plogtransform

Performs logarithmic and exponential transforms of the gray-levels.

Synopsis

plogtransform inverse min max [-m mask] [im_in|-] [im_out|-]

Description

plogtransform expands the gray-levels of the input image using a logarithmic or an exponential transform of the gray-levels. The parameter inverse specifies whether the transform is logarithmic or exponential.

The effect of the logarithmic transform is to map a narrow range of low gray-level values in the input image into a wider range of output levels. The inverse exponential transform is true for higher values of input levels.

The logarithmic transform of pixel 'p' has the form:

im_out[p]=(c*log(im_in[p]-smin+1.0)) + min;
c=(max-min) / (log(smax-smin+1.0))

where smin and smax are the minimum and the maximum values of the input image, and c is a normalization factor for stretching output values between min and max.

The exponential transform of pixel 'p' has the form:

im_out[p]=exp((im_in[p]-smin)/c) -1.0 + min;
c=(smax-smin) / (log(max-min+1.0))

where smin and smax are the minimum and the maximum values of the input image, and c is a normalization factor for stretching output values between min and max.

For color and multispectral images, the transform uses the vectorial approach: the min and max values are calculed from all the bands, and then each band is stretched with the same transform.

Parameters

inverse is an integer in [0, 1] that specifies whether the transform is logarithmic (inverse=0) or exponential (inverse=1).
min and max specify the bounds of the output pixel value. They are related to the type of the input image.
Note: if min < max then min and max are set with the minimum and maximum values of the input image type; for example, 0 and 255 for Uchar images.

Inputs

im_in: an image.

Outputs

im_out: an image with the same properties as im_in.

Result

Returns SUCCESS or FAILURE in case of invalid parameter values.

Examples

Applies a logarithmic transform followed by an exponential transform to create image b.pan. Because exp is the inverse transform of log, b.pan is (almost) equal to tangram.pan (due to rounding error).

   plogtransform 0 0 255 tangram.pan a.pan
   plogtransform 1 28 165 a.pan b.pan

Applies a logarithmic transform to create a.pan and uses the min and max values of the image type as new bounds.

   plogtransform 0 1 -1 tangram.pan a.pan

C++ prototype

Errc PLogTransform( const Img2duc &im_in, const Img2duc &im_out, int inverse, float min, float max );

Version française

Transformations des niveaux de gris par loi logarithmique ou exponentielle.

Author: Régis Clouard