Make NaNs transparent with pcolor

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V.D-C
V.D-C on 27 Feb 2020
Answered: V.D-C on 27 Feb 2020
Hello,
I have a matrix with NaNs and other values. I plot in the background a land mask on which the values overlay. But, because of this the NaNs also overlay the background and I end up not seeing it anymore. I would therefore like to make the NaNs transparent so we can see the background.
So far I was using imagesc but because of a georeference issue, pcolor is more adapted. I had the following code to maks NaNs transparent with imagesc:
f2 = figure('Visible',true);
% Plot the background image
ax1 = axes;
imagesc(ax1, demx, demy, maskLand);
hold on;
% Plot the overlay image
ax2 = axes;
% Make NaNs transparent
imAlpha = ones(size(overlay));
imAlpha(isnan(overlay)) = 0;
imagesc(ax2, demx, demy, overlay, 'AlphaData', imAlpha);
Now, with pcolor I have been setting:
pcolor(maskLand...)
hold on
e1 = pcolor(overlay)
set(e1,'facealpha',0.3)
But it sets the whole overlay as transparent while I only want the NaNs of the overlay to be transparent. What can I specify to make this happen ?
Have a nice day !
  2 Comments
V.D-C
V.D-C on 27 Feb 2020
Thank you for your answer !
I didn't keep the first strategy because by using imagesc, I use a georeferenced system that is tilted and at a specific scale.
I wanted to plot everything in a UTM coordinate system (and have the figures straight) but it looks like I have to go through pcolor which is able to handle inputs as matrices (for example, pcolor(x,y,data) with x & y size 330x300).
And because of the huge amount of data involved, I can't put my detailed code in here without importing everything.

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Accepted Answer

V.D-C
V.D-C on 27 Feb 2020
Ok I managed to find where it was blocking everything.
The issue is that I was setting a black background for the second figure. To make the overlay disappear for NaNs values, then the solution is:
figure()
ax1 = axes;
pcolor(ax1,lat,lon,mask)
set(gca, 'Color', 'black'); %sets black background
colormap(ax1,cm_mask); %colormap I created which makes the land mask brown
shading flat %get rid of the lines on the figure
hold on
ax2 = axes;
imAlpha = ones(size(smb));
imAlpha(isnan(smb)) = 0; %create NaN mask the size of the overlay
e1 = pcolor(ax2,lat,lon,overlay);
set(e1,'alphadata',0) %make NaNs transparent. CAUTION: Use "Alphadata" and not "facealpha" or everything
%will be transparent
colormap(ax2,cm_overlay) %colormap I created for the overlay
shading flat
hold on
set(gca, 'Color', 'none'); %IMPORTANT: set the second background as transparent otherwise it covers the land mask
axis tight
My mistake was to use a black background and "facealpha".

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