Problem with surface shader

[mitchbraggart]

Hi everybody!

I want to use Pictures as Backgound images in my 3d Szene! Therefore i project the images i want to use, on a plane. Since i do not want that the images are affected by the renderer i used an surface shader as material.

Everythings fine so far.

As i want  part of the images to be transparent, i created an alpha channel in my pictures and saved them out as a Tiff file.

For some reason i don't understand, this alpha channel does not affect the transparency of the surface shader.

In the picture below you can see 2 planes with the same picture applied.

On the left plane i used a lambert shader and on the right a surface shader.

Every Material has the same Tiff file applied to both its color and transperancy channel.

As you can see the surface shader does not show any transparency in the render view.

Although the transparency of the surface shader is showing in the viewport.

Since i couldn't find any useful help in the maya documentations i thought i post the problem here.

Thanks in advance

Greeting Braggart

[joojaa]

Well it actually does, however maya assumes the image is premutilied on black so if you want it to actually LOOK black then you must flood the alpha with black. So anything other than black transparent is just very colorful transparent stuff. YOu can use them as color filters.

(normal photoshop colro computation really IS color - alpha*color + alpha*layer below )

This is actually desirable since you MIGHT want to color areas even tiugh you make the object transparent, surface shader is just that it just dumps the values into calculation as is, it doesn't think or do ANYTHING for you. Its raw data so you can actually do all the tricks in the book otherwise you couldn't.

So you neo deal with the basics of basics, pedal to the theory.

[mitchbraggart]

Hi Joojaa!

I am quite confused here.

But thats why i stored a black and white alpha channel in the  tiff file. (Black where it should be transparent and white where it should be opaque).

I also deleted every color information where it should be transparent.

So now if you say the alpha is premultiplied the equation ends up like that: 0 color * 0 Alpha = 0 (transparent)

But the rendering is still white where i want it to be transparent.

I discovered this! If i plug my tiff file in a lambert shader and then plug the lambert shader in a surface shader the puicture will be rendered right.

Well not really beacause if i look at the alpha channel the parts where it should be transparent are still opaque.

What am i missing here?

[joojaa]

Yes, you hit on the snag that Adobe almost solely is responsible for making. See they made it easy, but they also made it hard to understand whats going on at the same time. Now everybody does this but you cant do any basic arithmetic in any imaging application anymore.

http://www.teamten.com/lawrence/graphics/premultiplication/

What am i missing here?

Just about everything about how images and maya really work, unfortunately.

So now if you say the alpha is premultiplied the equation ends up like that: 0 color * 0 Alpha = 0 (transpar

Oops i had typo in the formula. sorry. Its color - alpha*color + alpha*layer below (fixed in source)

But thats why i stored a black and white alpha channel in the  tiff file.

No you used photoshop and assumed layering is somehow fundamenta, well yeah sure but what if you want to do something even more fundamental such as addition? Surface shader is a do it yourself function, but that means you must DO EVERYTHING YOURSELF, including computing the alpha mask on everything. That means control but also responsibility

I also deleted every color information where it should be transparent.

Each pixel in your image HAS 4 values. Theres no such thing as a pixel with no color. Even pxels you think as having no color, do have a color. There quite literary is no such thing as apixel in image that has no color information, that is impossible. Your transparent pixels are white, simple enough.

See thast not what alpha is about. Alpha is just a metric on how much to show the layerand how much to delete of the intensity of layer below.

And by the way if you sue photoshop, then you can never really know what particular color phostoshop chose to flood this time. Mostly its white but some formats also are black. But reload this tiff into PS and behold it is white (assuming you didnt use CS, CS2 before they fixed the functionality of auto alpha loading back, because then PS will never let you know unless you go really tech).

Suggest you to Re-evaluate everything you think you know about images.

I discovered this! If i plug my tiff file in a lambert shader and then plug the lambert shader in a surface
shader the puicture will be rendered right.

Not quite you get a white fringe.

Lol, yes becasue it actually does multiply the colors to black. You could have put the color onto the lamberts ambient channel and kill color too. But also not quite you get a white fringe.

In any case theres a much easy  way to do this with the surface shader, and thats simply to do it how shading is actually done.

Connect Out Transparency to Out Matte Opaciy

Connect Out Alpha to Out Transparency R G and B

Now the color part follows
Simple arithemtic is if premutiplied by black:
Connect your image Out Color to a Out Color

Premultiplied by white on the otherhand reduces to:
Connect your image Out Color to a plus minus average node input [0]

Connect Out Alpha to same plus minus average node input [1] r g and b

Connect output 3d to Out Color

set mode to minus, Now you have the 100% correct composition

Other premultiplication colors are possible but have a bit more to implement

[mitchbraggart]

Thx a lot Joojaa! The hole Alpha thing is now much more clearer to me! (I think, Need to retrace every step again)

So how i understand that it is like that:

The out alpha which is connected to the out transperency produces my transparency in my rendered picture.

The out transparency connected in the Matte opacity creates my Alpha channel for the rendered image.

Since both of these connections do not produce my color i need to plug the out color into the out color of the surface shader.

So far for the basic understanding.

If the File is Premultiplied with white i have to substract the alpha channel so that my transparent parts will actually be transparent in my rendered image.

So far so good.

I also managed to get what i wanted.

To not have that problem with the Premultiplied white i used a file premultiplied with black.

(Therefore i saved the same image with a black backround.)

That fixed the Problem for me

But i did not understand how you pluged in the out alpha and out color in the plus minus average node.

I simply can't connect the  Images outColor into input[0]. Since out color has 3 attributes (RGB) i only can plug it in Input3D. Also i can plug the out alpha only in input1D[0]

What am i doing wrong? or did i missunderstand you?

Also i needed to insert a revers node vor the outMatte Opacity and Out Transperency. Why?

Sorry for all the nooby questions, i am still getting started. The whole technical side of 3D and Compositing can get really confusing some times. But its fun to understand these process.

Therefore i will continue to ask simple questions!

Hope you won't mind!

[joojaa]

Nono this is important, this is the sott of thing that will burn you if you dont know it.

If the File is Premultiplied with white i have to substract the alpha channel so that my transparent parts will actually be
transparent in my rendered image.

No see you images get mixed with teh premultiplication color, you just nee dto remove the color. The layering is just a addition, maya does the subtraction fo the background for you becasue otherwise nothing could be semitransparent. but you need to take care of your adding, when your fully transparent you just want to add noting, in other word 0 0 0 or in english black.

Also i needed to insert a revers node vor the outMatte Opacity and Out Transperency. Why?

Alpha and out transparency are inverse in meaning.

Ok lets make this in mel (mentalray reads tif files a bit wonky):

file -f -new;
shadingNode -asShader surfaceShader;
sets -renderable true -noSurfaceShader true -empty -name surfaceShader1SG;
connectAttr -f surfaceShader1.outColor surfaceShader1SG.surfaceShader;
shadingNode -asUtility plusMinusAverage;
connectAttr -f plusMinusAverage1.output3D surfaceShader1.outColor;
shadingNode -asTexture file;
// Result: file1 //
shadingNode -asUtility place2dTexture;
connectAttr -f place2dTexture1.outUV file1.uvCoord;
connectAttr -f place2dTexture1.outUvFilterSize file1.uvFilterSize;
connectAttr -f place2dTexture1.coverage file1.coverage;
connectAttr -f place2dTexture1.translateFrame file1.translateFrame;
connectAttr -f place2dTexture1.rotateFrame file1.rotateFrame;
connectAttr -f place2dTexture1.mirrorU file1.mirrorU;
connectAttr -f place2dTexture1.mirrorV file1.mirrorV;
connectAttr -f place2dTexture1.stagger file1.stagger;
connectAttr -f place2dTexture1.wrapU file1.wrapU;
connectAttr -f place2dTexture1.wrapV file1.wrapV;
connectAttr -f place2dTexture1.repeatUV file1.repeatUV;
connectAttr -f place2dTexture1.vertexUvOne file1.vertexUvOne;
connectAttr -f place2dTexture1.vertexUvTwo file1.vertexUvTwo;
connectAttr -f place2dTexture1.vertexUvThree file1.vertexUvThree;
connectAttr -f place2dTexture1.vertexCameraOne file1.vertexCameraOne;
connectAttr -f place2dTexture1.noiseUV file1.noiseUV;
connectAttr -f place2dTexture1.offset file1.offset;
connectAttr -f place2dTexture1.rotateUV file1.rotateUV;
connectAttr -force file1.outColor plusMinusAverage1.input3D[0];
connectAttr -f file1.outAlpha plusMinusAverage1.input3D[1].input3Dx;
connectAttr -f file1.outAlpha plusMinusAverage1.input3D[1].input3Dy;
connectAttr -f file1.outAlpha plusMinusAverage1.input3D[1].input3Dz;
connectAttr -f file1.outAlpha surfaceShader1.outTransparencyR;
connectAttr -f file1.outAlpha surfaceShader1.outTransparencyG;
connectAttr -f file1.outAlpha surfaceShader1.outTransparencyB;
connectAttr -f file1.outTransparency surfaceShader1.outMatteOpacity;
setAttr plusMinusAverage1.operation 2;

PS: be a bit carefull with tiff images and mentalray weid things do happen, especially if the image wasnt flat in photoshop befroe you exported.

[mitchbraggart]

Hello

Back again and more confused than ever!

I completely retraced your steps and i think i understand it why i have to plug everything like you said i should.

So here is my final result.

As you can see the shading network looks good. (at least the preview of the surface shader.)

Also is looking good in the viewport.

But still the rendered image is wrong.

It seems that i killed all the color!

But why? especially because its looking fine in the Shader preview.

Also i inverted the alpha channel in the Tiff file, so that i don't have to use a reverse node.

3 days of exploring the alpha channel and still it seems that i don't get it!

Looking forward to be enlightend

Greetings Mitch

[mitchbraggart]

Ok here is what i found out so far:

If i do not inverse my alpha channel in my tiff file but instead invers both my alpha out connections and my out transparency connection it is working. But honestly i don't understand why!

Its finnaly working what im trying to do here.

But still its not completely clear to me.

[joojaa]

Test using Maya software see they work differently sometimes, because mentalray reads TIFF files differently than maya*. TIFF isnt really ONE format is a bundle of many formats, this si why many studios refuse to wrok with TIFF files. As a result You can get tiff files that get very different results from each other, depend on what options you had used when you SAVED the TIFF file, TIFF is source of constant frustration to bigger studios because of this. Users never get this, since they dont you can never know what kind of tiff file you get, even IF everybody uses same version of photoshop.

Depends on wether your application is saving transparency or opacity. May be thet the sample file i was using happened to have upside down transparency, i didnt do it myself just picked a random file and random art that had this done.

Pixels are just data you know how you interpret them is up the the shading TD. Nothing actually says even the colro channels have to come in the order we think they ought to, even they can come mixed, thats up to the profile.

In any case to circumvent the TIFF format porblem i manually make all the channels myself with a action most of the time, this allows me to specify whether to use straight of premultiplied alpha, what color to premult with and so on. This actually gives me much better composite too.

*that is mentlary doesnt support all tif formats, and some differently then others.

PS: remeber to unchek alpha is luminance! on the file node, this makes MR work differently form maya sw.