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Silvo

OpenGL Simple OpenGL Setup Sequence Oddity

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G'day! I am wondering why this works:
	glEnable(GL_TEXTURE_2D);
	glClearColor(0, 0, 0, 1);
	glMatrixMode(GL_PROJECTION);
	glOrtho(0, 100, 100, 0, -1, 1);
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
But this doesn't:
	glEnable(GL_TEXTURE_2D);
	glMatrixMode(GL_PROJECTION);
	glMatrixMode(GL_MODELVIEW);
	glClearColor(0, 0, 0, 1);
	glOrtho(0, 100, 100, 0, -1, 1);
	glLoadIdentity();
I am drawing white boxes, and using the latter causes them to not appear. Why is this so?

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In the first example, the projection matrix is set to an orthographic projection with range [0, 100] on X and Y axis, and the modelview matrix is the identity matrix.

The second example leaves the projection matrix untouched, and sets the modelview matrix to the same orthographic projection matrix as above but then immediately resets it to the identity matrix.

All matrix operations modifies the active matrix stack. So in the second example, the orthographic projection matrix ends up in the modelview matrix stack.

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Ohhhhhhhh.....

Thank you so much. I get it now. I had a strange misconception that by calling glMatrixMode() I was doing something like "enabling" the matrix... So, by calling glMatrixMode(), I am setting which matrix I will affecting when I use functions like glOrtho() and glLoadIdentity(), right?

Thanks for that!

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Yes, OpenGL is designed as a state machine.

You set modes and those modes stay set until they are changed.

Like if you bind a texture, that texture will stay bound until another texture is bound or textures are disabled. It is the same with Matrices, you set the mode, do your stuff, and set another mode.

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