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2010.07.20 18:04 "fromskew and toskew parameters", by Richard Nolde
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