What are midtones exactly (mathematically)?

Posted By

p8mode

Jun 10, 2004

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Hello,

Terms like midtones (and graytones) are often used in relation to colour images, and understanding them seems to be important in correcting/adjusting colour images proficiently.

However, Im not sure I understand what these terms mean exactly in terms of the underlying rbg or hsv colour model.

Presumably hsv is the best model for understanding what "midtones" are. Are midtones, pixels with a medium s (in hsv colour model), or those with a medium v, or a combination of both?

In and rgb image, is {255,0,0} a midtone (since the average of the rgb component s is ~ 50% of 256×3) or is rather only pixels with an rgb value ~ {128,128,128} ?

Presumably the answer in easier to express in terms of hsv? If so how can one translate hsv to rgb in order to answer the same question in terms of the rgb values of an image?

Similarly what does the term "graytones" mean exactly?

I would be very grateful for any help in clearing this up, or any links you could give me to online discussion forums that might be able to answer such questions.

Thanks very much in advance.

Bart van der Wolf

Jun 11, 2004

"Peter Williams" wrote in message

Hello,

Terms like midtones (and graytones) are often used in relation to colour images, and understanding them seems to be important in correcting/adjusting colour images proficiently.

However, Im not sure I understand what these terms mean exactly in terms of the underlying rbg or hsv colour model.

It may depend a bit on the context, but one could loosely divide the brightness range in shadows, mid-tones and highlights (which is then different from specular highlights), each occupying roughly 1/3rd of the brightness range. That would mean that the HSV’s V, the HSB’s B, or the L from the Lab model will be a guide.

For RGB usage it is often much more informal, because one could just use the RGB values from say 85 to 171 and call them medium (as opposed to dark or light) for the channel, e.g. medium red and light blue. It would be technically more correct to use the luminosity (=weighted average) of all three channels, and classify by that, and it’s easy enough to use the color picker or the info panel to simultaneously read RGB and e.g. Lab or HSB values.

Bart

p8mode

Jun 22, 2004

Hello Bart.

Thanks for the helpful reply.

It may depend a bit on the context, but one could loosely divide the brightness range in shadows, mid-tones and highlights (which is then different from specular highlights), each occupying roughly 1/3rd of the brightness range. That would mean that the HSV’s V, the HSB’s B, or the L from the Lab model will be a guide.

For RGB usage it is often much more informal, because one could just use the RGB values from say 85 to 171 and call them medium (as opposed to dark or light) for the channel, e.g. medium red and light blue. It would be technically more correct to use the luminosity (=weighted average) of all three channels, and classify by that, and it’s easy enough to use the color picker or the info panel to simultaneously read RGB and e.g. Lab or HSB values.

In Photoshops "Image"->"Histogram" dialog you get "Luminosity" as the
combined display parameter. This is the weighted average of the RGB components you say. What’s the weighting factor? How does this differ from brightness in the hsv model?

In the "Image"->"Adjustments"->"Levels" dialog one sees "rgb" as the
combined display parameter. This doesnt seem to show the same graph as the histogram did so I presume that its not luminosity. Do you know what exactly this is displaying?

Thanks in advance for any help.
Cheers

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