HDRI: High Dynamic Range Imaging

14 flashcards · Computer Science · by lakesare

What's the common example of low dynamic range images losing information?
Room with the window. Humans are easily able to see details of both the indoor part and the outside part. A conventional photograph typically does not capture this full range of information—the photographer has to choose whether the indoor or the outdoor part of the scene is properly exposed.
What does HDR stand for?
High-dynamic-range imaging (HDRI)
What's tone-mapping?
Compressing the range of values of an image for the purpose of display on a low dynamic range display device. The process of reducing the range of values in an HDR image such that the result becomes displayable in some meaningful way is called “dynamic range reduction.” Specific algorithms that achieve dynamic range reduction are referred to as “tone-mapping operators” or “tone-reproduction operators.” The display of a tone-mapped image should perceptually match the depicted scene.
Name 2 types of tone reproduction.
Spatially uniform - apply the same transformation to every pixel. Also known as single-scale or global.This often results in a tone mapped image that looks 'flat', having lost its local details.Spatially varying - apply a different scale to different parts of an image. Also known as multi-scale or local. Local tone mapping operators consider pixel neighbourhood information for each individual pixel, which simulates the adaptive and local properties of the human vision system. This can lead to b…
What are HDR images? (High Dynamic Range)
Images that store a depiction of the scene in a range of intensities commensurate with the scene are called HDR. However, images suitable for display with current display technology are called LDR (Low Dynamic Range). HDR images and video are matched to the scenes they depict, rather than the display devices on which they are meant to be displayed. This benefits most image processing that may be applied during the lifetime of an image.
If each channel (red, green, blue) is represented by 1 byte, in a pixel, - how many colors can this pixel display?
8 bits * 3 channels = 24 places where 0 or 1 can occur. 224 = 16 777 216 colors. Currently, the vast majority of color images are represented with a byte per pixel for each of the red, green, and blue channels. With three bytes per pixel, about 16.7 million different colors can be assigned to each pixel. This is known in many software packages as "millions of colors".
What's white balancing?
Cameras often operate under the assumption that the scene is lit by a specific light source, such as a D65. White balancing is the act of adjusting to some other light source. White balance (WB) is the process of removing unrealistic color casts, so that objects which appear white in person are rendered white in your photo. Proper camera white balance has to take into account the "color temperature" of a light source, which refers to the relative warmth or coolness of white light. Humans are ca…
What does this graph demonstrate? Scatter plots of RGB pixels randomly selected from the image at the top.
For natural images, if a pixel of a natural image has a large value for the red component, - you're likely to have a large value for the green and blue components too. Thus, the three channels are highly correlated.
What does it mean to have a perceptually uniform color space? The aim of both CIE 1976 L∗u∗v∗ and CIE 1976 L∗a∗b∗ (abbreviated to CIELUV and CIELAB) is to provide perceptually uniform color spaces.
This means that Euclidian distances in such spaces can be used to assess perceived color differences. Both CIELUV and CIELAB are perceptually uniform to a good degree, but based on the newer psychophysical data, it has become apparent that some nonuniformities remain.
The aim of both CIE 1976 L∗u∗v∗ and CIE 1976 L∗a∗b∗ (abbreviated to CIELUV and CIELAB) is to provide perceptually uniform color spaces. Why do both CIELAB and CIELUV exist?
The reason for the existence of both of these color spaces is largely historical. Both color spaces are in use today, with CIELUV more common in the television and display industries, and CIELAB in the printing and materials industries. Both CIELUV and CIELAB are perceptually uniform to a good degree, but based on the newer psychophysical data, it has become apparent that some nonuniformities remain. For this reason, the more recent IPT color space was developed.
What is the observer adapted to when viewing: natural scene photo of a natural scene on display
natural scene - adapted to scene. natural scene on display - to the environment AND to the scene on display. In the case of viewing a scene directly, the observer will be adapted to the scene. When looking at an image of a display, the observer will be adapted to the light emitted from the display, as well as the environment in which the observer is located.
Why will there always be a need for dynamic range reduction?
Although HDR display technology will become generally available in the near future, it will take time before most users have made the transition. At the same time, printed media will never become HDR, as this would entail the invention of light-emitting paper. As a result, there will always be a need to prepare HDR imagery for display on LDR devices.
What's the ratio of luminous intensity the eye can percept in a single view VS displayable on LCD monitor?
Luminous intensity – the power of a light source – is measured in candelas per square metre (cd/m2 ). The human visual system can accommodate a wide range of luminance in a single view, around 10 000:1 cd/m2 , and our eye adapts to our surroundings, changing what we see over time. The dynamic range in the real world is far greater than the range that can be produced on most electronic displays. Luminous intensity – the power of a light source – is measured in candelas per square metre (cd/m2 ).…
Name 3 HDR file formats.
Radiance RGBE - [.hdr] OpenEXR - [.exr] Tiff/Psd - [.tiff, .psd]