[fitsbits] Images getting shifted and wrapping on themselves...?

[William Pence]

gberz3 wrote:
> On Aug 27, 9:33 am, William Pence <William.Pe... at nasa.gov> wrote:
> 
>>A phase shift like you describe in the image could be caused if the byte
>>offset in the file to the start of the image bytes is calculated
>>incorrectly.  The image always starts at the beginning of a 2880-byte
>>FITS block, e.g., at byte 2880, 5760, 8640, ..., etc.  and is the first
>>FITS block following the header block that contains the END keyword.
> 
> 
> 
> Right, and I've verified the 2880 lengths.  However, if it were an
> offset issue, then it would simply chop off the remainder, not wrap it
> around.  Is that not correct?  

That is not correct.  If there are extra bytes at the beginning of the 
image data array, this will shift the image in all the rows to the 
right, and the pixels that fall off the right hand edge will be wrapped 
around and appear on the left side of the image on the next row.

When you display the image, there are probably some non-image pixels 
(maybe all with zero value) in the first or last row(s) of the image. 
If you count the number of these extraneous pixels, and multiply by the 
pixel byte size, this will tell you the size of the byte offset error. 
If the blank pixels are at the beginning (bottom) of the image, your 
calculated byte offset to the start of the image is too small, and the 
opposite if the blank pixels are in the last row (top) of the image.