Point Grey Grasshopper - Lookup Table and Gamma

Point Grey Research Grasshopper Technical Reference
Camera Operations and Features
Revised 26-Nov-10
Copyright (c) 2010 Point Grey Research Inc.
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section for further information on acquiring images using these modes. An alternative to this is to
use the Bayer_Mono_Ctrl bit [24] of the IMAGE_DATA_FORMAT register 0x1048. Setting this bit
to 1 enables raw Bayer output in non-Format_7 Y8 / Y16 modes, or Format_7 Mono8 / Mono16
modes.
The actual physical arrangement of the red, green and blue "pixels" for a given camera is
determined by the arrangement of the color filter arrays on the imaging sensor itself. For
example, the CFA format of a Sony ICX204AK color CCD can be found in the "Block Diagram
and Pin Configuration" section of its
datasheet. The format (i.e. order) in which this raw color data
is streamed out, however, depends on the specific camera model and firmware version. This
format can be queried using the BAYER_TILE_MAPPING register 0x1040 that is implemented on
all PGR cameras.
Raw image data can be accessed programmatically via the pData pointer in the FlyCaptureImage
structure (e.g. FlyCaptureImage.pData). In Raw8 modes, the first byte represents the pixel at
(row 0, column 0), the second byte at (row 0, column 1), etc. In the case of a 1600x1200 color
Grasshopper that is streaming out Raw8 image data in RGGB format, if we access the image
data via the pData pointer we have the following:
• pData[0] = Row 0, Column 0 = red pixel (R)
• pData[1] = Row 0, Column 1 = green pixel (G)
• pData[1600] = Row 1, Column 0 = green pixel (G)
• pData[1601] = Row 1, Column 1 = blue pixel (B)
Related Knowledge Base Articles
ID Title URL
33 Different color processing
algorithms.
www.ptgrey.com/support/kb/index.asp?a=4&q=33
37 Writing color processing software
and color interpolation
algorithms.
www.ptgrey.com/support/kb/index.asp?a=4&q=37
89 How is color processing
performed on my camera’s
images?
www.ptgrey.com/support/kb/index.asp?a=4&q=89
4.6.2. Lookup Table and Gamma
The Grasshopper supports lookup table (LUT) and gamma functionality. CCD manufacturers
strive to make the transfer characteristics of CCDs inherently linear, which means that as the
number of photons hitting the imaging sensor increases, the resulting image intensity increases
will be linear.
The Grasshopper also provides an 11-bit input lookup table that produces 9-bit outputs. Although
the camera uses a 14-bit A/D converter, the Grasshopper is only able to accommodate 11-bit
images. The LUT therefore allows the user to map the 2
11
, or 2048, different possible pixel values
to any one of 512 (2
9
) possible output values. For example, the LUT would allow the user to map
any pixel with a value of 2047 (white) to any value between 0 (black) and 512 (white). On color
cameras, there are separate LUT channels for each entry, representing red, green, and blue
values. Monochrome cameras have a single grayscale channel per entry. Color cameras in a
monochrome (Y8) mode still require a three-channel RGB LUT because the monochrome image
comes from a weighted sum of the red, green and blue pixel values (see Color and Greyscale
Conversion) after the LUT has been applied.