How to make one color transparent in a PNG?

Hi, I am making maps and want to have the land transparent to then overlay over Google maps. The way around it I think is to creat the figure with land in black (as nothing else in the image is black) and the have that color made trasnaprent. I have not been able to do this so far... Here's the code, I appreciate anyone pointing me in the right direction. thanks

[the cyclist]

It's better to paste code than an image of code. Otherwise, if someone wants to try it out, they need to retype everything.

[Marina]

yes sorry,

fh=figure %('Menu','none','ToolBar','none '); 
ah=axes('Units','Normalize','Position',[0 0 1 1])
axis(ah, 'square')
fh.Position(3) = fh.Position(4);  %set width equal to height
m_proj('mercator','longitudes',[-96,-76.5],'latitud',[1,20]); 
hold on
fig=m_contourf(long,lati,speed,V,'LineColor','none');
colormap(jet)
caxis([0 0.7])
fig = m_quiver(long,lati,outstrct.u(idx_lon,idx_lat),outstrct.v(idx_lon,idx_lat),8,'k','LineWidth',0.5);
m_gshhs_h('patch',[0 0 0]); % searches for the GSHHS database in /m_map/private. Sets land to black.
m_grid('box','off', 'LineStyle','none','xticklabels',[],'yticklabels',[],'xtick',[],'ytick',[]);
print('/Users/marinam_ar/Desktop/OPERATIONAL/MAPS_JEFF5_TRANSP/CURRENTS_Oct272019','-dpng','-r1200') 
 

Answer 1

[Sahithi Kanumarlapudi]

You can change the transparency of a color in an image using ‘AlphaData’ property. The following link has an example

https://www.mathworks.com/help/matlab/creating_plots/changing-transparency-of-images-patches-or-surfaces.html#ChangingTransparencyExample-3

In order to change the transparency of an object on an axis you can use ‘alpha’ property. For more information refer to the following link

https://www.mathworks.com/help/matlab/ref/alpha.html#d117e21041

Hope this helps!

Answer 2

[Walter Roberson]

print() to the png file. Then read in the file and write out a new copy of it that has the appropriate color set as transparent. See

https://www.mathworks.com/matlabcentral/answers/423418-resize-and-save-transparent-png-file-in-matlab#comment_620927