OS window size adaptive to Imgui window size

I am looking to build a “tool application”, just one single Imgui (no frame, etc) window at once.
My problem is that I would like to have different “pages” (windows) from different sizes and I would like that the OS windows fits the Imgui window size. Ideally it will automatic adjust to the Imgui window size.

An example would be something like:
welcome window with a “Press OK to continue”, then after press OK, a new window different size will appear and the welcome window will disapear.

Will this be possible to do with Imgui and other lib ( glfw, SDL2, etc ) what would be the best way to do it?
Can I do it by resizing a windows, etc or will I need to close and create a new OS window / GL context ever time?

Here are my findings on this subject.

IMGUI related:

Create a ImGui window in the position (0,0), with flags that remove not need stuff:

ImGui::SetNextWindowPos( ImVec2(0,0) );
ImGui::Begin( “MainWindow” , 0 , ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoCollapse | ImGuiWindowFlags_NoSavedSettings | ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoScrollWithMouse );

winSize = ImGui::GetWindowSize();

This winSize will be the need size for resize the window.

GLFW related:

Create a window not resizable, without decoration (if desired), also it is possible on latest glfw version to make the window transparent.

//glfwWindowHint(GLFW_TRANSPARENT_FRAMEBUFFER, GL_TRUE); // Only on latest versions
const GLFWvidmode* mode = glfwGetVideoMode(glfwGetPrimaryMonitor());
GLFWwindow* window = glfwCreateWindow(360, 161, “ImGui GLFW+OpenGL2 example”, NULL, NULL);

mode will be used to know the screensize in order to center the window.

if( ( (int)winSize_old.x != (int)winSize.x ) || ( (int)winSize_old.y != (int)winSize.y ) )
winSize_old = winSize;
glfwSetWindowSize( window, (int)winSize.x, (int)winSize.y ); // Resize
glfwSetWindowPos( window, (mode->width - (int)winSize.x) / 2, (mode->height-(int)winSize.y) / 2 ); // Center window

Mind there could be differences while resizing the window due the OS frame decoration size vs actual frame buffer size.

Yes that would work. You may also want to check the “viewport” which is a generalization of the concept of imgui interacting with the underlying windowing system to manipulate multiple os windows/graphic contexts, which include moving and resizing os windows.

Thanks for the tip, I will have a look.

btw, I found that the first time it loops, the GetWindowSize is returning 32,32
I guess that is the default WindowMinSize.
Is it the way it is expected to work or there is some way I can use to know the size early?

Most windows are hidden during the first frame they appear and don’t have a proper size yet, they are using this frame to do a first pass measuring the expected window size. So yes, that’s expected.

1 Like