Get Rid of glClear(GL_STENCIL_BUFFER_BIT)

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C++

Windows

OpenGL

Hello everyone!

I am try to Limit OpenGL draw to part of the window. In order to do it I am trying to use GL_STENCIL_TEST option. Here is my code:

// Enable stencil and clear the stencil buffer
glClear(GL_STENCIL_BUFFER_BIT);
glEnable(GL_STENCIL_TEST);

// Enable func and Op so drawing will effect the stencil buffer
glStencilFunc(GL_NEVER, 0x0, 0x0);
glStencilOp(GL_INCR, GL_INCR, GL_INCR);

// Draw a rect to the stencil
DrawSolidRect(rectDrawArea);

// Enable func and Op so drawing will not effect the stencil buffer
// but will only effect places where the stencil was drawn to in the previous step.
glStencilFunc(GL_EQUAL, 0x1, 0x1);
glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);

The problem is that glClear(GL_STENCIL_BUFFER_BIT) function is very time consuming, and since I am drawing with 25 fps, it really slows down the application.

I tried to remove this function - this solves the slowness problem but it causes flickering when I initialize the application - For about 10 seconds. Than it disappears and works ok.

Can someone suggest why the flickering is or what other function may I use that not so time-consuming?

Thanks!