Contents > Colour basics 9 / 10
 

The two modes of conversion Relative and Perceptual in photo

 

Perceptual and relative colorimetric rendering intent
Updated on November 22, 2018

 

Again, this is a vocabulary specific to color management. Conversion is essential because it serves to keep the same colors (i. e. L*a*b*) from one device to another. An image is converted when its RGB values are modified for slightly different ones to take into account the defects of the destination device. If you are a beginner in color management, the most important thing is to know that Photoshop can take care of everything. But what satisfaction if you take the time to know how to do it yourself: your prints, your images on your site will only be more beautiful !


We have just seen on the previous page the different roles of conversion. When you choose to do a conversion, what really happens to the RGB values of my image ? How can the conversion tool, the CMM, best preserve the L*a*b* colors, i.e. the visual sensation of the photo if the destination device does not allow it, as is unfortunately often the case with printers ? The conversion being in a way a translation whose mission is to distort the original as little as possible...

What is the difference between conversion and rendering intents ? The conversion takes care of changing the RGB values - origin to destination - and the conversion mode also called rendering mode takes care of knowing what to do with the out-of-the-box values - keep, delete, replace or transform -.


 

 

Why is there a need for two rendering intents ?

3D comparison of two ICC profilesAs we have seen on the page dedicated to colour spaces, the colorimetric spaces called peripheral or work spaces, are more or less large. Some are so big that they include all the others (ProPhoto RGB or DonRGB). Some are so small that they are encompassed by all the others. But sometimes, however, some are a little larger in one color or another and vice versa for another. This can be seen in the illustration on the right. Overall, the color space contained in the ICC profile of this printer/paper pair is smaller than the gamut of the screen except towards the blue-green ones. These colors are printable but invisible on the screen or rather, "replaced" by less saturated colors. More often than not, the opposite is true.

In other words, some colors may be contained in the original file but not printable. So we observe two scenarios :

  • Either the original colors are printable (in the destination gamut),
  • or the colors of the original file are not printable (out-of-gamut).

In the first case, if the colors of the photo are printable, the conversion is only used to change the RGB values of the photo to obtain the same printed color on the printout, taking into account the defects of the printer. So far, so good ! But what if the second case or if the gamut of the image is much larger than the gamut of the printer or... the opposite ?

What should be done with non-printable or out-of-gamut colours ?

Out of gamut conversion in Relative mode or PerceptualLet's assume that the profile of your image is the ICC profile 1 (red line) - and that of your printer the 2 (white line) -. Your image has green/yellow colors that the printer will not be able to reproduce because they are said to be out of range for this one. No combination of CMYK can reproduce exactly this L*a*b* color belonging to my image. So how do we do that ? The conversion then consists in "bringing", as with a shoehorn, these greens into the printer's space so that they can still be printed when normally the printer does not know how to do it. However, the visual impression must remain as close as possible to the visual sensations of the original image. To do this "shoehorn work", the color management and conversion tools use four conversion rules - only two of which are used for photographers :

  • The relative colorimetric
  • The perceptual one 

They are discussed below. In both cases there will inevitably be losses but a good engine will be able to reduce them to the maximum (the losses!) without distorting the visual sensations of the original image. Photoshop's is particularly powerful if you choose the right conversion mode. Of course, some image editing software, which is much cheaper, also has editing tools, such as stamps and other high quality tools, but none have such good color management. Unfortunately, this has a price...

   
 
 

Non-printable colours... really ?


There are of course no non-printable colours. They are just replaced by the nearest printable color in saturation. However, in Photoshop, if you display the so-called "non-printable" colors (below), you will see a plain grey area appear on your photo without distinguishing between a non-printable color for 1% or 20%. So it's spectacular but not very accurate. That's why, when you print your photo, you often feel like you're finally so close to your screen. A fright for nothing because the display of these bloody non-printable colors has an ON/OFF side. When will there be a progressive display ?


Unprintable colors in Photoshop

 
     
   
 

Colorimetric rendering intents : relative or perceptual ?

When you do a conversion in Photoshop in particular, you must therefore :

  • First, make sure that the original image has an ICC profile,
  • then, choose the destination ICC profile - your printer and paper -
  • But also, as we have just seen, choose the rendering intent.

What is it about ? Simply the way out-of-gamut colors are managed during a conversion. Do we "delete" the colours out of gamut - logical because they are not printable ? Problem : it's destructive. Are they replaced by other colours ? Which ones ? What about the colours used for replacement ?

Let's see it now.... Depending on the rendering mode chosen, the conversion rules will change. The most suitable for photography is very often, in my opinion, the relative rendering intent but the most "logical" because so-called photography is the perceptual rendering intent. I will describe them now.

Converting a point A to a point B in a photoLet's take an example to feel the problem : let's imagine that I want to print an image. It has an independent color space - red line on the illustration opposite - and I know that my printer's color space is much smaller for these green/yellow colors - white line -. Colors from my original, around point A, should be non-printable - the associated L*a*b* color is not printable. I want to try to print these colors anyway so I have to convert my image in another space, the destination space - inside the white line - to translate them into the right RGB values BUT HERE THE GOOD RGB VALUES DO NOT EXIST BECAUSE THEY ARE NOT PRINTABLE.
In addition, point B is common to both spaces, so the associated L*a*b* color belongs to both spaces. This L*a*b* color can therefore be converted to other RGB values.
Under these conditions, how can I still print these yellows / greens of my image that are outside the reproducible space of the printer so as not to completely distort my image once printed ? What does the out-of-range color printer do ? Does it remove them ? In addition, on my original image, the distance between A and B gives me a certain visual "sensation"; how can I keep it after the conversion ?

But this raises several problems :

1 - 1 - If the out-of-space greens, which are normally not printable, are placed in the printer's space - inside the white line - many colours will, in fact, be "superimposed". One might think then that the only choice available to us is to keep only color B and eliminate A. In other words: what is the decision to keep and eliminate ? Do we really eliminate it ?
2 - Moreover, if I put (replace) the color A on the color B and the colors of the red line on those of the white line, it will be impossible to differentiate them on the print while they were different in the original space.
3 - Finally, I would like to remind you that the yellows of point A were at a certain distance from the yellows of point B in my original image, which gave me a certain visual sensation.

To solve these problems, mathematical researchers have therefore invented conversion rules called rendering intents : the perceptual and relative rendering intents for photographers.

In perceptual colorimetric rendering intent

Conversion to perceptual modeThe ACE Photoshop conversion engine will work a lot ! It will indeed "bring" the greens out of space so as not to completely remove these nuances and preserve my visual impression, my visual perception. To achieve a conversion that respects the visual sensations of the original image, Photoshop's ACE conversion engine will proceed in two steps. First of all, it will "place" the colors of the red line (boundary of the image space) on the white line (boundary of the printer space) because the colors of the white line are the closest visually to the colors of the red line. The colors of the red line are the most saturated colors of my original and the colors of the white line are the most saturated colors of my printer. The most saturated colors of my original are preserved and replaced by less saturated colors.
Then, to maintain a visual sensation close to the original sensations, it will slightly and gradually "shift" the colors inside the printer space as shown in the figure above. B will move to B' and so on... B's place being "liberated", he will be able to put a color out of gamut in its place. So it's a progressive game of musical chairs! As he could not do this indefinitely otherwise colors would disappear from the "other side", therefore towards the reds, he makes "disappear" some nuances of the original image in the gamut as he "pushes" the colors inwards but while trying to keep the same perception, hence the name of this mode. The image will lose some of its original saturation but the visual sensation will remain very close on the print.

In brief :

  • The red line goes on the white line = the most saturated colors of the image are printed with the most saturated colors of the printer; There is a loss of saturation on some colors.
  • The A color "disappears" because it cannot be printed = it is replaced by the A' color, the closest but printable color. At the same time, A' is replaced by A''. From time to time two different colors in the original space are replaced by the same color but this time it can be printed.
  • The relative distance between A and B is replaced by a smaller distance A' and B' so the "spirit" of the original photo is preserved as best as possible. This is why an image must be processed as long as possible in its original space. It's the only way not to lose information.

Conversions in this mode are destructive since all colors are modified, even those that were printable. They must be limited and done at the last minute. Finally, the converted image will appear slightly desaturated and only slightly less contrasted if the printer space is really too small for these colors. It is then sufficient to apply a level or contrast layer to restore - almost - the original L*a*b* colors.

In relative colorimetric rendering intent

Conversion in relative modeThe ACE conversion engine of Photoshop works much less because it is then a question of simply "eliminating" all the original colors out of gamut. Did you say eliminate ? obviously not quite ! In fact, all the colors out of gamut are replaced by a single color, the most saturated color that the printer is able to print. All colors between the red line and the white line are replaced by the only colors of the white line. Phew ! On the other hand, the L*a*b* colours common to both spaces remain unchanged.

Summary :  Thus, if the out-of-gamut colors "disappear" or rather are replaced by the most saturated colors that the printer is able to print, it is necessarily at the expense of the loss of all the nuances of the most saturated colors on the original image. The original drape partially disappears as many shades disappear. On the other hand, the common colors are not modified at all as in the perceptual rendering intent. This mode is therefore perfectly suitable when the original image contains very few out-of-gamut colors or when an original color is printable and must not move at all.

top

 

 

How to choose between perceptual and relative colorimetric ?

The most important thing is that there is not a better way of rendering than the other, it depends on the photo to convert and more precisely of Lab colors contained in the file.

Reminder: In nature, the colors are not always very saturated and that is why the choice of working only in sRGB is not "catastrophic". While it is fashionable to work in larger color spaces to show that you know a lot about color management, I find even smarter to stay in sRGB in most cases because the sRGB color space already contains enough colors, to be only rarely forced to work in a larger color space - but it all depends on what you photograph -. Each picture we take does not contain millions of colours and not all photographers take pictures of turquoise blue seas, designer dresses with intense reds with flash or even leaves in the sun in proxi photo !
Thus, if your original does not contain any or only a few colours out of gamut for your printer, they will not be "lost" by a relative colorimetric rendering intent ! It is then the most judicious because it does not transform - and therefore does not deform - the colors within the common gamut. As if by chance, it is the one that works best most often and is perfectly suitable ! Even if you work in ProPhoto RGB, it is amazing how well this rendering mode often works very well. Your image did not necessarily contain many out of gamut colors for the printer whose gamut is much smaller.
If your original contains non-printable colors in significant quantities, it may be wise to use the perceptual rendering intent. Certainly it changes all the values of your file but it preserves much better some materials, some drapes or gradients, some relative distances between two colors as explained above.


 

How does it get actually materialized ?

Where the relative rendering could transform non-printable colours into a solid without shades since all the out-of-gamut colours are replaced by a single colour, the closest printable colour, the perceptual mode tries to keep these shades, even if it is obliged to replace them by others, very close visually but less saturated. I will try to show you, I hope, with the illustrations below.

   
Converting photos in relative and perceptual mode
  One point is very interesting : if you look at the whole picture EXCEPT the circled area, you will see that the picture does not "move" between the original, the perceptual and relative colorimetric rendering intents. The "movement of colors" will take place for some colors only!
   
 

Now that my image is correctly displayed and in a neutral and large working space, I can touch it up as I wish - possibly with a eyedropper -.

In fact, each device, each image etc.... has its own ICC profile and can be interpreted by software such as Photoshop. It can assign a profile to an image or "translate" the colors, we say convert an image, from one device to another. The devices can therefore communicate via the CMM - color conversion engine - and their ICC profile. In Photoshop since version 6, it is called: ACE color engine. There wasn't any before this version in Photoshop (so it was a long time ago).

To be able to work, the CMM needs to know which ICC profile is assigned to an image - source ICC profile - to know which colours L*a*b* it has to do for given RGB signals and to which device to send it - destination ICC profile -, thus converting it into R'V' B' signals.

The CMM is the central element and is based on the L*a*b* colours (the universal standard) and not on RGB or CMYK signals because, as we have seen several times, the L*a*b* colours are absolute. It knows it must transmit as information such or such color L*a*b* (therefore absolute) and not such or such RGB value (relative) thanks to the ICC profiles and knows to which RGB or CMYK signal it corresponds for this one and only this one. It will therefore translate this L*a*b* color, a different RGB value for each device - even if they are obviously close enough - into another signal R'V' B' or C' M' J' N' so that the destination device reproduces the same L*a*b* color. And the strongest thing is that if it cannot do it directly, it will replace it with another one without betraying the perceived visual sensation. This is especially his great strength ! This "translation" operation is called a conversion and there are four different ways to do it according to the desired rendering. We will discuss this in detail on the next page

When does a photo need to be converted ?

So a image need to be converted at least once in its lifetime to match what our Photoshop eyedropper measures and what we see. Thereafter, we will automatically make or undergo further conversions to modify the RGB values of our photo - so always without changing the colors or as little as possible - to broadcast this photo on the Internet (sRGB color space), to be displayed correctly on our calibrated monitor (it is done on the fly in the graphic card) and therefore take into account its own display defects, or even each time it is printed.

Important ! When converting to a neutral color space, you should choose it a little larger than the ICC profile of the device you used. That's why there are many of them, from the smallest sRGB to the largest ProPhoto RGB. 

top

 

 

How is the conversion done ?

The paragraph above has just seen the different roles of conversion. When you choose to do a conversion, what really happens to the RGB values of my image? How can the conversion tool, the CMM, best preserve the L*a*b* colors, i.e. the visual sensation of the photo if the destination device does not allow it, as is unfortunately often the case with printers ? The conversion being in a way a translation whose mission is not to distort the original...

More or less large colour spaces...

Differences between ICC profiles in 3DAs we have seen on the page dedicated to color spaces, the colorimetric spaces called peripheral or work spaces, are more or less large. Some are so large that they include all the others (ProPhoto RGB or some digital cameras). Some are so small that they are encompassed by all the others like some printers on matte paper. But sometimes, however, some are a little larger in one colour or another and vice versa for another. This can be seen in the illustration on the right. Overall, the color space contained in the ICC profile of this printer/paper pair is smaller than the gamut of the screen except towards the blue-green ones. These colors are then printable but invisible on the screen (not displayable as they are, therefore displayed but less saturated than their original saturation). More often than not, the opposite is true.

In other words, some colors may be contained in the original file but not printable (or even displayable). So we observe two scenarios :

  • Either the original colors are printable (in the destination gamut)
  • Either the original colors are printable (in the destination gamut)

In the first case, if the colors of the photo are printable, the conversion is only used to change the RGB values of the photo to obtain the same printed color on the print. So far, so good! But what if the second case or if the gamut of the image is much larger than the gamut of the printer or... the opposite ?

 

 
 

Are they really impossible to display ?


Of course not ! In fact they are "replaced" by the color or colors - see the next page on conversion modes - the closest and therefore the most saturated one on this screen. On a print run, the colors don't disappear either! They are just printed with the closest and most saturated colors that the printer and its inks can make. So where we eventually had a gradient of colors on the screen (for example), we will have a solid, less saturated, on the print. In fact, with Glossy papers, it's more often the other way around !

 
     

 

What to do with non-printable or out-of-gamut colors ?

Out of gamut conversion in Perceptual or Relative modeLet's assume that the profile of your image is the profile - 1 (red) - and that of your printer - 2 (white) -. Your image has green/yellow areas that the printer will not be able to reproduce because they are called out-of-gamut. No combination of CMYK can reproduce exactly this L*a*b* color belonging to my image (whether I can display it on the screen or not). So how do we do that ? The conversion then consists in "bringing", as with a shoehorn, these greens into the printer's space so that they can still be printed when normally the printer does not know how to do it. However, the visual impression must remain as close as possible to the visual sensations of the original image. To do this "shoehorn work", the color management and conversion tools use four conversion rules - only two of which are used for photographers :

  • relative rendering intent
  • perceptual rendering intent

They are studied next page: modes of Relative and Percpetual conversion. There will necessarily be losses but a good engine will reduce them to the maximum (losses!) Without denaturing the visual sensations of the original image. Photoshop's is particularly powerful if you choose the right conversion mode. of course some image editing software, much cheaper, also have editing tools, such as stamps and other very good quality, but none has such good color management. This unfortunately has a price ...

... And concretely?

Conversion can be done in three different ways:

  • Directly in your camera case (in Jpeg),
  • In your demolition software (Camera Raw, Lightroom, DXO, Capture One, etc.) in RAW,
  • In Photoshop (all image files). 


1 - In your camera case (in JPEG)

Choose your color space on your cameraThe assignment of the ICC profile of your box has necessarily taken place automatically in your box. The conversion to a neutral color space like the famous sRGB will also be done directly in your camera if you shoot in Jpeg. To do this, go to the menus of your camera and choose your color space between two options (very, very rarely three):

   sRGB (default) 
   Adobe RGB (or Adobe RGB but it's the same). 


Attention ! Ce choix ne s'applique qu'aux fichiers Jpeg, même si vous photographiez en RAW + Jpeg. 


2 - In your demo software (Camera Raw, DXO, etc.) for RAW

Assigning your box's ICC profile automatically applies when you open your RAW file in your software. You have nothing to do and nothing else you can do at this point! If your box is not recognized by your software because it is very recent or you have not updated your software database, you will simply not be able to open your RAW file.

   
 
   
 

Once opened with the right ICC profile, you must choose the neutral color space of destination, as on your box (see above), but with a difference in size: you can choose more options including the very interesting and very useful ProPhoto, in some cases. Example with Photoshop Camera Raw below:

   
   
 

At the very bottom of the window, you can choose the destination color space. In Camera Raw, you can choose between the classic sRGB, the largest Adobe RGB and ColorMatch RGB and the very large ProPhoto.

Importante update!

 Since the update of Photoshop CC 14.1, it is now possible to choose your destination space from all its profiles! If you have installed them on your computer, you can choose a DON RGB, BEST RGB, MÉLISSA RGB, etc. and no longer only among four possibilities and in addition, you now have the choice of conversion mode. (Until now, the default choice was relative, which is quite logically explained). We can say that now the situation is perfect and finally leaves us the choice of controlling our color management in Camera Raw.

When you click on the "Open an image" button, your photo will necessarily have this color space.

3 - In Photoshop...

The conversion in Photoshop is processed on a dedicated page: Manage colors with Photoshop Suivre

   
  To finish this series, let's see what exactly the calibration consists of : What's calibration? - 10 / 10    Suivre
   
 
 

To remember!


 A true color (L*a*b*) can be "translated" by a very wide variety of RGB values, depending on the device.  An RGB color is therefore not a "true" color but only a color for a given device and therefore in a given color space.

 To communicate the same color (meaning L*a*b*), two devices must exchange RGB values through a conversion: the RGB values of one color are converted into R'V' B' values for another device and therefore for the same color L*a*b*. 

 This conversion therefore strives to preserve the appearance and colors of a photo as best as possible, even if some colors are not printable, for example.

 There are two "ways" to solve these non-printable color problems: the relative and perceptual conversion modes studied on the next page.

 This conversion can be done directly once in your camera if you work in Jpeg, in your demo software if you work in RAW and in Photoshop or your image editing software afterwards.

 
     
 
 
Through these 10 pages we will learn all the vocabulary related to colour management: colour spaces, ICC profiles, gamuts, etc...
 
- Introduction to colour management
- Eye and colour perception
- Colours and computer science
-
Gamma
- Colour spaces
- ICC profiles
- Assigning an ICC profile to an image
- Convert an image
-
Perceptual and relative colorimetric rendering intent - 9/10
  - Why is there a need for two rendering intents ?
- How to choose between perceptual and relative colorimetric ?
- How is the conversion done ?
- To be remembered...

- What is calibration ?!

 

- 2018 monitors buying guide !
- My 21 full monitor review


 

Calibrate your photo printer with the best quality/price ratio: i1 Studio !

Read my full review...  

Amazon Good Deals £338.67

Calibrate your monitor with the best
quality/price ratio: Colormunki Display !

Read my full review...

Wex Good Deals ! £99.00

   
 

 

     

 

 

 
 

 

       

From 2002, this Website offers...
 

This site dedicated to colour management for photographers and videographers, beginners or professionals, visited by more than 180,000 people last year, offers on the one hand to help you easily understand and put into practice the colour management of your photos and on the other hand helps you make the best investments with more than 100 hardware or software reviews since April 2002 in French and 2014 in English! It is the result of a patient work and a long experience shared by a professional but above all passionate photographer !

 

Where do the tested products come from ? Is my opinion completely independent ?

All the products I talk about on this site have been tested by me either following a personal purchase, a friendly loan, during a training session at a customer's site or after a manufacturer's loan.
they thank me for the precision of my remarks which will help their future products to progress. So things are moving and my impartial opinion is more and more appreciated.... If I believe your emails, your trust marks and even some brands.

 

You often ask me how this site is financed since its content is free...

Here is how I have been doing, historically since 2002 (in French), to make it easier for you to contribute: the affiliation and sale of my PDFs (in French).

 

Here's how affiliation works. Each time you click on the links on this site to my partner shops, for example from my test pages or on the thumbnails on the right, they know that you come from me (thanks to a piece of code in the link) and give mea small percentage on the whole of your basket (my advice of purchase as well as all your purchases in this shop). Nothing prevents you from thinking about my site when you buy from these online stores even if you don't want to buy a monitor or a colorimeter!

 

... And how to participate!

So my proposal when you don't know how to thank me for the free content: think about using my links to buy in these different partner shops because they play the game!

 

 

Legal informations

Legal information is available on my page Legal information - Contact me...

 
 

 

     

 

 

Top



 

US Version  |   Sitemap  |   About   |   Legal information