Wading Deeper Into Photoshop Luminosity

[thebestcpu]

If you want a deeper (waist deep only) understanding of Photoshop RGB Luminosity the info below "may" be helpful.

The following image was created with two simple Layers set to Lighter Blend Mode which is one of the Luminosity based Layer Blend Modes

xxxxxxxxxxxThe bottom Layer is an exact Color Spectrum Gradient from Left to Right.

xxxxxxxxxx
The upper Layer (set to Lighter Blend) is an exact grayscale gradient from bottom to top:






and here is the resulting image:


Nice colorful mountain range yet many may be scratching their heads so a little more detail is in order.

RGB Luminosity is a simple equation that combines each of the three color channels into one grayscale value via the following equation and Excel Table"

The range of color values for this equation and result is from 0 to 1 for each channel which corresponds to the 8 bit values of 0 to 255.

I have also included the resulting Luminosity values (in the left column) for the fully saturated Primary and Secondary colors.

If you look at the Luminosity of the Rainbow gradient by using the Luminosity Blend against any totally grayscale image (no color) as one Layer below, you get the following luminosity with different weighting so yellow area of the gradient is the highest Luminosity at .89 gray value and the blue area of gradient is the lowest Luminosity at .11 gray value

Each color gets a

What the Lighter Color Blend does is evaluate the RGB luminosity for each pixel of both Layers and picks the higher luminosity pixel.

The Luminosity of the vertical grayscale image is just the grayscale image itself as all channels since R=G=B.

Show the color mountain range image shows that each fully saturated color has:
- a different luminosity
- Above the max luminosity of the color (color dependent), then the Blend chooses the grayscale gradient
- The Luminosity changes linear between the primary and secondary colors
- And the peaks at each of the primary and secondary colors match the values calculated in the Excel Table above (check against the percent scale ruler)
- It gives you a feel which color will win out in the Blend over other colors (the ones with a higher peak):



I hope these little tidbits help in the basic understanding of Photshop RGB Luminosity and some insight into how he Luminosity based Blends work.

Questions are welcome as I know even if it is clear to me, I may have made only as clear as mud to others

John Wheeler

As ad added edit, I have included a zip file with the enclosed PSD file for those who want direct access to play with it or try different blends

 

[Rich54]

John,
This is interesting. Without thinking it through, I would have guessed that the luminosity level was independent of Hue. Instead of a mountain shape, I would have guessed a smooth gradient.

 

[thebestcpu]

John,
This is interesting. Without thinking it through, I would have guessed that the luminosity level was independent of Hue. Instead of a mountain shape, I would have guessed a smooth gradient.

Hi @Rich54
I believe the attempt by Adobe and others is to simulate a perceived brightness of a saturated Hue.
I would expect it to peak in the yellows and diminish at deep blues and deep reds similar to how the human eye does not detect ultra violet and infrared.
Yet how the eye perceives luminosity also depends on the overall luminosity. At brighter levels the peak is in the yellows (photopic vision) yet at lower brightness levels (e.g. at night) we are more sensitive too blues and cannot discriminate colors (scotopic vision).
That's not to mention that magenta is not a true frequency of any light, it's just a combination of red and blue. See picture from: Eye spectral sensitivity chart:




So I imagine it is a compromise by Adobe and others to approximate the eye spectral sensitivity in a simple to compute formula for computation speed making it "close enough."

I also did not want to wade into the deep waters too much yet Photoshop is basing the calculations on the color numbers without taking into account the Color Space your are in. My image was done in sRGB. If the sRGB colors were converted to other spaces (e.g. Adobe RGB or ProPhoto RGB), the response curve would look different. So its an approximation at best.

I wanted to at least show what Adobe means by Luminosity in the Blend modes as it goes hand in hand with the associated Blends e.g. Color, Hue, Saturation, Lighter Color, Darker Color.

The Luminosity, Hue, and Saturation Trio actually defines a complete color model similar to RGB, HSB/HSV, and can come in handy in certain operations for images. Understanding the Adobe's Luminosity in more detail allow me to create Actions that I have found useful using the associated Blends.

For better for for worse, the majority of the Luminosity adjustment procedures or actions that separate out the high tones, mid tones, low tones, for separate operations is the same Adobe model.

Going back to just waist high water, this post points out which colors would more likely win out in the Blends of Lighter Color and Darker Color.

John Wheeler