[gdal-dev] Weird GCPs behavior when flipping an image (left-to-right or upside-down)

[Bugbuster]

Dear all,

I would like to flip a "plain TIFF" (no geolocation information) image with
respect to either lines or columns.
After several tests, I finally found out I have two ways to do it :
1) with a temporary TIFF image
First create a temporary TIFF image with 3 GCPs with gdal_translate
Then create the final "plain TIFF" flipped image with gdalwarp
2) with a temporary VRT image
First create a temporary VRT image with 3 GCPs with gdalbuiltvrt 
Then create the final "plain TIFF" flipped image with gdalwarp

Both ways give the same results, although the 2nd one is more efficient with
large images.

However, I bumped into some weird results. Here is a small example to
illustrate :
1) I create a TIFF image with 3 GCPs which simulate the identity
transformation :
*gdal_translate -of GTiff -gcp 0 0 0 0  -gcp 100 0 100 0 -gcp 0 100 0 100
TIF_plain.tif TIF_identity_gcp.tif*
2) I warp the intermediate image with the folowing command :
*gdalwarp -of GTiff -co PROFILE=BASELINE TIF_identity_gcp.tif
TIF_identity_warped.tif*

Surprisingly, the final image image TIF_identity_warped.tif is flipped with
respect to lines (upside-down).

Did I miss something ?

Thanks for your hints
Philippe




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