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Thread: two direction transparency

  1. #1
    Junior Member Newbie techwinder's Avatar
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    two direction transparency

    Hi everyone,
    Need some help after having struggled unsuccessfully with this problem for some time.
    Click image for larger version. 

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    The transparency and light are OK seen from one side, but not from the other.
    The objects are arrays of triangles drawn using
    Code :
    glDrawArrays(GL_TRIANGLES,pos,3)
    Am I missing something?
    Thanks in advance for any help.

    The fragment shader:
    Code :
    #version 410
     
    in vec3 Position_viewSpace;
    in vec3 Normal_viewSpace;
    in vec3 EyeDirection_viewSpace;
    in vec3 LightDirection_viewSpace;
    in vec3 vPosition;
    in vec4 vertexcolor;
     
    uniform int lightOn;
    uniform vec3 LightPosition_viewSpace;
    uniform vec4 LightColor;
    uniform float LightAmbient, LightDiffuse, LightSpecular;
    uniform float Kc, Kl, Kq;
    uniform float MaterialShininess;
    uniform vec4 clipPlane0; // defined in view-space
     
    out vec4 fragColor;
     
    void main()
    {
    	// Material properties
    	vec4 MaterialAmbientColor, MaterialDiffuseColor, MaterialSpecularColor;
     
    	if (vPosition.z > clipPlane0.w)
    	{
    		discard;
    		return;
    	}
     
    	if(lightOn==1)
    	{
    		MaterialAmbientColor  = vec4(vertexcolor.rgb * LightAmbient, vertexcolor.a);
    		MaterialDiffuseColor  = vec4(vertexcolor.rgb * LightDiffuse, vertexcolor.a);
    		MaterialSpecularColor = vec4(1.0, 1.0, 1.0, 1.0);
     
    		// Distance to the light
    		float distance = length(LightPosition_viewSpace - Position_viewSpace);
     
    		// Normal of the computed fragment, in viewSpace
    		vec3 N = normalize(Normal_viewSpace);
     
    		// Direction from the fragment to the light
    		vec3 L = normalize(LightDirection_viewSpace);
     
    		// Cosine of the angle between the normal and the light direction, clamped above 0
    		float cosTheta = clamp(dot(N,L), 0.0, 1.0);
     
    		// Direction from the vertex to the eye, in view space.
    		vec3 E = normalize(EyeDirection_viewSpace);
     
    		// Direction in which the triangle reflects the light
    		vec3 R = reflect(-L,N);
     
    		// Cosine of the angle between the Eye vector and the Reflect vector,
    		float cosAlpha = clamp(dot(E,R), 0.0, 1.0);
     
    		float attenuation_factor = clamp(1.0/(Kc + Kl*distance + Kq*distance*distance), 0.00001, 1.0);
     
    		gl_FragColor =
    			  MaterialAmbientColor  * LightColor +
    			 (MaterialDiffuseColor  * LightDiffuse  * cosTheta)                         * LightColor * attenuation_factor
    			+(MaterialSpecularColor * LightSpecular * pow(cosAlpha, MaterialShininess)) * LightColor * attenuation_factor;
    	}
    	else
    	{
    		gl_FragColor  = vertexcolor;
    	}
    }

  2. #2
    Senior Member OpenGL Guru
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    Quote Originally Posted by techwinder View Post
    The transparency and light are OK seen from one side, but not from the other.
    Translucent polygons usually need to be drawn in a particular order (either back-to-front or front-to-back, depending upon the blending mode). BSP trees are one common solution to this issue.

  3. #3
    Senior Member OpenGL Guru Dark Photon's Avatar
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    Location
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    Search the archives of these forums for "order independent transparency" and you'll come up with a number other options for dealing with this. Examples: A-buffer, Weighted Average blending, Depth Peeling, MSAA Alpha (ALPHA_TO_COVERAGE), etc.

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