[gdal-dev] Contrast, Brightness and Gamma

[Saâd HESSANE]

Hi,

I think the method described by Chaitanya in the VRT : "<LUT>[src value
1]:[dest value 1],[src value 2]:[dest value 2]</LUT>"  is equivalent to do
"-scale srcMin srcMax dstMin dstMax", right?
So why when I do "-scale 0 65535 0 120" the output have pixels values
greater than 120 ?

Thank you.



2012/3/20 Chaitanya kumar CH <chaitanya.ch at gmail.com>

> Saâd,
>
> Gamma correction is more related to displaying the data than the data
> itself.
> You can, however, modify the contrast and brightness of the dataset.
>
> Using LUT is not hard to learn as long as you know what you want.
> A simple method is the stretch the data linearly along the whole range.
> For that, your LUT element looks like this:
>
> <LUT>[src value 1]:[dest value 1],[src value 2]:[dest value 2]</LUT>
>
> There are only four values.
> src value 1: This should be the least value of the pixels in the source
> raster band.
> src value 2: This should be the greatest value of the pixels in the source
> raster band.
> dest value 1: This should be the least value the data type of the raster
> band can hold. That would usually be zero.
> dest value 2: This should be the greatest value the data type of the
> raster band can hold. That would be 255, 65535 and 4294967295 for 8bit,
> 16bit and 32bit raster bands.
>
> The VRT driver calculates the intermediate values. The result here would
> be similar to using the scale ratio and offset parameters.
>
> On Tue, Mar 20, 2012 at 9:25 PM, Saâd HESSANE <saad.hessane at gmail.com>wrote:
>
>> Hi list,
>>
>> I use gdal_translate to convert 16bits images to 8bits images. The
>> gdal_translate have the -scale argument to specify the convertion range
>> (source->destination).
>> If I change the destination range (dst_min and dst_max), I can do
>> manually a brightness correction (also a contrast correction).
>>
>> But is there any way to do that with a Photoshop like method?
>> For example set a brightness to -50, a contrast to +8 and let
>> gdal_translate do the job ?
>>
>> Also can I apply a gamma correction?
>>
>> I saw the VRT format and there's a way to apply a LUT. But I don't think
>> a LUT of 65536 values is the best way to apply the correction.
>>
>> Also the VRT have the scale ratio and scale offset elements. I think it
>> can be useful but I don't understand how use it...
>>
>> So simply put, my question : can I do a contrast, brightness or gamma
>> correction to a raster with gdal?
>>
>> Thank you !
>>
>> PS : I apologize for my broken English !
>>
>> _______________________________________________
>> gdal-dev mailing list
>> gdal-dev at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/gdal-dev
>>
>
>
>
> --
> Best regards,
> Chaitanya kumar CH.
>
> +91-9494447584
> 17.2416N 80.1426E
>
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