7/25/2019 Optimized Visual Rendering

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Optimized Visual Rendering

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Optimized Visual Rendering(OVR) is the latest option for radiometric enhancement of Pleiadesimagery. OVR is an option perfectly suited to display the imagery within applications and directintegration into GIS. The OVR process includes:

Radiometric adjustment: color stretching and contrast enhancement

Atmospheric offset subtraction

Process and Encoding:

The process is done for each individual product for optimum results (the continuity of visualrendering is not ensured between separate products). For tiled products, OVR rendering ishomogeneous on all tiles.

For JPEG 2000 12-bit / GeoTIFF 16-bit products without OVR, the dynamic rangecorresponds to the dynamic range at acquisition (12-bit contains 4096 values).

With OVR option, images are encoded in 16 bits with 16-bit depth dynamic range (65536values).

The pixel values of images delivered with no OVR are raw digital counts as imaged and coded at thesatellite sensor. These values are useful for customers whose applications require detailedknowledge of the sensor. In contrast to a visual application, their proper rendering on the screen canbe difficult to achieve with standard display software. OVR formats the image with immediate visualrendering for customers who do not want to, or cannot, carry out elaborate display adjustments. Themain goals of OVR are to:

- Standardize the display color rendering in consistency with its spectral band characteristics.- Simplify the user's final brightness/contrast adjustment via a simple, standard tool such as a

cursor.- Distribute the values of the whole display range used by customer software (256 values per

channel for 8-bit, 65535 values per channel for > 8-bit) and thus use all the colors available inthis range.

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Enhance the image brightness and contrast for an initially balanced direct display.- Minimize some of the atmosphere content related effects to lend towards ground visual

rendering.

OVR uses the radiometric calibration of each band, in other words, OVR is based on the spectralresponse in the visible spectrum (R, G, B) and near infrared (NIR) spectrum. This calibration is notstrictly the colorimetry seen on the ground, as the characteristics of the atmosphere are not known atthe time the acquisition is taken (the calibration is known as Top of Atmosphere). The atmosphere is amajor contributor to the colorimetry seen on the ground as it acts as a filter that can disrupt lightradiation. As OVR also performs dynamic matching to optimize visual rendering, the pixel values nolonger have any meaning for conversion into a lighting brightness or color physical unit (radiance,luminance, etc.). Some radiometric information thus becomes obsolete after OVR application and it isno longer provided in the DIMAP file (i.e. Radiometric_Calibration, Histogram_Band_List).

One aim of OVR is to use the full display range of software. This avoids manual adjustments. Sincecommercial off-the-shelf software handles bits coded internally as bytes, this range is a binary powerwhich is a multiple of 8: whence 256= 2^8, 65536=2^16, 16777216=2^24, etc. If we had kept the 12-bit dynamic range, we would have only used the first 1/16th of the useful range (65536/4096) andthus only the dark shades of a standard Look Up Tables (LUT) would have been displayed. Bydistributing these over 16 bits, the whole display range is used and thus all the shades of a standardLUT, from dark to light. There is no change for 8-bit products as they already use the full availablerange.

The native characteristics are altered with a nonlinear correction, which depends on the image. Thiscorrection cannot be characterized and the original digital number values cannot be recovered. Forthis reason, OVR option is not recommended for radiometric measurement and/or processing for

applications such as agriculture, forestry, hydrology, automatic change detection, automaticclassification and feature extraction.