OpenGL ES 2.0: Controlling Transparency in Fragment Shader

Question

The following is the OpenGL ES 2.0 simple GLSL Fragment Shader, I use to place textures on polygons, to render 2D sprites.

varying mediump vec2 TextureCoordOut; uniform sampler2D Sampler; void main() { gl_FragColor = texture2D(Sampler, TextureCoordOut); //gl_FragColor = vec4(texture2D(Sampler, TextureCoordOut).xyz, TextureCoordOut.w * 0.5); }

The fragment shader places voxels with alpha information taken from the source 2D texutre(.png image). Apart from alpha information, I need to control overall polygon/sprite transparency to achieve Fade In/Fade Out effects.

Could you show me, please, how to modify the above shader to control the overall transparency, besides the alpha information?

Note: The commented out line is used for my attempts to achieve the transparency. I wish to combine both the alpha information with the overall polygon/sprite transparency.

Thanks.

Notabene · Accepted Answer · 2011-05-27 01:57:12Z

4

This is probably cleaner solution, which also doesn't require to ask texture unit two times.

void main() { gl_FragColor = texture2D(Sampler, TextureCoordOut); gl_FragColor.a *= 0.5; }

(maybe compiler does that for you, but what if not. Especially on the opengl-es device performance difference may be observed)

answered May 27, 2011 at 1:57

Notabene

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\$\begingroup\$ +1, Agreed, and I will modify my code according to your idea. Let me mar yor answer as the Accepted Answer as well. \$\endgroup\$

Bunkai.Satori
– Bunkai.Satori

2011-05-27 02:29:25 +00:00
Commented May 27, 2011 at 2:29
\$\begingroup\$ I was under the impression that you cannot set gl_FragColor multiple times. Once you set it, the color is fed into the frame buffer. Is that a limitation on newer version of OpenGL? Maybe ES? \$\endgroup\$

Utkarsh Sinha
– Utkarsh Sinha

2013-06-01 22:50:44 +00:00
Commented Jun 1, 2013 at 22:50

Add a comment |

Bunkai.Satori · Accepted Answer · 2011-05-26 23:06:50Z

Apparently, I found the soultion.

This is the correct line in Fragment Shader:

 gl_FragColor = vec4(texture2D(Sampler, TextureCoordOut).xyz, texture2D(Sampler, TextureCoordOut).w * 0.5);

Thanks anyway.

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