![]() ![]() The tricky part is figuring out the two additional pieces of critical information: when encoding, how the original light is related to the samples, and when decoding, how image samples are related to the display's actual output (i.e., the reproduced light). Storing the image samples themselves is easy. A decoder that has both that information and knowledge of how the user's display system behaves can then deduce how the image samples must be transformed in order to produce the correct output. PNG's role is to provide a way to store not only the image samples, that is, the color components of each pixel but also the information needed to relate those samples to the desired output of the display. We'll refer to the combination of the encoding and decoding processes as the end-to-end process. ![]() Clearly this involves both the encoding process performed by the editor or conversion program that writes the image file, and the decoding process, perfromed by the viewer or browser that reads and displays the image, as well as aspects of human physiology and psychology. Alternatively, for images created with an image-editing application, the goal is for your display to produce the same perception (and basically the same light) as the artist's monitor produced while he was creating the image. The ultimate goal of the entire process is for the light that leaves your monitor to produce the same perception as the light that originally entered the camera would have if it had entered your eyeballs instead. I may even mention some physics and an equation or three, but you shouldn't need a technical degree to be able to understand the basic ideas. But I will give a brief overview of the main issues and explain how some of the features of the Portable Network Graphics format fit into the picture. I won't attempt to cover the subject in detail an entire book could be written on it-and, indeed, Charles Poynton has done just that. To understand the solutions, one must first become acquainted with the problems. As recently proposed standards are approved and implemented in hardware, from graphics cards, to monitors, to printers and scanners, there is reason to expect that platform-independent color will become the norm, not the exception, in the new millennium. PNG certainly doesn't solve all of these problems, but it does provide image authors with the means to minimize many of them, as long as the editing and viewing software is written properly. And in the absence of tedious calibration procedures and high-end color-conversion software, what comes out of the printer is, at best, only a vague approximation of what the screen shows. Even on a single machine there are usually obvious changes in brightness and color as the monitor (CRT) warms up, not to mention when the user adjusts the screen controls. A pure yellow on one machine may have an orange or greenish tint on another. Images created on Macs tend to look too dark on PCs images created on PCs tend to look too bright and washed out on Macs. Gamma Correction and Precision ColorĪnyone who has transferred images between a PC and a Macintosh-or even simply viewed on one platform an image created on another-has probably noticed one of the little gotchas of the computer world: images don't look the same on all systems.
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