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• Understanding how OpenGL blending works

  - by yuumei

  I am attempting to understand how OpenGL (ES) blending works. I am finding it difficult to understand the documentation and how the results of glBlendFunc and glBlendEquation effect the final pixel that is written. Do the source and destination out of glBlendFunc get added together with GL_FUNC_ADD by default? This seems wrong because "basic" blending of GL_ONE, GL_ONE would output 2,2,2,2 then (Source giving 1,1,1,1 and dest giving 1,1,1,1). I have written the following pseudo-code, what have I got wrong? struct colour { float r, g, b, a; }; colour blend_factor( GLenum factor, colour source, colour destination, colour blend_colour ) { colour colour_factor; float i = min( source.a, 1 - destination.a ); // From http://www.khronos.org/opengles/sdk/docs/man/xhtml/glBlendFunc.xml switch( factor ) { case GL_ZERO: colour_factor = { 0, 0, 0, 0 }; break; case GL_ONE: colour_factor = { 1, 1, 1, 1 }; break; case GL_SRC_COLOR: colour_factor = source; break; case GL_ONE_MINUS_SRC_COLOR: colour_factor = { 1 - source.r, 1 - source.g, 1 - source.b, 1 - source.a }; break; // ... } return colour_factor; } colour blend( colour & source, colour destination, GLenum source_factor, // from glBlendFunc GLenum destination_factor, // from glBlendFunc colour blend_colour, // from glBlendColor GLenum blend_equation // from glBlendEquation ) { colour source_colour = blend_factor( source_factor, source, destination, blend_colour ); colour destination_colour = blend_factor( destination_factor, source, destination, blend_colour ); colour output; // From http://www.khronos.org/opengles/sdk/docs/man/xhtml/glBlendEquation.xml switch( blend_equation ) { case GL_FUNC_ADD: output = add( source_colour, destination_colour ); case GL_FUNC_SUBTRACT: output = sub( source_colour, destination_colour ); case GL_FUNC_REVERSE_SUBTRACT: output = sub( destination_colour, source_colour ); } return output; } void do_pixel() { colour final_colour; // Blending if( enable_blending ) { final_colour = blend( current_colour_output, framebuffer[ pixel ], ... ); } else { final_colour = current_colour_output; } } Thanks!

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• Could someone explain why my world reconstructed from depth position is incorrect?

  - by yuumei

  I am attempting to reconstruct the world position in the fragment shader from a depth texture. I pass in the 8 frustum points in world space and interpolate them across fragments and then interpolate from near to far by the depth: highp float depth = (2.0 * CameraPlanes.x) / (CameraPlanes.y + CameraPlanes.x - texture( depthTexture, textureCoord ).x * (CameraPlanes.y - CameraPlanes.x)); // Reconstruct the world position from the linear depth highp vec3 world = mix( nearWorldPos, farWorldPos, depth ); CameraPlanes.x is the near plane CameraPlanes.y is the far. Assuming that my frustum positions are correct, and my depth looks correct, why is my world position wrong? (My depth texture is of format GL_DEPTH_COMPONENT32F if that matters) Thanks! :D Update: Screenshot of world position http://imgur.com/sSlHd So you can see it looks nearly correct. However as the camera moves, the colours (positions) change, which they shouldnt. I can get this to work, if I do the following: Write this into the depth attachment in the previous pass: gl_FragDepth = gl_FragCoord.z / gl_FragCoord.w / CameraPlanes.y; and then read the depth texture like so: depth = texture( depthTexture, textureCoord ).x However this will kill the hardware z buffer optimizations.

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• If I project a sphere in 3D will it be a circle?

  - by yuumei

  Assuming I have infinite vertices to represent the sphere, if I project the sphere from any position/scale in 3D to 2D, will it be a circle? I know it will not be a circle on the screen, because of scaling and different resolutions. But do field of view and aspect ratio effect the results? Edit: Sorry yes, I am talking about perspective projection. Seems the answer is no then, perspective will distort the sphere. Thanks!

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