2009.09.10 13:51 "Re: [Tiff] Manually convert 16-bit grayscale image data to 8-bit grayscale in Borland c++", by Evgenia Gurova

For converting 16-bit TIFF images to 8-bit I successfully use ImageMagic: convert -depth 8 ifile ofile.

The only option - ImageMagic "kills" geo-tag, so if you have geo-tiff, you need first write geo-tag to a txt file (with "listgeo") and after processing with ImageMagic - write it back (with "geotifcp"). I do it on Ubuntu Linux, works perfect!

I have been using this website and mail archieve exclusively finding answers to the questions I had recently. Thanks a bunch for your knowledge. However, I am stuck with this issue i cannot find and answer to because everyone is using the libtiff library to perform the action.

I am creating an application to manually read in the bits from the file into memory to be displayed in Borland (I have to read certain segment of a file given an offset and extract an (x,y) size sub-image of an image. What I mean by manually, is I parse through the header and footer myself using fseek() and fread(), and use the stripOffset and StripByteCount to perform a fread() for each strip of image data.

How on earth may I be able to convert my 16-bit grayscale image data to 8-bit? I have been able to take 2 bytes to a short value, then truncate it to a single byte like so:

 unsigned short value16 = *TmpBits++;
value16 = value16 + (*TmpBits++ << 8)
pBits[curPix++] = (value16/257. + .5) //convert value to double and round up.

where pBits is my BYTE array to hold image data and TmpBits is another BYTE array of size StripByteCount.

When I dislpay my data in Borland through a TImage object, I will recieve my image (meaning, you can make out my object I am looking at, but the gray values for each pixel is way off. Sometimes too much white and some pixels are correct for black) but the gray coloration is way off. I have to do this because in Borland, to create my HBITMAP from a bmp object it must be in 1,4,8 or 24 bpp image. I then pass this created HBITMAP to an TBitmap object and associate this Bitmap to an TImage for display purposes.

It works well with 8-bit images, but I cannot get the 16-bit data to work. Here is my tag information to help out what my issue is:

NewSubfileType 254 LONG 1 0

ImageWidth 256 LONG 1 14592

ImageLength 257 LONG 1 6940

BitsPerSample 258 SHORT 1 16

Compression 259 SHORT 1 1

PhotometricInterpretation     262 SHORT 1 1

ImageDescription 270 ASCII 13 Space Imaging

StripOffsets 273 LONG 6940 56040 85224 114408 143592...

SamplesPerPixel 277 SHORT 1 1

RowsPerStrip 278 SHORT 1 1

StripByteCounts 279 LONG 6940 29184 29184 29184 29184...

MinSampleValue                280 SHORT 1 0

> MaxSampleValue               281        SHORT         1         2047

> XResolution                        282        RATIONAL      1         1/1

> YRresolution                      283        RATIONAL      1         1/1
> ResolutionUnit                    296        SHORT         1         1

> DateTime                           306        ASCII         20        
> 2001:01:11 18:50:28
> Copyright                          33432      ASCII         24        

> (c) Space Imaging LLC |
>
> It is a geotiff image, but that is irrelevent. Please help me
> understand this issue.
> Thanks in advance.
> Robert
>
>
>
> ------------------------------------------------------------------------

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