highly technical questions, I would recommend one of the following:

     

    http://www.photoshopsupport.com/aperture/books.html

  • léonie Level 8 Level 8 (46,355 points)

    Hello Kirby,

    sofar I refrained from answering your interesting question - I came back to your post expecting to find some brilliant answers by now.

    I am not sure about the level brick you are talking about; when I open the "Adjustments" tab in the Inspector panel and open the levels brick, then I see a selector for channels, with the options "Luminance", "RGB", "Red", "Green", "Blue", no brightness controls.

     

    Assuming you mean these levels-control, I'll head straight for a part of your very hard question.

    My last question is hard.  As with _every_ control in Aperture, I want to know (and strongly feel the right to know) what operation is actually being done (at the pixel level of luminance/channel). 

     

    I can only guess from the effects these controls have on the histogram displayed (reverse engineering, since Apple withholds all information), but what I observe is the following:

     

    luminance.png

    You can move the little triangles at the top of the graphics independently of the triangles at the bottom. The lines connecting them with their counterparts at the bottom define the mapping between the original Luminance values in the input image to the final values in the resulting image.

    • The three triangles on the top boundary of the histogram split the luminance values of the resulting image into four intervals: shadows, low midvalues, high midvalues, and highlights.
    • The five  triangles at the bottom split the luminance values of the input image into six intervals: ignored and mapped to black, shadows, low midvalues, high midvalues, and highlights,  ignored and mapped to white.

     

    So what happens at the pixel level: Determine the luminosity (or whatever the channel selected is) of the pixel and map it to an output value according to the sliders and the corresponding interval.

     

    Each of these input intervals (shadows, low midvalues, high midvalues, and highlights) will be mapped independently onto its corresponding output interval, to me this mapping seems to be linear, but it might be a weighted mapping, I cannot tell that from the visible effects.

    In the example above all luminances between 0 and 0.19 are cut off and mapped to black, values between 0.19 and 0.34 are mapped and squeezed (linearly?)  onto the interval 0 to 0.18, values between 0.34 and 0.49 are mapped  and stretched (linearly?)  onto the interval 0.18 to approx. 0.4 and so on. Pixels with luminance values greater 0.87 will be cut off and mapped to white.

     

    As you probably observed yourself, the contrast of the output image will increase if you enlarge the  "ignored and mapped to black or white" sections, and the same effect will apply to he midtones, if you enlarge the shadows/highlights intervals.

    Generally speaking, a small intervall at the bottom of the histogram mapped onto a larger interval at the top of of the histogram will result in an improved intensity resolution and more detail for pixels within this range of luminance values, while mapping it to a smaller interval will reduce contrast and loose detail.

     

    Cheers

    Léonie

  • léonie Level 8 Level 8 (46,355 points)

    oh, I forgot to mention the effect of the command key. The command key allows you to shift a luminance level slider so that the relatve size of the intervals to its left and right are preserved

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