programming objectspace-normal mentalray shader



From an animation i’d like to render a pass that shows the final normals (calculated from object and bumpmapping and so on) in object-space.

As i couldn’t find a way to do this in the software, i tried to write a mentalray shader for this.

the shadercode seems to work in a simple testscene, but not in the one i want to use it with.

This is my shadercode:

struct mrNormalOutputShader_params {
	miBoolean normalize;
	miBoolean mapTo01;
	miBoolean flipYZ;

extern "C" DLLEXPORT int mrNormalOutputShader_version(void) {return(2);}

extern "C" DLLEXPORT miBoolean mrNormalOutputShader(
	miColor		*result,
	miState		*state,
	struct mrNormalOutputShader_params *paras)

	miVector internalNormal = state->normal;
	miVector objectSpaceNormal;


		miScalar tmpy = objectSpaceNormal.y;
		objectSpaceNormal.y = objectSpaceNormal.z;
		objectSpaceNormal.z = -tmpy;


	result->a = 1.0;
	result->r = objectSpaceNormal.x;
	result->g = objectSpaceNormal.y;
	result->b = objectSpaceNormal.z;


I use this shader in a “mr Shader Element” render element to output a objectspace-normals pass. the direction of the output normals seems to be right qualitatively, but theyr ammount is wrong - as if some gammacorrection was applied to the values (but i did try to set different Gamma/LUT values, this only affected the visual appearance on screen, not the actual pixelvalues)

So, this is what I get:

in the middle top you see the normalpass output, with a pixel of the left wall testet (this wall is aligned to the world coordinate system and has no transformation applied to it). i would expect 0.5 in the green and blue channel, but i get the value 0.217

Even more strange: if i hit the “clone rendered Frame Window” button, then i get the result in the bottom right corner, where the pixelvalues deviate even more (there green and blue of the left wall have the value 0.035)

I don’t see what’s wrong in my shadercode… anyone can spot an error or knows how i could get that rendered correctly?


it must be connected to gamma-correction somehow, as 0.5 ^ 2.2 = 0.217

and when i clone the window, turns out that 0.217 ^ 2.2 = 0.035

why is there a gamma of 2.2 applied to the values in the framebuffer? (and why is it applied again when i clone the window??)

i get exactly the same results (value wise, on the screen it will fairly look different) independant of Gamma/LUT settings

this is confusing me.


I know nothing about c and MR shaders programming but the logic of your shader looks right (for me)).
The only thing i want to mention is your mapto01 func will work right only if normals have been already normalized. But you have separate check boxes for these ops.


And could you please explain why do you expect left wall normal to be [1,0.5,0.5]. For me it should be [1,0,0] ?

EDIT: got it - mapto01 func)


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