Re: [Gimp-user] Histogramm -- values
OK, colour spaces are very confsing, but a very basic point is that the human spectral sensitivity has a pronounced peak in the green region, see http://en.wikipedia.org/wiki/Luminosity_function. This is why any function deriving the perceived brightness or luminance from RGB, such as Rec709Luminance (0.212656 * R + 0.715158 * G + 0.072186 * B) weights the green far more than red and blue. This just reflects the spectral sensitivity of the human eye. The perceived brightness of purely red image at, say, 100 bit will be far less than that of a purely green image at 100 bit. Correspondingly, the latter should result in a leighter equivalent grey than the former. Gimp proceeds this way when converting a colour image to grey, as I have just tested: 100 R -- 21 G 100 G -- 72 G 100 B -- 7 G This is very close to Rec709Luminance. I just suggest that it should offer such a weighting in the histogramm for a colour image, i.e. the equivalent grey value at Gimp itself would calculate it. Wolfgang Hugemann ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
Re: [Gimp-user] Histogramm -- values
On 12/02/2013 02:31 PM, Wolfgang Hugemann wrote: human spectral sensitivity has a pronounced peak in the green region, see http://en.wikipedia.org/wiki/Luminosity_function. This is why any function deriving the perceived brightness or luminance from RGB, such as Rec709Luminance (0.212656 * R + 0.715158 * G + 0.072186 * B) weights the green far more than red and blue. This just reflects the spectral sensitivity of the human eye. The perceived brightness of purely red image at, say, 100 bit will be far less than that of a purely green image at 100 bit. Correspondingly, the latter should result in a leighter equivalent grey than the former. Gimp proceeds this way when converting a colour image to grey, as I have just tested: 100 R -- 21 G 100 G -- 72 G 100 B -- 7 G Gimp 2.8 uses the values from Poynton, which are the right values to use in a non-color-managed application when sending signals to a D65 Rec709 display device. The correct sRGB values for an ICC profile color-managed application are slightly different: R 22%, G 72%, and B 6%. See: http://ninedegreesbelow.com/photography/srgb-luminance.html This is very close to Rec709Luminance. Gimp 2.8 operates on the nonlinear sRGB RGB values, so it computes luma rather than luminance. See Poynton's ColorFAQ #9 and #11 (http://www.poynton.com/PDFs/ColorFAQ.pdf). Calculating true luminance requires that you first linearize the RGB values. Gimp 2.10 will use linear sRGB to calculate true luminance rather than luma. I just suggest that it should offer such a weighting in the histogramm for a colour image, i.e. the equivalent grey value at Gimp itself would calculate it. I concur 100%. I hope Gimp 2.10 includes exactly such a histogram, except personally I would prefer luminance (calculated using linear sRGB) rather than luma. Luminance is a mathematically expression of how we actually perceive relative light and dark, based on what you said above about how we perceive colors (very sensitive to green, less to red, even less to blue). Luma is mathematically easier to calculate than Luminance, but as far as I know that's its only virtue. Elle ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
Re: [Gimp-user] Histogramm -- values
OK, colour spaces are very confsing, but a very basic point is that the human spectral sensitivity has a pronounced peak in the green region, see http://en.wikipedia.org/wiki/Luminosity_function. This is why any function deriving the perceived brightness or luminance from RGB, such as Rec709Luminance (0.212656 * R + 0.715158 * G + 0.072186 * B) weights the green far more than red and blue. This just reflects the spectral sensitivity of the human eye. The perceived brightness of purely red image at, say, 100 bit will be far less than that of a purely green image at 100 bit. Correspondingly, the latter should result in a leighter equivalent grey than the former. Gimp proceeds this way when converting a colour image to grey, as I have just tested: 100 R -- 21 G 100 G -- 72 G 100 B -- 7 G This is very close to Rec709Luminance. I just suggest that it should offer such a weighting in the histogramm for a colour image, i.e. the equivalent grey value at Gimp itself would calculate it. Wolfgang Hugemann ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
Re: [Gimp-user] Histogramm -- values
On 11/25/2013 01:51 PM, Elle Stone wrote: On 11/25/2013 12:16 AM, scl wrote: Hi, thank you, Wolfgang, for your reply and sorry I couldn't answer earlier. On 19.11.2013 at 5:14 AM Wolfgang Hugemann wrote: I'm for using the words value, tonal value, brightness, lightness etc. consistently to avoid further confusion. What are the 'officially defined' or 'most commonly used' names and meanings in these contexts for you, the users? Do you think it is the right approach to ask the users? I would rather suggest asking someone who has a thorough understanding of colour spaces and alike. I could ask Fred Weinhaus (www.fmwconcepts.com/imagemagick) to give that a look, as he has a deep understanding of the matter. Asking someone with a thorough understanding of the topic is the rationale behind asking the users. The large amount of GIMP users and the assumption that there must be at least some among them with a thorough knowledge led me to my proposal. Yes, it's not perfect, but at least one step into the right direction. If you have an expert I'd be glad if you asked him and reported back. Another point that needs clarification is the use and meaning of the word 'Value' in the Layer and Painting modes. Elle Stone and Nicolas Robidoux, can you please also have a look on this topic? Thank you in advance. This post is only about the word Value as used in Gimp's Histogram and Levels dialog. I don't know if the Value in the blending mode is the same as the Value in the histogram and Levels. Apologies in advance for a long post. XYZ is a model of how humans actually perceive color. RGB refers to a convenient subset of all possible XYZ values. The simplest possible subset of all possible XYZ values is determined by picking red, green, and blue primaries in XYZ space, plus a black point and a white point, plus a tone response curve (TRC) to create a matrix RGB color space such as sRGB. Above is covered in http://ninedegreesbelow.com/photography/xyz-rgb.html So RGB is mathematically derived from XYZ. In turn the various HSL/HSV/HSI color space models are mathematically derived from RGB. So first you pick an RGB color space. Then you calculate HSL/HSV/HSI values. The question is why? Why not just use the RGB values directly? According to Wikipedia (https://en.wikipedia.org/wiki/HSV_color_space#Motivation), the answer is because apparently RGB isn't intuitively obvious to us end-users of graphics programs, and also (here's the real reason) HSL/HSV calculations are computationally cheaper than RGB calculations, which made a difference back in the 1970s and 80s and 90s, but not so much today because computer hardware is exponentially faster than it used to be. The question is Gimp's term Value. The HSL/HSV/HSI models start with RGB and then define more or less adequate measures of: *Hue (whether it's red, blue, green, or some in-between color) *Saturation (whether it's very close to or far from the XYZ neutral axis connecting the RGB color space's white point and black point) *Some measure of Luminance. Luminance is a physical measure (based on real properties of actual light) of how bright something is (speaking loosely), or how much light reflects off of it in the direction of the viewer's eyeballs (speaking more precisely). Luminace happens to be equal to the Y of the equivalent XYZ value for the given RGB values in any specified RGB color space. If your goal is to represent what people see out there in the world, rather than to make computationally cheaper calculations, none of the HSX models are any good. See the nice illustration in the disadvantages section of the Wikipedia article: https://en.wikipedia.org/wiki/HSV_color_space#Disadvantages, and then read the paragraph following the illustration. To illustrate the problem with numbers, let's pick an RGB color space, sRGB. Then pick some colors and calculate Value, Lightness, and Intensity. Then divide each value by 255 to put them on a scale from 0 to 1 so we can compare them to the equivalent XYZ Y value. The L, I, and V equations are from the Wikipedia article: Lightness L = The sum of the maximum (M) and minimum (m) RGB values for the RGB values for the color, divided by two = (M + m)/2 Intensity I = one third of the sum of R, G, and B = (R + G + B)/3. Value V = the maximum of R, G, and B = M. This is what Gimp uses in the Histogram and Levels. I'm not sure what Curves does. Below the word value (lower case) means the numerical value of the RGB coordinates of a color as specified in a particular RGB color space. Let's pick the reddest possible red in the sRGB color space, at 8-bits, which is (255,0,0). The maximum RGB value of reddest sRGB red is 255. The minimum RGB value is 0. The Lightness is (0+255)/2 =127.5. Divide by 255= 0.50 The Value is 255. Divide by 255= 1.00 The Intensity is (255+0+0)/3 = 85. Divide by 255= 0.33 Now let's pick sRGB middle
Re: [Gimp-user] Histogramm -- values
On 11/25/2013 12:16 AM, scl wrote: Hi, thank you, Wolfgang, for your reply and sorry I couldn't answer earlier. On 19.11.2013 at 5:14 AM Wolfgang Hugemann wrote: I'm for using the words value, tonal value, brightness, lightness etc. consistently to avoid further confusion. What are the 'officially defined' or 'most commonly used' names and meanings in these contexts for you, the users? Do you think it is the right approach to ask the users? I would rather suggest asking someone who has a thorough understanding of colour spaces and alike. I could ask Fred Weinhaus (www.fmwconcepts.com/imagemagick) to give that a look, as he has a deep understanding of the matter. Asking someone with a thorough understanding of the topic is the rationale behind asking the users. The large amount of GIMP users and the assumption that there must be at least some among them with a thorough knowledge led me to my proposal. Yes, it's not perfect, but at least one step into the right direction. If you have an expert I'd be glad if you asked him and reported back. Another point that needs clarification is the use and meaning of the word 'Value' in the Layer and Painting modes. Elle Stone and Nicolas Robidoux, can you please also have a look on this topic? Thank you in advance. This post is only about the word Value as used in Gimp's Histogram and Levels dialog. I don't know if the Value in the blending mode is the same as the Value in the histogram and Levels. Apologies in advance for a long post. XYZ is a model of how humans actually perceive color. RGB refers to a convenient subset of all possible XYZ values. The simplest possible subset of all possible XYZ values is determined by picking red, green, and blue primaries in XYZ space, plus a black point and a white point, plus a tone response curve (TRC) to create a matrix RGB color space such as sRGB. Above is covered in http://ninedegreesbelow.com/photography/xyz-rgb.html So RGB is mathematically derived from XYZ. In turn the various HSL/HSV/HSI color space models are mathematically derived from RGB. So first you pick an RGB color space. Then you calculate HSL/HSV/HSI values. The question is why? Why not just use the RGB values directly? According to Wikipedia (https://en.wikipedia.org/wiki/HSV_color_space#Motivation), the answer is because apparently RGB isn't intuitively obvious to us end-users of graphics programs, and also (here's the real reason) HSL/HSV calculations are computationally cheaper than RGB calculations, which made a difference back in the 1970s and 80s and 90s, but not so much today because computer hardware is exponentially faster than it used to be. The question is Gimp's term Value. The HSL/HSV/HSI models start with RGB and then define more or less adequate measures of: *Hue (whether it's red, blue, green, or some in-between color) *Saturation (whether it's very close to or far from the XYZ neutral axis connecting the RGB color space's white point and black point) *Some measure of Luminance. Luminance is a physical measure (based on real properties of actual light) of how bright something is (speaking loosely), or how much light reflects off of it in the direction of the viewer's eyeballs (speaking more precisely). Luminace happens to be equal to the Y of the equivalent XYZ value for the given RGB values in any specified RGB color space. If your goal is to represent what people see out there in the world, rather than to make computationally cheaper calculations, none of the HSX models are any good. See the nice illustration in the disadvantages section of the Wikipedia article: https://en.wikipedia.org/wiki/HSV_color_space#Disadvantages, and then read the paragraph following the illustration. To illustrate the problem with numbers, let's pick an RGB color space, sRGB. Then pick some colors and calculate Value, Lightness, and Intensity. Then divide each value by 255 to put them on a scale from 0 to 1 so we can compare them to the equivalent XYZ Y value. The L, I, and V equations are from the Wikipedia article: Lightness L = The sum of the maximum (M) and minimum (m) RGB values for the RGB values for the color, divided by two = (M + m)/2 Intensity I = one third of the sum of R, G, and B = (R + G + B)/3. Value V = the maximum of R, G, and B = M. This is what Gimp uses in the Histogram and Levels. I'm not sure what Curves does. Below the word value (lower case) means the numerical value of the RGB coordinates of a color as specified in a particular RGB color space. Let's pick the reddest possible red in the sRGB color space, at 8-bits, which is (255,0,0). The maximum RGB value of reddest sRGB red is 255. The minimum RGB value is 0. The Lightness is (0+255)/2 =127.5. Divide by 255= 0.50 The Value is 255. Divide by 255= 1.00 The Intensity is (255+0+0)/3 = 85. Divide by 255= 0.33 Now let's pick sRGB middle gray: (119,119,119). The maximum value of sRGB
Re: [Gimp-user] Histogramm -- values
Elle Stone and Nicolas Robidoux, can you please also have a look on this topic? Thank you in advance. Wow, this was a very elaborate answer ... Fred Weinhaus also answered me and he is pointing to some transfer functions that ImageMagick uses, see http://www.imagemagick.org/script/command-line-options.php#intensity. He also gives some helpful illustrations what effects different transfer functions have on the image: http://www.fmwconcepts.com/imagemagick/color2gray/index.php I understand what Elle says in regard to histogram clipping, i.e. max(R, G, B) being very helpful for finding the white point -- while min (R, G, B) in regard to the black point is missing. Perhaps one should give the user just a few more options what to display in the histogram and how to transfer R, G, B into a single 'value'. I would be very lucky with 'Rec709Luma' as defined by ImageMagick. I understand that GIMP internally uses the sRGB colour space at the moment (?). A standalone feature for GIMP would be the possibility to display a cumulated histogram, which would for instance demonstrate the effect of equalisation better than an ordinary histogram. This should be easy to implement, I guess, and would be a simple check box for the user. BTW: Besides equalisation, one could also try to meet other cumulative histogram distribution, like a Gaussian bell curve, see http://www.fmwconcepts.com/imagemagick/redist. Wolfgang Hugemann ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
Re: [Gimp-user] Histogramm -- values
Am 13.11.2013 22:16, schrieb scl: I'm for using the words value, tonal value, brightness, lightness etc. consistently to avoid further confusion. What are the 'officially defined' or 'most commonly used' names and meanings in these contexts for you, the users? Do you think it is the right approach to ask the users? I would rather suggest asking someone who has a thorough understanding of colour spaces and alike. I could ask Fred Weinhaus (www.fmwconcepts.com/imagemagick) to give that a look, as he has a deep understanding of the matter. Another approach would be to have a look on what these things are called in Photoshop, if you dare ;-). Normally, I would also suggest to have a look at ImageMagick's nomenclature, but this is also rather questionary in some regards ... In regard to the HSV colour space, you probably have no other option than calling the V component value, as this is its name, whether it makes sense or not. In regard to the histogram of a colour image, I would regard the y-axis as the equivalent grey-value of the colour, i.e. its brightness, lightness or luminance, whatever you would like to call that. I think the word unspecific value should be avoided as far as possible. Wolfgang Hugemann ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
Re: [Gimp-user] Histogramm -- values
Hi, thank you, Wolfgang, for your reply and sorry I couldn't answer earlier. On 19.11.2013 at 5:14 AM Wolfgang Hugemann wrote: I'm for using the words value, tonal value, brightness, lightness etc. consistently to avoid further confusion. What are the 'officially defined' or 'most commonly used' names and meanings in these contexts for you, the users? Do you think it is the right approach to ask the users? I would rather suggest asking someone who has a thorough understanding of colour spaces and alike. I could ask Fred Weinhaus (www.fmwconcepts.com/imagemagick) to give that a look, as he has a deep understanding of the matter. Asking someone with a thorough understanding of the topic is the rationale behind asking the users. The large amount of GIMP users and the assumption that there must be at least some among them with a thorough knowledge led me to my proposal. Yes, it's not perfect, but at least one step into the right direction. If you have an expert I'd be glad if you asked him and reported back. Another point that needs clarification is the use and meaning of the word 'Value' in the Layer and Painting modes. Elle Stone and Nicolas Robidoux, can you please also have a look on this topic? Thank you in advance. Another approach would be to have a look on what these things are called in Photoshop, if you dare ;-). I'm not feared of Photoshop ;-) Can we assume for sure that the wording there IS proper (using a wrong word just because PS uses it is no convincing reason for me)? Unfortunately I only have the German version and switching languages in PS is not as easy as in GIMP. So if we want to do this somebody with an English version is needed. I think the word unspecific value should be avoided as far as possible. I agree with you. Kind regards, Sven ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
Re: [Gimp-user] Histogramm -- values
On Tue, Nov 12, 2013 at 4:39 AM, Wolfgang Hugemann a...@hugemann.de wrote: ... However, there seems to be no way to display the histogramm in terms of brightness in Gimp (?). The value seems to be just max(R,G,B). But luminance is generally caculated by Y = 0.2126 R + 0.7152 G + 0.0722 B see http://en.wikipedia.org/wiki/Luminance_%28relative%29. Wouldn't it make sense to offer this option? In my opinion, the (still) widespread use of value = max(max(r,g),b) is a remnant of 8bit image processing centric performance short-cuts. We would be better off by using luminance in most or all the cases where this approach is currently used. /pippin ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
Re: [Gimp-user] Histogramm -- values
On 11/14/2013 08:16 AM, scl wrote: On 13.11.2013 at 9:38 PM �yvind Kol�s wrote: On Tue, Nov 12, 2013 at 4:39 AM, Wolfgang Hugemann a...@hugemann.de wrote: ... However, there seems to be no way to display the histogramm in terms of brightness in Gimp (?). The value seems to be just max(R,G,B). But luminance is generally caculated by Y = 0.2126 R + 0.7152 G + 0.0722 B see http://en.wikipedia.org/wiki/Luminance_%28relative%29. Wouldn't it make sense to offer this option? In my opinion, the (still) widespread use of value = max(max(r,g),b) is a remnant of 8bit image processing centric performance short-cuts. We would be better off by using luminance in most or all the cases where this approach is currently used. Hi, we had a similar discussion about the inconsistent use of the word 'value' in IRC ca. 4 months ago. Currently this word appears at various places in GIMP: - in the Levels Tool as a channel, - in the Curves Tool as a channel, - in the Colors Menu as item 'Invert value' - in the dockable Histogram dialog as a channel (but there's also a channel 'RGB'), - in the dockable Pointer dialog as component of the HSV color model, - in the dockable Sample Points dialog as component of the HSV color model, - in the Color Picker tools info window as component of the HSV color model, - in the Change Color dialog as component of the HSV color model (V slider), - as result layer in Color/Decompose/HSV, HSL, - as parameter in Filters/Distorts/Value propagate, - as parameter in Filters/Decor/Add border, - in various places all of the program in its original meaning (set/increase/decrease/minimize/etc. value) and - in various places of the help. ... and not all seem to have the same meaning as we found out that time. There also seems to be no consensus about the meaning of this word in the graphics industry, even when used as abbreviation for 'tonal value': some mean the brightness of a given pixel, others the perceptual brightness of each color (yellow-light, blue-dark). I'm for using the words value, tonal value, brightness, lightness etc. consistently to avoid further confusion. What are the 'officially defined' or 'most commonly used' names and meanings in these contexts for you, the users? For me, value means how white or black the pixel is. This assumes an implicit map to gray scale. I think that this meaning remains the same even if you choose different ways to map to gray. So, users should be made aware of, and be able to select a mapping/a default mapping. ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
[Gimp-user] Histogramm -- values
I have to make some histogram corrections to nighttime photographs where it is essential to keep the average brightness (luminance) constant. (The photos illustrate the human perception in nighttime traffic, as far as this is possible by means of photography.) However, there seems to be no way to display the histogramm in terms of brightness in Gimp (?). The value seems to be just max(R,G,B). But luminance is generally caculated by Y = 0.2126 R + 0.7152 G + 0.0722 B see http://en.wikipedia.org/wiki/Luminance_%28relative%29. Wouldn't it make sense to offer this option? BTW: The histogram window always switches back to value when I start to modify the graduation curve. At the moment, I want to keep it at RGB, as this comes as close as (momentarily) possible to the above equation. Wolfgang Hugemann ___ gimp-user-list mailing list List address:gimp-user-list@gnome.org List membership: https://mail.gnome.org/mailman/listinfo/gimp-user-list
