How to make one color transparent in a PNG?

Question

Marina 2019-11-1

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https://ww2.mathworks.cn/matlabcentral/answers/488761-how-to-make-one-color-transparent-in-a-png

回答： Walter Roberson 2019-11-4

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

Screen Shot 2019-11-01 at 1.25.15 PM.png

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the cyclist 2019-11-1

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 2019-11-1

在 MATLAB Online 中打开

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') 
 