24.2.3 Monitors Are Nonlinear, Renderers Are LinearĬRTs do not behave linearly in their conversion of voltages into light intensities. They will not multiply, subtract, or interfere with each other in unobvious ways. The illumination contributions from two light sources in a scene will sum. The output scales by the same factor as a scale of the input (for a scalar k): f( k x x) = k x f ( x).The output of the sum of inputs is equal to the sum of the outputs of the individual inputs.In the mathematical sense, a linear transformation is one in which the relationship between inputs and outputs is such that:
However, its effects on rendering, particularly in scenes with plenty of dynamic range like Figure 24-1, are quite noticeable, and the simple steps needed to correct it are well worth the effort.įigure 24-1 The Benefit of Proper Gamma Correction 24.2.2 Digression: What Is Linear? The nonlinearity is subtle enough that it is often unintentionally or intentionally ignored, particularly in real-time graphics. In particular, both the capture (scanning, painting, and digital photography) and the display (CRT, LCD, or other) are likely not linear processes, and this can lead to incorrect and unrealistic images if care is not taken at these two steps of the pipeline. The answer to these questions is, surprisingly, probably not. Does the display accurately turn the digital image back into light? If a pixel's numerical value is doubled, will the CRT, LCD, or other similar device display an image that appears twice as bright?.Does a producer of synthetic images, such as a renderer, create digital images whose pixel values are proportional to what light would really do in the situation they are simulating?.Does the capture of the light (by a camera, scanner, and so on) result in a digital image whose numerical values accurately represent the relative light levels that were present? If twice as many photons hit the sensor, will its numerical value be twice as high?.You may be surprised to learn that there are several steps in the digital image creation pipeline where things can go awry. If you are interested in high-quality rendering, you might wonder if the images displayed on your CRT, LCD, film, or paper will result in light patterns that are similar enough to the "real world" situation so that your eye will perceive them as realistic. 24.2 Light, Displays, and Color Spaces 24.2.1 Problems with Digital Image Capture, Creation, and Display Proper gamma correction is probably the easiest, most inexpensive, and most widely applicable technique for improving image quality in real-time applications. However, a subtle nonlinear property of almost every device that captures or displays digital images necessitates careful processing of textures and frame buffers to ensure that all this lighting and shading is computed and displayed correctly.
The performance and programmability of modern GPUs allow highly realistic lighting and shading to be achieved in real time. You can also subscribe to our Developer News Feed to get notifications of new material on the site.Ĭhapter 24. The CD content, including demos and content, is available on the web and for download.
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Behavior Research Methods, Instruments, & Computers, 18, 587–594.GPU Gems 3 GPU Gems 3 is now available for free online! Use of a raster framebuffer in vision research. K., Poirson, A., Fitzhugh, A., Bilson, A., Nguyen, K., & Ahumada, A. Using color dimensions to display data dimensions. Color Research & Application, 11, S57-S62. Factors in using color video monitors for assessment of visual thresholds. Behavior Research Methods, Instruments, & Computers, 21, 502–524. Lookup tables for linear trajectories through color space. Benson (Ed.), Television engineering handbook (pp. Monochrome and color image-display devices. Cambridge, England: Cambridge University Press. Numerical recipes: The art of scientific computing. Color Research & Application, 11, S47-S51. Minimizing quantization errors in digitally-controlled CRT displays. Optical characteristics of cathode ray tube screens (Tube Engineering Panel Advisory Council Publication No. Color Research & Application, 11, S34-S38.Įlectronic Industries Association. On the gun independence and phosphor constancy of colour video monitors. An inexpensive scheme for calibration of a colour monitor in terms of CIE standard coordinates. Color Research & Application, 14, 23–34.Ĭowan, W. Calibration of a computer controlled color monitor.
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