Rendering Intents: The How and Why
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Rendering Intents: The How and Why

Rendering intents, another key component of a Color Management System, are ways of dealing with colors that are out-of-gamut ≠ colors present in the source space that our destination space or output device is incapable of producing. We'll aim to help you wrap you thoughts around this theory.

By Johnny Shell, Vice President, Technical Services, SGIA

In todayís color workflow, we deal with a very diverse selection of input, display and output devices, resulting in a number of possible color conversions between them. Each device we encounter has its own color gamut, or range of reproducible color.

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  • A Color Management System (CMS) can provide consistent and accurate color interpretation among multiple input, display and output devices so that perceived color differences between these devices are minimized. A CMS basically assigns a meaning to the color numbers and changes them based on the specific device being used, so that the final color produced is consistent. Device profiles provide a description of the color behavior of a specific device to the CMS when these color conversions are performed. Rendering intents, another key component of a CMS, are ways of dealing with colors that are out-of-gamut ≠ colors present in the source space that our destination space or output device is incapable of producing.

    Many graphic software applications and raster image processors (RIPs) allow you to select rendering intents to be used when color conversions are performed. But there is confusion over what rendering intents are and how one goes about choosing amongst them. The answers to these problems are not always clear, but this article attempts to describe the different types of rendering intents and the impact each one has on color.

    To get from here to there
    When converting color in a CMS, two profiles are needed: a source and a destination. The source profile provides the CMS with information on the colors represented in its gamut, and the destination profile provides the CMS with its color capabilities. The CMS then determines how to best reproduce the original colors in the new color space. Colors present in a source space that canít be reproduced in the destination space must be replaced with colors that are reproducible in the destination space. Rendering intents help specify how the new colors are determined. The intent you choose will depend on the graphical content of the image thatís being converted and the color precision desired for the final outcome.


    In Figure 1, Iíve superimposed two color space modelsí RGB and the CMYK gamut of my output device. Immediately, one notices the much smaller gamut of my CMYK device. Now, letís add data points from an image. Figure 2 illustrates six points representing colors in my image and where they fall in each gamut. Those colors that fall outside the CMYK gamut are beyond the capability of my device. They are out-of-gamut. Rendering intents will adjust the out-of-gamut colors so that they are still represented in the final print. Each type of rendering intent preserves certain color characteristics. Itís important to understand how out-of-gamut colors are treated in each method and the resulting impact this has on in-gamut colors.

    Four flavors, two sizes
    Rendering intents are available in four versions: saturation, perceptual and two types of colorimetric (relative and absolute). These can be further characterized by how they generally treat out-of-gamut colors. Saturation and perceptual rendering intents use gamut compression which essentially desaturates all the colors in the source space so they will fit in the destination space. Relative and absolute rendering intents use gamut clipping, which clips out-of-gamut colors and moves them to the closest color the destination space can reproduce.

    Saturation rendering intent
    Saturation rendering intent is probably the least-used in our industry as it merely tries to produce vibrant color without any concern for color accuracy. Individual colors are treated with no such consideration given to relationships with surrounding colors. If I have an image with three out-of-gamut colors very close in hue, the saturation intent will re-map those colors to the closest in-gamut color of my destination space, Figure 3. Unfortunately, this may be the same color for all three colors because of their hue and relationship to each other. Because out-of-gamut colors are re-mapped to the most saturated in-gamut color rather than the closest hue, this intent is not recommended for photographic images because the primary goal is to maintain saturation rather than hue. Saturation rendering intent is acceptable to use when an exact color match is not as important as having a vivid image.


    Figure 4 shows the result of using a saturation rendering intent after converting the image from an RGB gamut to the CMYK gamut of the device in the SGIA Digital Lab.

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    Perceptual rendering
    Perceptual rendering intent is probably the most applicable rendering intent for our daily needs as graphics producers. The perceptual rendering intent tries to preserve color appearance and overall color relationships by adjusting all of the colors in the source space so they fit in the destination space. Surrounding colors are taken into consideration so that the relationship between all colors in the image is preserved in the final output, which helps to maintain smooth gradations and tonal blends. But, it also causes more colors to shift than any other rendering intent, Figure 5. Some out-of-gamut colors are re-mapped well inside the closest hue in the destination space in order to maintain the relationship for those out-of-gamut colors, where the closest reproducible hue is on the edge of the color gamut in the destination space.

    So this does mean every color will be wrong in the final print. But the human eye is much more sensitive to relationships between colors rather than absolute color values. Because the perceptual rendering intent maintains the relationship between colors and adjusts them proportionally, the chance that a viewer will notice the colors have been modified is unlikely because of the proportionate adjustments made to all of the colors.

    Figure 6 shows the results of using Perceptual rendering intent.

    Colorimetric intents
    Colorimetric intents adjust all in-gamut colors for accuracy. Out-of-gamut colors are compressed to the closest available hue in the destination space, similar to how out-of-gamut colors were adjusted in the saturation rendering intent. The only real difference between the relative colorimetric intent and absolute colorimetric intent is how the white point is treated.

    Relative or absolute
    Relative colorimetric intents rely on the premise that our eyes adapt to the white in whatever image weíre viewing. Relative colorimetric causes the white in the source space to be re-mapped to the white point of the destination space, Figure 7.

    All of the colors maintain their relative position to the white point. White in the original image effectively becomes the paper white of the media that will be printed. Figure 8 shows some samples that were generated by using the relative colorimetric rendering intent.

    Absolute colorimetric intents treat color the same as relative colorimetric intents but they do not map the source space white point, Figure 9, to the destination space white point. That means if the white in my source space has a bluish tint, cyan and magenta will be added in my final print to simulate the white point of the original. Absolute colorimetric is suitable for trying to simulate the output of one device by using another device, Figure 10.

    Therefore, only use the absolute colorimetric to simulate an output on your monitor or on your proofing system. You should never use it for final output conversion.

    In summary
    A rendering intent is really nothing more than a method of mapping color that will be applied between a source and destination profile. It specifies how color in an image is treated during conversion to a different color gamut. Take comfort in the knowledge that once you have your head wrapped around these rendering intents, you will understand one of the most complicated aspects of color management.

    This article appeared in the SGIA Journal, 3rd Quarter 2006 Issue and is reprinted with permission. Copyright 2006 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.

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