How ClearBackground works ?

[madamac]

Why do I see a red square on the screen in this simple example?

int main() {

InitWindow(640, 480, "Clear background");

while (!WindowShouldClose()) {

	BeginDrawing();

	DrawRectangle(0, 0, 40, 40, RED);

	ClearBackground(GREEN);

	EndDrawing();

}

}

[purepelmen]

That's intresting. I also wanna know. Even though this was posted last month, but I anyway write the anwser for others.

Shape drawing functions emit vertices using rlVertex*, rlTexCoord*, rlColor* functions. For older OpenGL APIs they immediately call gl* equivalents (immediate mode). Vertices are emitted between rlBegin() and rlEnd() calls which on old OpenGL with immediate mode just forwards to glBegin() and glEnd(). On newer OpenGL it justs modifes current Raylib rendering state. glEnd() is what actually performs rendering to the backbuffer. But on newer OpenGL raylib doesn't call that. It postpones rendering.

EndDrawing(), before swapping screen buffers, will call rlDrawRenderBatchActive() which only on newer OpenGL draws active render batch (all queued verticies of rlVertex*, rlTexCoord*, rlColor* calls) manually without OpenGL's immediate mode.

So it works like this on older OpenGL:

BeginDrawing()
    -> Drawing setup
    
DrawRectangle(0, 0, 40, 40, RED)
    -> rlBegin() -> glBegin()
    -> rlVertex*... -> glVertex*...
    -> rlEnd() -> glEnd() -> ACTAULLY RENDERS TO THE BACKBUFFER

ClearBackground(GREEN)
    -> rlClearColor() -> glClearColor()
    -> rlClearScreenBuffers() -> glClear()
    -> RESULT: Clears everything rendered before immediately in the buffer.
    
EndDrawing()
    -> SwapScreenBuffer() 

But on newer OpenGL:

BeginDrawing()
    -> Drawing setup
    
DrawRectangle(0, 0, 40, 40, RED)
    -> rlBegin() -> changes internal Raylib state
    -> rlVertex*... -> changes internal Raylib state (adds vertices)
    -> rlEnd() -> changes internal Raylib state -> BUT DOES NOT PERFORM ANY OPENGL DRAW CALLS

ClearBackground(GREEN)
    -> rlClearColor() -> glClearColor()
    -> rlClearScreenBuffers() -> glClear()
    -> RESULT: Clears everything rendered before immediately in the buffer.
    
EndDrawing()
    -> rlDrawRenderBatchActive() -> Actually draws all queued vertices using non-immediate OpenGL draw calls
    -> SwapScreenBuffer() 

What's also interesting.. Is that rlDrawRenderBatchActive() is called inside BeginMode2D(), EndMode2D(), BeginMode3D(), EndMode3D(), BeginTextureMode(), EndTextureMode(), BeginScissorMode(), EndScissorMode(). So if you call any of these after DrawRectangle() call, queued verticies will be rendered immediately.

But you can just flush them manually:

#include <raylib.h>
#include <rlgl.h>

int main() {

InitWindow(640, 480, "Clear background");

while (!WindowShouldClose()) {

	BeginDrawing();

	DrawRectangle(0, 0, 40, 40, RED);
	rlDrawRenderBatchActive();
        
	ClearBackground(GREEN);

	EndDrawing();

}

}