# GMT.jl: How to use a cpt with imshow?

**URL:** <https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571>\
**Category:** GMT.jl Q&A\
**Created:** [April 15, 2021, 11:25am UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571 "2021-04-15T11:25:45Z")\
**Posts on this page:** 15\
**Page:** 1

<div class="post-metadata">

**Author:** ![KristofKoch](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/kristofkoch/32/314_2.png) [@KristofKoch](https://forum.generic-mapping-tools.org/u/KristofKoch)\
**Post date:** [April 15, 2021, 11:25am UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/1 "2021-04-15T11:25:45Z")

</div>

I’m a Julia beginner and try to figure out how to get `imshow` to color my terrain:

`imshow(G3, fmt=:png, color="faa.cpt")`

Doesn’t work for me I end up with a grayscale image and the following message:

```
Warning: the following options were not consumed in grdimage => [:color]

```

How do I correctly specify a CPT?

---

<div class="post-metadata">

**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\
**Post date:** [April 15, 2021, 12:45pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/2 "2021-04-15T12:45:58Z")

</div>

Hmm, it should have worked. Must investigate why it didn’t.  
Notice, `imshow` is wrapper to `grdimage` and `grdview` and it does lots of guessing. Apparently some wizard power is failing in this case. I’m betting that’s because you are trying to plot an image and setting the cpt, which probably GMT itself does not accept. Now that I’m writing this it does ring a bell.

As a general advise to learn the GMT,jl way, use the option **Vd** to print the GMT command that is generated under the hood. **Vd=1** prints and executes, **Vd=2** just prints it. E.g.

```auto
imshow(G3, color="faa.cpt", Vd=2)
Warning: the following options were not consumed in grdimage => [:color]
"grdimage -R10.5098611111111/11.4101388888889/46.6298611111111/47.1401388888889 -JX14c/0 -Baf -BWSen -n+a -D -P -K > C:\\TMP\\GMTjl_tmp.ps"

```

The other advise is to read at least this

> **[Quick Learn · GMT](https://www.generic-mapping-tools.org/GMT.jl/dev/quick_learn/)**

---

<div class="post-metadata">

**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\
**Post date:** [April 15, 2021, 1:13pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/3 "2021-04-15T13:13:26Z")

</div>

Yes, that’s on purpose. You cannot set a CPT in `grdimage` when input is an image.  
But this case should caught earlier and print a meaningful error message. Please open an issue in GMT.jl for this

Besides, the solution for this case is failing too. And that would be to use the `image_cpt!` function. This should have worked

```auto
C = gmtread("faa.cpt");
image_cpt!(G3, C)
imshow(G3)

```

---

<div class="post-metadata">

**Author:** ![KristofKoch](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/kristofkoch/32/314_2.png) [@KristofKoch](https://forum.generic-mapping-tools.org/u/KristofKoch)\
**Post date:** [April 15, 2021, 2:42pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/4 "2021-04-15T14:42:25Z")

</div>

[Issue #574](https://github.com/GenericMappingTools/GMT.jl/issues/574) created.

---

<div class="post-metadata">

**Author:** ![KristofKoch](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/kristofkoch/32/314_2.png) [@KristofKoch](https://forum.generic-mapping-tools.org/u/KristofKoch)\
**Post date:** [April 15, 2021, 2:48pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/5 "2021-04-15T14:48:16Z")

</div>

> [@Joaquim](#):
>
> The other advise is to read at least this
> 
> ### [Quick Learn · GMT](https://www.generic-mapping-tools.org/GMT.jl/dev/quick_learn/)

Wise words … I skimmed it but more in-depth reading & learning is definitely necessary. Thank you for the `Vd=1` and `Vd=2` tips!

---

<div class="post-metadata">

**Author:** ![KristofKoch](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/kristofkoch/32/314_2.png) [@KristofKoch](https://forum.generic-mapping-tools.org/u/KristofKoch)\
**Post date:** [April 15, 2021, 6:34pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/6 "2021-04-15T18:34:54Z")

</div>

> [@Joaquim](#):
>
> Besides, the solution for this case is failing too.

Is that the error you are talking about?

```
G = gmtread("alps2.grd", layout="TRB");
G2 = GMT.texture_img(G);
makecpt(cmap=:dem4, cptname="my_dem4.cpt");
C = gmtread("my_dem4.cpt");
hill = gdaldem("alps2.grd", "hillshade", zfactor=2.5);
color = gdaldem("alps2.grd", "color-relief", C);
G3 = blendimg!(G2, hill, new=true, transparency=0.61);
G4 = blendimg!(color, G3, new=true);
imshow(G4, fmt=:png)

```

`ERROR: LoadError: MethodError: no method matching helper_run_GDAL_fun(::typeof(gdaldem), ::String, ::String, ::GMT.GMTcpt, ::String)`

---

<div class="post-metadata">

**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\
**Post date:** [April 15, 2021, 7:04pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/7 "2021-04-15T19:04:22Z")

</div>

Nope. The error in my example is much more illusive. Everything seems to run fine but the produced PS is corrupted. I’m afraid I’ll have to dig into the GMT guts to see what is failing.

Note that these two

```auto
makecpt(cmap=:dem4, cptname="my_dem4.cpt");
C = gmtread("my_dem4.cpt");

```

can be collapsed in

```auto
C = makecpt(cmap=:dem4);

```

All modules that produce files can be used to save that file in disk or just return the data to be used in Julia, wrapped in the [appropriate type](https://www.generic-mapping-tools.org/GMT.jl/dev/types/)

Will look why your example fail.

---

<div class="post-metadata">

**Author:** ![KristofKoch](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/kristofkoch/32/314_2.png) [@KristofKoch](https://forum.generic-mapping-tools.org/u/KristofKoch)\
**Post date:** [April 15, 2021, 7:22pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/8 "2021-04-15T19:22:40Z")

</div>

Ok, this is the current version:

```
using GMT
Gt = gmtread("alps2.grd", layout="TRB");
Gh = gmtread("alps2.grd");
G2 = GMT.texture_img(Gt);
C = makecpt(cmap=:dem4);
hill = gdaldem(Gh, "hillshade", zfactor=2.5);
color = gdaldem(Gh, "color-relief", C, Vd=1);
G3 = blendimg!(G2, hill, new=true, transparency=0.61);
G4 = blendimg!(color, G3, new=true);
imshow(G4, fmt=:png)

```

And this the full error message:

```
> julia alps.jl
ERROR: LoadError: MethodError: no method matching helper_run_GDAL_fun(::typeof(gdaldem), ::GMT.GMTgrid{Float32, 2}, ::String, ::GMT.GMTcpt, ::String, ::Pair{Symbol, Int64})
Closest candidates are:
  helper_run_GDAL_fun(::Function, ::Any, ::String, ::Vector{String}, ::String, ::Any...) at /home/kristof/.julia/packages/GMT/dCcJn/src/gdal_tools.jl:90
  helper_run_GDAL_fun(::Function, ::Any, ::String, ::Vector{String}) at /home/kristof/.julia/packages/GMT/dCcJn/src/gdal_tools.jl:90
Stacktrace:
 [1] gdaldem(indata::GMT.GMTgrid{Float32, 2}, method::String, opts::GMT.GMTcpt; dest::String, kwargs::Base.Iterators.Pairs{Symbol, Int64, Tuple{Symbol}, NamedTuple{(:Vd,), Tuple{Int64}}})
   @ GMT.Gdal ~/.julia/packages/GMT/dCcJn/src/gdal_tools.jl:85
 [2] top-level scope
   @ ~/Dokumente/GMT/teximg/alps.jl:7
in expression starting at /home/kristof/Dokumente/GMT/teximg/alps.jl:7
>

```

The line 7 it is complaining about is this one:

```
color = gdaldem(Gh, "color-relief", C, Vd=1);
```

---

<div class="post-metadata">

**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\
**Post date:** [April 15, 2021, 7:36pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/9 "2021-04-15T19:36:53Z")

</div>

First, in my defense, I’m trying to make the use of `gdaldem` more user friendly. Specially in light of GDAL claims that they support GMT CPT’s. which is basically false.

I think I know where problem is. `gdaldem(..., "hillshade", ...)` was tested and use smoothed out but `gdaldem(..., "color-relief", ...)` much less. I think that if you use `color="cptfilename.cpt"` it will work but passing the GMTcpt structure is not being recognized (or some shit around that)

Last, The **Vd** mechanism works only for the GMT modules calls, not for GDAL’s (different machinery and solution, if possible, not yet invented).

---

<div class="post-metadata">

**Author:** ![KristofKoch](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/kristofkoch/32/314_2.png) [@KristofKoch](https://forum.generic-mapping-tools.org/u/KristofKoch)\
**Post date:** [April 15, 2021, 8:34pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/10 "2021-04-15T20:34:24Z")

</div>

No need for defensive actions – I tip my hat for your work that went into GMT.jl and your ongoing support. Thank you! 👍

`color="cptfilename.cpt"` does indeed work.

Not that happy with the blending of the colored terrain and the hillshading and for some reason I can’t use `blendimg!(color, G3, new=true, transparency=0.4);` It seems to dislike the `transparency` part and throws an error.

---

<div class="post-metadata">

**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\
**Post date:** [April 15, 2021, 8:50pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/11 "2021-04-15T20:50:33Z")

</div>

No, you can’t use transparency when blending a color image and texture. It was not in the equation that Andreas posted. Nothing than cannot be changed but so far it’s like that.

Update your GMT.jl and you can now do

```auto
color = gdaldem("alps.grd", "color-relief", color=C);

```

To update the Julia wrapper you need to be using its master version. Don’t know if you have to uninstall the version you have (`] rm GMT`) before installing the master with  
`] add GMT#master`

---

<div class="post-metadata">

**Author:** ![pwessel](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/pwessel/32/30_2.png) [@pwessel](https://forum.generic-mapping-tools.org/u/pwessel)\
**Post date:** [April 15, 2021, 9:07pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/12 "2021-04-15T21:07:13Z")

</div>

You may be able to mix in some transparency via grdmix.

---

<div class="post-metadata">

**Author:** ![KristofKoch](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/kristofkoch/32/314_2.png) [@KristofKoch](https://forum.generic-mapping-tools.org/u/KristofKoch)\
**Post date:** [April 15, 2021, 9:07pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/13 "2021-04-15T21:07:14Z")

</div>

It works like a charm @Joaquim – thank you.

---

<div class="post-metadata">

**Author:** ![KristofKoch](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/kristofkoch/32/314_2.png) [@KristofKoch](https://forum.generic-mapping-tools.org/u/KristofKoch)\
**Post date:** [April 15, 2021, 9:32pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/14 "2021-04-15T21:32:26Z")

</div>

That’s a great idea – thank you @pwessel. It is not really transparency what I’m after, it is more of a weighting/mixing thing.

Prior to my first Julia steps I did quite some modifications to the intensity grid file with `grdmath` like compressing the light areas (\>0) into the (0, 0.1) range and the dark areas (\<0) into the (-0.7, 0) range.

I’m hoping that you and the core devs see _texture shading_ as a useful permanent addition to GMT leading to an easier integration into my existing scripts. That Julia language seems to be a quite useful rabbit hole to explore …

---

<div class="post-metadata">

**Author:** ![Joaquim](https://yyz1.discourse-cdn.com/flex047/user_avatar/forum.generic-mapping-tools.org/joaquim/32/16_2.png) [@Joaquim](https://forum.generic-mapping-tools.org/u/Joaquim)\
**Post date:** [April 15, 2021, 9:55pm UTC](https://forum.generic-mapping-tools.org/t/gmt-jl-how-to-use-a-cpt-with-imshow/1571/15 "2021-04-15T21:55:05Z")

</div>

> [@KristofKoch](#):
>
> That Julia language seems to be a quite useful rabbit hole to explore …

The hole texture thing (excluding the blending) in Julia boils down to

```auto
function texture_img(G::GMTgrid; detail=1.0, contrast=2.0, uint16=false)
	texture = deepcopy(G.z)
	terrain_filter(texture, detail, size(G,1), size(G,2), G.inc[1], G.inc[2], 0)
	(startswith(G.proj4, "+proj=merc")) && fix_mercator(texture, detail, size(G,1), size(G,2), G.range[3], G.range[4])
	terrain_image_data(texture, contrast, size(G,1), size(G,2), 0.0, (uint16) ? 65535.0 : 255.0)
	mat = (uint16) ? round.(UInt16, texture) : round.(UInt8, texture)
	Go = mat2img(mat, hdr=grid2pix(G), proj4=G.proj4, wkt=G.wkt, noconv=true, layout=G.layout*"a")
	Go.range[5:6] .= extrema(Go.image)
	Go
end

```

with these two wrapper functions to _talk_ directly with the lib.  
This is why I think Julia brings another dimension to GMT expansion

```auto
terrain_filter(data, detail, nrows, ncols, xinc, yinc, coord_type, center_lat=0.0, progress=C_NULL) =
	ccall((:terrain_filter, thelib), Cint, (Ptr{Cfloat}, Cdouble, Cint, Cint, Cdouble, Cdouble, Cint, Cdouble, Ptr{Cvoid}), data, detail, nrows, ncols, xinc, yinc, coord_type, center_lat, progress)

terrain_image_data(data, contrast, nrows, ncols, image_min=0., image_max=65535.0) =
	ccall((:terrain_image_data, thelib), Cint, (Ptr{Cfloat}, Cint, Cint, Cdouble, Cdouble, Cdouble), data, nrows, ncols, contrast, image_min, image_max)

```