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NVIDIA HPDR: Lighting Gets More Realistic

Previous generation graphics processors from NVIDIA didn’t support information output from the pixel shader to a few buffers simultaneously (Multiple Render Targets) and data rendering into a buffer in floating-point representation (FP Render Target). ATI graphics chips family supported these features from the very beginning, which made an advantageous difference from NVIDIA’s solutions.

NV40 has finally acquired full support of the Multiple Render targets and FP Render Target, which allowed the company marketing people to introduce a new term: NVIDIA HPDR. This abbreviation stands for High-Precision Dynamic-Range, i.e. the ability to build a scene with high dynamic lighting range (HDRI, High Dynamic Range Images).

The major idea of the HDRI is very simple: the lighting parameters (color and intensity) of the pixels forming the images should be described with real physical terms. To get what this actually means, you should recall the today’s approach to image description model.

RGB Model and Our Eyes

The today’s universal image description model is an additive hardware dependent RGB (Red, Green, Blue) model, which was first developed for such display devices as CRT (Cathode Ray Tube), i.e. the regular computer monitor. According to this model, any color can be represented as a sum of three basic colors: Red, Green and Blue with properly selected intensities. The intensity of each basic color is split into 256 shades (intensity gradations).

The number 256 is quite a randomly selected one and appeared as a compromise between the computer graphics subsystem performance, photorealistic image requirements and binary nature of all computer calculations. In particular, they found out that 16.7 million shades (256x256x256) are more than enough for images with photographic quality. Moreover, 256 can be easily codes in the binary system as 2^8, i.e. 1 byte.

So, according to the RGB model, black color looks like (0, 0, 0), i.e. there is no intensity at all, while white color looks like (255, 255, 255), which is the maximum intensity possible for all three basic colors.

Of course, any color in RGB model will be described with an integer triad. Note that floating point numbers (such as 1.6 or 25.4, for instance) cannot be used within this model, and the numbers used are kind of “fake”, i.e. they have nothing to do with real physical lighting parameters.

One more interesting feature of the 8-bit intensity representation is its discrete character. The maximum screen brightness of contemporary monitors is known to be around 100-120cd/m^2. If we split this value into 256 shades, we will get about 0.47cd/m^2, which is the brightness interval between the two nearest shades. This way, the monitor brightness is discreet and this sampling rate (which we can also call a threshold of sensitivity to brightness gradients) equals 0.47cd/m^2 if we set the monitor brightness to the maximum, and around 0.4 cd/m^2 is the brightness is set to 70-80%.

On the other hand, the dynamic range of human eye lies between 10^6 and 10^8 cd/m^2, i.e. it makes 100,000,000,000,000:1 or 14 orders. Although human eye cannot see the light from this entire range at the same time: the maximum intensity level visible for a human eye at a time makes around 10,000:1. And since human eyesight tracks the light intensity and color separately, the entire color gamma your eye can perceive makes 10,000 brightness shades x 10,000 color shades, which equals 10^8 colors.

Another important peculiarity of human eyes is the threshold of sensitivity or the minimal change of the lighting intensity perceivable by the human eye (brightness resolution). The value of this threshold depends on the light intensity and grows up as the latter increases. From 0.01 to 100cd/m^2 the dependence of the intensity on the threshold value is constant (Weber’s law) and equals 0.02cd/m^2. In other words, the threshold of sensitivity for 1cd/m^2 light intensity makes 0.02cd/m^2, for 10 – 0.2cd/m^2, for 50 - 1cd/m^2, and for 100 - 2cd/m^2. The remaining part of the intensity range doesn’t follow this rule and the dependence in this case can be described with a more complicated rule.

Of course, the dynamic monitor range (and the RGB model description) is not enough to represent all real world images or at least that part of it, which a human eye can perceive. The typical consequence of that is the “removal” of all intensities from the upper and lower part of the range. An example here could be a room with the open window on a sunny summer day. The monitor will correctly display either the room interior or the part of the outdoor scene, which you can see through the window.

 
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