Extracción de RNA y RT-PCR

The use of red, green and blue (RGB) primaries is the basis of all common colour displays. The chromaticities of the primaries will determine the gamut – the range of displayable colours. There are many different RGB colour spaces (S¨usstrunk et al., 1999), differing in the chromaticities of the primaries, the white point (which variation of “white” results when the maximum output is being generated by all ofR,GandB)

17_{Not the author: this impromptu experiment arose as a result of the author purchasing “daylight”}

compact fluorescent lamps and using one to replace a failed incandescent lamp in a bedroom of another member of the household.

and the gamma (a measure of the non-linearity of the relationship between excitation and luminosity). All of these can vary from one RGB colour space and one display technology to another, so the specification of RGB values such as RGB = (250, 192, 110) only loosely specifies the displayed colour.

To more precisely define the displayed colour, the ambient lighting conditions, the overall gamma, and the characteristics of the display must be known. If these can be (even partially) standardised, there will be a closer match between the intended and rendered colours.

The visual system’s tolerance of white point differences is fortunate, as in most environments, controlling the illumination is impractical. In contrast, significant differ- ences in the chromaticities of the display and the system gamma can cause noticeable and objectional errors, especially in the mid-tones. A pragmatic agreement between the manufacturers of display hardware and those creating operating systems was in- strumental in reducing this source of error.

As the cost of memory and high-resolution display technology decreased, the use of colour displays capable of displaying millions of simultaneous colours became common- place. With systems capable of displaying high resolution photographs and the growing availability of affordable digital cameras and scanners, the lack of standardisation in colour representation became problematic: users expected, quite reasonably, that the colours in a photograph or scan would appear to be the same when displayed on the screen as when printed. Without standardising the colour space being used or the mapping between different known colour spaces, there is insufficient information for this to be possible18_{. To address this, Hewlett Packard and Microsoft proposed the}

use of a standard known as sRGB as a common standard for consumer-level colour representation. The primaries used by sRGB are the same as the ITU-R BT.709-5 HDTV (high definition television) standard, with a 6500K white point and a gamma very close to 2.2 (Anderson et al., 1995; Stokes et al., 1996).

The sRGB colour space has been widely adopted by manufacturers and has sim- plified the representation of colour in the domestic and small-business market. The standard specifies the primaries, the white point, the gamma, and the viewing condi- tions. If devices adhere to the chromaticity, white point and gamma standards, even without “correct” viewing conditions (due to adaptation in the human visual system), generally acceptable colour rendering is achieved. This allows users to create documents using uncalibrated systems in whatever lighting is convenient or comfortable, and other users to view these documents with similarly ill-specified equipment and uncontrolled viewing conditions, and it to be rare for there to be anything obviously wrong with the colours. The standard was not intended for the professional market, where both abso- lute colour matching and the largest possible gamut are expected. These require very strict constraints on viewing conditions and calibrated equipment, neither of which can be realistically expected from domestic and small business users.

Even though the gamut of the sRGB colour space is a subset of those colours rep- resentable by cameras, scanners, displays and printers, sRGB has been very successful.

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A user with a digital camera producing images to the sRGB standard can display these images on their computer and have them printed by a photo-lab to their satisfaction, without being aware of any colour management taking place.

Almost without exception, all current non-professional computer displays default to an sRGB rendering, although the white point may be set higher than 6500K standard. This has the effect of making the images appear brighter and more colourful (the Hunt effect Fairchild (1998)). Most users rarely adjust their monitors, and the fact that images from Internet sites around the world are rendered in acceptable colour testifies to the success of the combination of the sRGB standard and the human ability to adapt to widely-varying white points. This level of standardisation is useful when algorithmically creating colour schemes. If the colours within a schemes are created to the sRGB standard, it is likely that the colours seen will be close to those intended. Should more precisely controlled colours be desired, it is possible to use colour management.

2.6.1 Colour management

The use of a standard such as sRGB significantly reduces the error that might be expected due to the display technology. However, even with colour values corresponding to the sRGB standard and sRGB-specified equipment, manufacturing tolerances and device adjustments result in the displayed colour differing (in an absolute sense) from that intended. To ensure the display is rendering correct colours, it is possible to characterise the display by displaying a set of known colours, measuring these colours to find out what colour is actually being displayed and creating a mapping table to correct for any discrepancies. A standardised method of representing these mappings has been defined by the International Color Consortium (ICC) (International Color Consortium, 2004).

The mappings (Color Profiles) define a transform between a source or destination colour space and a standardised intermediate colour space is called the Profile Con- nection Space (PCS). The CIEXYZ colour space is often used as the PCS. For colour spacess1 and s2, if transforms exist between any both colour spaces s1 and CIEXYZ,

and s2 and CIEXYZ, then as long as the colour being transformed is within gamut,

colourimetrically correct transforms are possible betweens1 ands2.

By measuring the chromaticities of the display, it is possible to determine its gamut and produce transforms to correctly render colours specified in CIEXYZ coordinates (as long as the colour is within gamut). There are, however, many sources of colour data (cameras, scanners, etc.) and many destination devices, and their gamuts differ. When mapping from one device to another, it is useful to have an intermediate colour space capable of representing all visible colours. This limits the number of transforms required for m source colour spaces and n destination colour spaces to m+n rather thanm×n.

ICC mappings are useful when it is necessary to ensure that stored colour values refer to a specific colour in an absolute, rather than a device-dependent, manner. It enables a display (or printer) to reflect the colours of the originally encoded image. Nielsen and Stokes (1998) detail the derivation and transformations embodied in an

sRGB ICC profile using CIEXYZ as the profile connection space. ICC profiles, while desirable, are not widely used.

A harmonious colour scheme can be represented by sets of sRGB values. In the unlikely event that the user is using colour management and controlled viewing con- ditions, the displayed colours will be very close to those intended. However, for the more normal case – no colour management and arbitrary lighting, due to adaptation and the standardisation of display technology19 – we can be confident that the colours and balance of the colour scheme are seen as intended.

None of this guarantees that the viewer will think the scheme is harmonious. There are personal preferences to be allowed for, but if the colour scheme does not appeal, this will not be because of faults in the technology.

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Figure 2.13: The ordering and relationship of colours of Franciscus Aguilonius giving prominence to red, blue and yellow, and ordered by the lightness of the colours (from 1613), based on a diagram in Norman (1990).