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Paddon

OpenGL GLSL Question

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Hey all, I am using the Irrlicht3D engine to develop a Visualization application for my current research. However, since I am developing on a mac I am using OpenGL rendering with Irrlicht. I have written my own, basic shader to use with irrlicht but I am not quite sure how I communicate from within the program, to the shader. Essentially I want to be able to draw a 3d cube (or sphere) and control the color and alpha value of it from within my program. Does anyone know how I do this? Here are my working vertex and fragment shaders, and everything looks exactly how I would like it to, all that is left is figureing out how to mesh it with my code. Vertex Shader:
	void main() {
	
		vec3 normal, lightDir;
		vec4 diffuse;
		float NdotL;
		
		/* first transform the normal into eye space and normalize the result */
		normal = normalize(gl_NormalMatrix * gl_Normal);
		
		/* now normalize the light's direction. Note that according to the
		OpenGL specification, the light is stored in eye space. Also since 
		we're talking about a directional light, the position field is actually 
		direction */
		lightDir = normalize(vec3(gl_LightSource[0].position));
		
		/* compute the cos of the angle between the normal and lights direction. 
		The light is directional so the direction is constant for every vertex.
		Since these two are normalized the cosine is the dot product. We also 
		need to clamp the result to the [0,1] range. */
		NdotL = max(dot(normal, lightDir), 0.0);
		
		/* Compute the diffuse term */
		diffuse = gl_FrontMaterial.diffuse * gl_LightSource[0].diffuse;
		
		gl_FrontColor =  NdotL * diffuse;
		
		gl_Position = ftransform();
	}
Fragment Shader:
void main()
	{
		gl_FragColor = gl_Color;
	}
Any help would be greatly appreciated. Thanks.

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Uniforms In your case you want 1 vec4 uniform - colour + alpha

IIRC Irrlicht supports GLSL uniforms through video::IMaterialRendererServices::setVertexShaderConstant and video::IMaterialRendererServices::setPixelShaderConstant.

Irrlicht didn't treat sampler (texture) uniforms correctly last time I tried though, so watch out! It also has no VBO support...

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I think that is ok because I am really just looking for the basics. This application (probablly) will never invoke textures or anything beyond efficent alpha blending.

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So essentially, in my fragment program I can just have a uniform vec4 named "color". Then set it gl_FragColor = color. And use the setPixalShaderConstant() function to assign this variable color the color and alpha values I choose?

Also, which matricies do I have to set. I notice in the ittlicht tutorial they set the mInvWorld, mWorldViewProj, mLightPos, mLightColor, mTransWorld. What are the neccessary, analogous ones for GLSL?

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sounds like you have the gist of it. Though if you wanted to keep your per vertex lighting you'd want to have the frag colour = NdotL * colour

you could also pass through the colour as one of the material parameters - say GL_DIFFUSE. This would mean you could leave your shaders as they are.

I'm not sure how Irrlicht matrices map to opengl ones. Irrlicht is heavily based around d3d, so I'd guess it'd be something like that. If your current shader is working though, it means you have the correct modelview and projection matrices already.

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Quote:
Original post by Paddon
So essentially, in my fragment program I can just have a uniform vec4 named "color". Then set it gl_FragColor = color. And use the setPixalShaderConstant() function to assign this variable color the color and alpha values I choose?

Also, which matricies do I have to set. I notice in the ittlicht tutorial they set the mInvWorld, mWorldViewProj, mLightPos, mLightColor, mTransWorld. What are the neccessary, analogous ones for GLSL?


I have no exp with ittlicht, but those matrices are for DX/HLSL. GL/GLSL doesn't use them per se. They are upload to GLSL from OpenGL and you call matrices like


worldMatrix = gl_ModelViewMatrix
worldInverseTranspose = gl_ModelViewMatrixInverseTranspose
worldViewProjection = gl_ModelViewProjectionMatrix


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You could always define those matrices in your GLSL shader and set them with Irclics functions.

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