wglMakeCurrent textures disappear - c

Question

Hi I'm trying to render 3 full screen windows on different monitors, until now I've successfully queried for existing monitors EnumDisplayMonitors to get the 4 parameters necessary to create 3 windows with WS_POPUP style applied.
In one frame I do the following:
for(int i=0; i<monitorsNum; i++)
{
wglMakeCurrent(hdcs[i], sharedHrc);
doRendering();
SwapBuffers(hdcs[i]);
}
Many websites suggest the same, however, when I go from 1 monitor to 2 or more monitors, textures disappear:
What you see is the same scene rendered 3 times, the slight different background clear color shows that at least I'm doing the stuff partially correct (gl clear color showed correctly, it even works with 3 monitors of 3 different sizes). I tried to intercept all the gl calls with glGetError() without getting any error. Is there a specific step I missed, or maybe it is a issue of my laptop?
If it helps, the 3 windows are created with an existing framework, so at creation each window has been given its own hrc, but then I just use one hrc for the other 2 windows. (so 3 hrc created, and 1 used, if it matters)

Answer 1

There are many reasons that a texture may not display correctly when rendering some geometry.
But assuming your problem isn't related to any of these things such as incorrect UVS, shader issues, texture creation etc the issue could be related to the fact that you are now managing multiple contexts.
To set up multiple windows you need to create a context for each window.
The wglMakeCurrent function allows you to switch the context for each window, rendering as you go.
https://learn.microsoft.com/en-us/windows/desktop/api/wingdi/nf-wingdi-wglmakecurrent
The wglMakeCurrent function makes a specified OpenGL rendering context the calling thread's current rendering context. All subsequent OpenGL calls made by the thread are drawn on the device identified by hdc. You can also use wglMakeCurrent to change the calling thread's current rendering context so it's no longer current.
An OpenGL context represents the default frame buffer (default place that your shader will output to when rendering) but it also stores all of the state associated with that instance of OpenGL.
Furthermore:
Each context has its own set of OpenGL Objects, which are independent of those from other contexts.
So this means that each context does not have access to the same resources unless explicitly told.
Any object sharing must be made explicitly, either as the context is created or before a newly created context creates any objects. However, contexts do not have to share objects; they can remain completely separate from one another.
So one reason you may be able to render the same texture(s) in each window is because that texture is not a shared resource. glClearColor works fine because it is not dependent on any resources that are associated with any particular context.