Details about OpenGL glViewport, glOrtho, and glScissor in windows

These two days I have been programming OpenGL for a long time and have forgotten a lot. Now I have come back and summarized my experiences:

First, some beginners may be confused about the viewpoint:

In Windows, the coordinate origin is in the upper right corner, while in OpenGL, the coordinate origin is in the lower right corner.

For rendering a 2D image, the viewpoint settings should be as follows:

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1. GlMatrixMode (GL_PROJECTION );
2. GlLoadIdentity ();
4. GlOrtho (0, (IPHONE_SCREEN_WIDTH), 0, SCREEN_HEIGHT,-I2F (100), I2F (1000); // here glOrtho (left, right, bottom, top, near, FAR)
5. // This is based on openGL coordinates
7. GlMatrixMode (GL_MODELVIEW );
8. GlLoadIdentity ();
10. GlViewport (0, 0, SCREEN_WIDTH, SCREEN_HEIGHT); // glViewport (x, y, width, height)
11. GlScissor (0, 0, SCREEN_WIDTH, SCREEN_HEIGHT); // glScissor (x, y, width, height)

Then, when you draw an object, for example, draw a square on the screen. If you want to draw it in the upper left corner, you need to perform this conversion.

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1. Void drawRect (x, y, width, height)
2. {
3. Y = SCREEN_HEIGHT-y-h;
5. // Draw a rectangle using GL
6. }