Currently Being ModeratedOct 29, 2011 4:17 PM (in response to Kirby Krieger)
You have so many highly technical questions, I would recommend one of the following:
Currently Being ModeratedOct 29, 2011 7:03 PM (in response to JohnTheAppleFan)
Appreciate the links -- but they are quite out-dated. Both books are from 2006, and have been superceded by the now-current Scoppettuolo book, which covers Aperture 3. All of them offer excellent overviews of Aperture, but I wouldn't recommend them for anyone seeking a more sophisticated understanding of the program.
I posted in the hope that one the several Aperture aficionados -- or other -- who frequent the forum might have some input.
Currently Being ModeratedOct 30, 2011 4:23 AM (in response to Kirby Krieger)
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:
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.
Currently Being ModeratedOct 30, 2011 4:31 AM (in response to JohnTheAppleFan)
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
Currently Being ModeratedOct 30, 2011 8:09 PM (in response to léonie)
Léonie -- many thanks for the long reply. I will respond fully once I can use Aperture again -- -- I'm doing my weekly back-ups and monthly repairs, and don't expect to be quarrying the whole hole until tomorrow.