gamma_imagegamma_imageGammaImageGammaImage (Operator)

Name

gamma_imagegamma_imageGammaImageGammaImage — Perform a gamma encoding or decoding of an image.

Signature

gamma_image(Image : GammaImage : Gamma, Offset, Threshold, MaxGray, Encode : )

gamma_imagegamma_imageGammaImageGammaImageGammaImage performs a general gamma encoding or decoding of the input image ImageImageImageImageimage and returns the resulting image in GammaImageGammaImageGammaImageGammaImagegammaImage. A generalized gamma encoding can be described as follows:

Here, G' is the gamma-encoded gray value, G is the linear gray value, , o = OffsetOffsetOffsetOffsetoffset, m = MaxGrayMaxGrayMaxGrayMaxGraymaxGray, t = ThresholdThresholdThresholdThresholdthreshold, and s is a factor that is computed from GammaGammaGammaGammagamma, OffsetOffsetOffsetOffsetoffset, ThresholdThresholdThresholdThresholdthreshold, and MaxGrayMaxGrayMaxGrayMaxGraymaxGray in such a way that the linear and exponential parts of the transformation are continuous.

Analogously, a generalized gamma decoding can be described as follows:

where the variables have identical meanings as for the gamma encoding and .

For example, the gamma encoding or decoding required by the sRGB standard can be obtained by setting GammaGammaGammaGammagamma = 1.0/2.4, OffsetOffsetOffsetOffsetoffset = 0.055, and ThresholdThresholdThresholdThresholdthreshold = 0.0031308. Similarly, the gamma encoding or decoding required by the HDTV video standard can be obtained by setting GammaGammaGammaGammagamma = 0.45, OffsetOffsetOffsetOffsetoffset = 0.099, and ThresholdThresholdThresholdThresholdthreshold = 0.018. In any case, MaxGrayMaxGrayMaxGrayMaxGraymaxGray should be set as appropriate for the image type of ImageImageImageImageimage (e.g., 255.0 for byte images).

Attention

gamma_imagegamma_imageGammaImageGammaImageGammaImage can be executed on an OpenCL device for byte, uint2 and real images. As the operation is performed in single precision floating point instead of double precision as on the CPU, the result of the OpenCL implementation can vary slightly from that of the CPU implementation.

Execution Information

Supports OpenCL compute devices.
Multithreading type: reentrant (runs in parallel with non-exclusive operators).
Multithreading scope: global (may be called from any thread).
Automatically parallelized on tuple level.
Automatically parallelized on channel level.
Automatically parallelized on domain level.

Parameters

ImageImageImageImageimage (input_object) (multichannel-)image(-array) → object (byte* / uint2* / real*) *allowed for compute devices

Input image.

GammaImageGammaImageGammaImageGammaImagegammaImage (output_object) (multichannel-)image(-array) → object (byte / uint2 / real)

Output image.

GammaGammaGammaGammagamma (input_control) real → (real)

Gamma coefficient of the exponential part of the transformation.

Default value: 0.416666666667

Suggested values: 0.416666666667, 0.45

OffsetOffsetOffsetOffsetoffset (input_control) real → (real)

Offset of the exponential part of the transformation.

Default value: 0.055

Suggested values: 0.055, 0.099

ThresholdThresholdThresholdThresholdthreshold (input_control) real → (real)

Gray value for which the transformation switches from linear to exponential.

Default value: 0.0031308

Suggested values: 0.0031308, 0.018

MaxGrayMaxGrayMaxGrayMaxGraymaxGray (input_control) number → (real / integer)

Maximum gray value of the input image type.

Default value: 255.0

Suggested values: 1.0, 255.0, 511.0, 1023.0, 4095.0, 16383.0, 65535.0

EncodeEncodeEncodeEncodeencode (input_control) string → (string)

If 'true'"true""true""true""true", perform a gamma encoding, otherwise a gamma decoding.

Default value: 'true' "true" "true" "true" "true"

List of values: 'false'"false""false""false""false", 'true'"true""true""true""true"

Alternatives

pow_imagepow_imagePowImagePowImagePowImage

References

Erik Reinhard, Erum Arif Khan, Ahmet Oguz Akyüz, Garret M. Johnson: “Color Imaging: Fundamentals and Applications.” A K Peters, Ltd., 2008.
CEI/IEC 61966-2-1:1999: “Colour management -- Default RGB colour space -- sRGB.”
ITU-R BT.709-5: “Parameter values for the HDTV standards for production and international programme exchange.”

Module

Foundation

Operators