# Window flickers

**URL:** <https://discourse.glfw.org/t/window-flickers/1471>\
**Category:** support\
**Created:** [January 7, 2020, 7:09pm UTC](https://discourse.glfw.org/t/window-flickers/1471 "2020-01-07T19:09:59Z")\
**Posts on this page:** 1\
**Showing post:** 10

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**Author:** ![dougbinks](https://yyz1.discourse-cdn.com/flex035/user_avatar/discourse.glfw.org/dougbinks/32/39_2.png) [@dougbinks](https://discourse.glfw.org/u/dougbinks)\
**Post date:** [January 19, 2020, 3:57pm UTC](https://discourse.glfw.org/t/window-flickers/1471/10 "2020-01-19T15:57:38Z")

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Hi again @DanielMendes!

The full rendering code would need to be available to debug this properly, but I can help with a few hints.

You might need to set the OpenGL Viewport with [glViewport](https://www.khronos.org/registry/OpenGL-Refpages/es2.0/xhtml/glViewport.xml). This defaults to the size of the window when it is first created, but if your window size changes you’ll need to set it.

I’m not sure how you’re rendering the vertices, but your vertex shader or glmatrix needs to be set to output the correct coordinates. For legacy OpenGL see this post: [A very basic issue with draw line](https://discourse.glfw.org/t/a-very-basic-issue-with-draw-line/804/2)

You might find that using a program like [RenderDoc](https://renderdoc.org/) can help you debug your code by looking at all the parameters along with the input and output vertex data.

Good luck - if you need more help on this it would be useful to see the entire program.

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_[View the full topic](https://discourse.glfw.org/t/window-flickers/1471)._