Datenbank Glossar – CGI

渲染

渲染 is the process of generating an image from a model, by means of computer programs. The model is a description of three-dimensional objects in a strictly defined language or data structure. It would contain geometry, viewpoint, texture, lighting, and shading information. The image is a digital image or raster graphicsimage. The term may be by analogy with an “artist’s rendering” of a scene. ‘Rendering’ is also used to describe the process of calculating effects in a video editing file to produce final video output

High dynamic range imaging – 动态范围图像

The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to shadows.

In real world some areas are too bright for the eye to look at , and some areas that are too dark to see. This is referred to as RANGE of light (also referred to as luminance) which is visible to the human eye. The human eye can see a range of approx. 10,000:1 but this range can adapt over a 1,000,000:1 range . Currently our monitors, cameras and TV’s have very limited range, approx. 100:1, which is 1/100 of what we can see. Original JPEG and TIFF images could only encode a limited brightness range of 0-255 (i.e. the brightest point of only 255 times as bright as the darkest point).

In the world of computers a BYTE is limited to values between 0 – 255, so let’s say you store all your image information as BYTES and you brighten your image by 20 value points, any value that was greater than 235 now becomes 255 and all the detail that was stored between 235 – 255 is now lost. So if the same image was darkened by 20 points the image would make any value that was 255 equal to 235 (rather than a range of values between 235 and 255). This always leaves your image with a contrast reduction effect.

High dynamic range image files are special formats that accurately encode the extreme luminance of very dark and very light areas of the scene without any visible data loss. HDR images address two issues, the first is that it can now store values ranging from 1x10exp(+/-37) and the second is that the range is so large that programmers never have to clamp the values of their image to a fixed max value such as 255. So by using the example above if the value was increased and then decreased by 20 points the original values would be restored leaving the image unchanged. This sort of editing is referred to as “Non-Destructive”, since any change can be reversed without data loss.

HDR images are normally created using digital cameras that can record different exposure levels (some cameras even come with Auto-Bracketing), so that they can be combined later to make a very detailed images storing data for both very dark and bright areas. This leaves the user with all the control to decide the look of the final result.

OpenEXR

OpenEXR is a high dynamic-range (人类发展报告) image file format developed by Industrial Light & Magic for use in computer imaging applications.

OpenEXR is used by ILM on all motion pictures currently in production. The first movies to employ OpenEXR were Harry Potter and the Sorcerers Stone, Men in Black II, Gangs of New York, and Signs. Since then, OpenEXR has become ILM’s main image file format.

OpenEXR’s features include:

1. Higher dynamic range and color precision than existing 8- and 10-bit image file formats.

2. Support for 16-bit floating-point, 32-bit floating-point, and 32-bit integer pixels. The 16-bit floating-point format, called “half”, is compatible with the half data type in NVIDIA’s Cg graphics language and is supported natively on their new GeForce FX and Quadro FX 3D graphics solutions.

Multiple lossless image compression algorithms. Some of the included codecs can achieve 2:1 lossless compression ratios on images with film grain.

1. Extensibility. New compression codecs and image types can easily be added by extending the C++ classes included in the OpenEXR software distribution. New image attributes (strings, vectors, integers, etc.) can be added to OpenEXR image headers without affecting backward compatibility with existing OpenEXR applications.

Standard data resolutions

The following figures are the industry accepted digital equivalents for various analogue formats, and the standard formats for several native digital formats.