Hi, all I use the GMT to plot the Sentinel-2 optical image data. The region is over the Arctic, so I choose some map projection suitable for there. -Q in grdimage could plot the NaN value as transparent. But in this projection (-JB), the transparent is not perfect. See: ps=bigmap.ps J=B80/82/80/8…

FYI, I opened the tif in PhotoShop to verify that those patches are 1,1,1.

Hi @yangleir , can you please check whether the result is the same when you put a space in between -Q and the redirect? Currently it looks like there is no space, which could cause problems.

[image] yangleir: gmt grdimage sub_low_byte.tif+b0,1,2, -Rsub_low_byte.tif -J$J -K -P -Q>$ps Crashes for me too so we will figure this out - but we also need to wrap up a proposal so it may take a few days.

What does -Q mean if input is a r,g,b image? Nothing I think and should not apply, no?

Perhaps he means black (0,0,0) should be transparent? But that is not what -Q is set up to do. (0,0,0) is a color, not a NaN.

Thanks! I use the GMT6.1.1. Not the latest version. Maybe that is why -Q crash for you. Also, the tiff data maybe in chaos. The black is 0,0,0 and not NaN. I will check it.

After fixing the crash with -Q I just made a plot using a linear projection so that there is no reprojection: This will check if the black pixels are turned to transparent. It mostly work but there are patches in the image that has pixels set to (1,1,1) instead - these show up as black below. So n…

Hi, @pwessel , Many thanks! Yes, I try the -JM projection, too. The black pixels are also there. So, it is the data problem. Not the GMT.

Now the map is nice. I use gdal to change the original tiff to Byte type. gdal_translate -ot Byte -q -scale 0 1 0 255 sub_seaice.tif sub_low_byte.tif The black pixels reason is that -scale 0 1 0 255 should be used instead of -scale 0 1 1 255. [bigmap]

There are still strange things in this case. See gmt grdimage -R39.27833884/120.843316067767/77.3124277327767/88.19456263 -JD80.0608274538835/82.75349518138835/79.12611688231391/90.0082517795372/12c -Baf -BWSen -Q sub_low_byte.tif -P > lixo.ps [image] but when converted gmt psconvert lixo.ps …

NaN color by grdimage with Arctic Polar pojection

Q&A

Joaquim January 11, 2021, 8:15pm 3

No, the -Q>$ps construct poses no problem but something in recent grdimage revamp may be causing a problem. Using -Q is crashing for me.

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NaN color by grdimage with Arctic Polar pojection

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