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2010.07.20 19:10 "Re: fromskew and toskew parameters", by Toby Thain
On 20-Jul-10, at 2:57 PM, Sergius Bobrovsky wrote:
> Hello Richard,
>
> Thank you for your answer.
> I think I should summarize information from you, Edward Grissom (he
> sent me private e-mail) and own findings:
>
> 1) toskew is an offset for 'uint32* cp' param in both
> DECLAREContigPutFunc and DECLARESepPutFunc.
>
> 2) fromskew is an offset for 'unsigned char* pp' param in
> DECLAREContigPutFunc and for 'unsigned char* r', 'unsigned char* g',
> 'unsigned char* b', 'unsigned char* a' in DECLARESepPutFunc.
>
> 3) fromskew is usually 0. it is greater than 0 if width of strip being
> converted is greater than image width or part of the tile being
> converted is outside the image (may be true for tiles on the right and
> bottom edge of the image). In other words, fromskew is used to make up
> for any padding on the end of each line of the input buffer.
>
> 4) toskew is 0 if width of tile being converted is equal to image
> width and image data should not be flipped vertically. In other
> circumstances toskew is used to make up for any padding on the end of
> each line of the input buffer and/or for flipping purposes.
>
^^ Might be a nice addition to the source commentary &/or doc.
--T
> Please correct me, if I am wrong. And thanks again to everyone.
>
> On Wed, Jul 21, 2010 at 1:04 AM, Richard Nolde <richard.nolde@cybox.com
> > wrote:
>> On Sun, Jul 18, 2010 at 6:46 AM, Sergius Bobrovsky
>>> <sergius.bobrovsky@gmail.com> wrote:
>>>
>>>> Hello!
>>>>
>>>> I'm trying to wrap my mind around "put routines" in
>>>> tif_getimage.c and
>>>> it looks like I can't understand what do fromskew and toskew
>>>> params in
>>>> DECLAREContigPutFunc mean.
>>>>
>>>> Can someone explain me the meaning of this parameters?
>>>>
>>>> --
>>>> Sergius Bobrovsky
>>>>
>> If you look at tiffcp and/or tiffcrop, you will see similar
>> definitions. The original author, Sam?, used a rather opaque
>> syntax to
>> provide function definitions for a variety of input/output modes
>> rather
>> than defining each one individually as I have chosen to do in
>> tiffcrop.
>> I believe that you will find the skew parameters are the buffer
>> offsets
>> between the start of successive read/write positions in a strip or
>> tile. The strip or tile width is NOT the same, as there can be end
>> of
>> line padding to fill a byte or as required for certain compression
>> schemes and tile layouts.
>>
>> In tif_getimage.c look at the function. Note that from/to skew are
>> only
>> used in the case of flipping the image vertically. Since data are
>> being
>> read from the start of the line to the end of the line, you back up
>> two
>> scanlines after having done a read to get to the position where the
>> next
>> read will take place if you are reading from the bottom of the image
>> back to the top of the image instead of in normal top to bottom
>> order.
>>
>> gtStripContig(TIFFRGBAImage* img, uint32* raster, uint32 w, uint32 h)
>> {
>> ...
>> if (flip & FLIP_VERTICALLY) {
>> y = h - 1;
>> toskew = -(int32)(w + w);
>> } else {
>> y = 0;
>> toskew = -(int32)(w - w);
>> }
>>
>> TIFFGetFieldDefaulted(tif, TIFFTAG_ROWSPERSTRIP,
>> &rowsperstrip);
>> TIFFGetFieldDefaulted(tif, TIFFTAG_YCBCRSUBSAMPLING,
>> &subsamplinghor, &subsamplingver);
>> scanline = TIFFNewScanlineSize(tif);
>> fromskew = (w < imagewidth ? imagewidth - w : 0);
>> for (row = 0; row < h; row += nrow)
>> {
>> rowstoread = rowsperstrip - (row + img->row_offset) %
>> rowsperstrip;
>> nrow = (row + rowstoread > h ? h - row : rowstoread);
>> nrowsub = nrow;
>> if ((nrowsub%subsamplingver)!=0)
>> nrowsub+=subsamplingver-nrowsub
>> %subsamplingver;
>> if (TIFFReadEncodedStrip(tif,
>> TIFFComputeStrip(tif,row+img->row_offset, 0),
>> buf,
>> ((row + img->row_offset)%rowsperstrip +
>> nrowsub) *
>> scanline) < 0
>> && img->stoponerr)
>> {
>> ret = 0;
>> break;
>> }
>>
>> pos = ((row + img->row_offset) % rowsperstrip) *
>> scanline;
>> (*put)(img, raster+y*w, 0, y, w, nrow, fromskew,
>> toskew, buf + pos);
>> y += (flip & FLIP_VERTICALLY ? -(int32) nrow :
>> (int32)
>> nrow);
>> }
>>
>> Richard Nolde
>>
>> _______________________________________________
>> Tiff mailing list: Tiff@lists.maptools.org
>> http://lists.maptools.org/mailman/listinfo/tiff
>> http://www.remotesensing.org/libtiff/
>>
>
>
>
> --
> Sergius Bobrovsky
> _______________________________________________
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> http://lists.maptools.org/mailman/listinfo/tiff
> http://www.remotesensing.org/libtiff/