Hi all I am new to OpenGL ES 2.0 . I am confused with gl_position and varying variable both will be the output from vertex shader. varying variable will be passed to fragment shader, what about gl_position. Does gl_position influence on varying variable in fragment shader. gl_position=vec4(-1); what is the meaning of this. PLease do help me to understand these things in a much better way.
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1 Answers
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gl_Position is special variable. It is used to calculate which fragment will fragment shader be calculating/shading (it calculates its position). All other varyings are directly interpolated across the primitive.
gl_Position is not available in fragment shader. But there is gl_FragCoord variable available which is calculated from gl_Position so, that x/y values of it changes from 0 to 1 (from one screen side to another), z is depth from 0 (near plane) to 1 (far plane). And w is something like 1/gl_Position.w (feel free to look what it is exactly in OpenGL|ES2 spec).
