Image plane,,pls help!

[limp]

I have an object,a camera...the problem is I want render out the object's alpha channel.the result is Maya renders out the image plane and the object's alpha channel.How can I just only render out the object alpha channel...please....reply....

[joojaa]

In reply to:
I have an object,a camera...the problem is I want render out the object's alpha channel.the result is Maya renders out the image plane and the object's alpha channel.How can I just only render out the object alpha channel...please....reply....

Two solutions:

Either connect your alpha to your objects color. But then you would need to disconect the alpha from the object before you do that, so that you wont get alpha affecting alpha.

Or (the easier way) uncheck color, and check alpha in the rendering menu... Maya does usually render both the color and the alpha in the same file so you could just extract the exsisiting alpha channel, with some image editting app like photoshop.

[limp]

[
Two solutions:

Either connect your alpha to your objects color. But then you would need to disconect the alpha from the object before you do that, so that you wont get alpha affecting alpha.

Or (the easier way) uncheck color, and check alpha in the rendering menu... Maya does usually render both the color and the alpha in the same file so you could just extract the exsisiting alpha channel, with some image editting app like photoshop.

I have to make it clear...
I have a sequence of images as my background(imageplane)the foreground is my character..the reason y I just want the character's alpha channel and z-channle is I want to do some post effect meanwhile do not effect the background...U will probably say that what I need is just give a simple background color and render the character out...if things like that...I will got blurring edge around the character...so,my solution is render out the background and the character in one file(but this file Must has the alpha channel&z channel of the character,the orther alpha channel must get rid of)then,,in the post programe I can only focous on the character....
the problem right now is Maya take the image plane as an object that's y I can not get the character's alpha channel seperated from the imageplane alpha channel and z-channel alone.your method propbaly work for a single frame...but as to an animation,....it would be crazy to get those alpha channels...I was wondering if maya has such function ,,u can set which channel of the object u want to render out...if it does has..it would solve this problem totally.....or any orther freaky tricks (:)to solve this problem,,,u know what?in Max...this is just a piece of cake....
anyway..thanx for your reply..
have fun...

[joojaa]

Ok, what comp app are you using.

Also note that maya uses premultiplied images, so set your background to black or white and render with alpha then set your image iterpretation or whatever settings to premultiplied and the color to your background color.

[robin-scher]

if i understand your problem right it is this:
you need to render over the image plane for the color, but you don't want the image plane to have any alpha.

if this is the case, you have several possible solutions:

1. Render both over the image plane (for your color fill) and not over the image plane (for matte and z-depth). advantages: easy to setup; disadvantages: you have to render the whole thing twice.

2. Change your image plane files to an RGBM format, and add a solid black matte channel to them. then use the RGBM display type for the image plane. In theory, the color should now render with the image plane's color, but since the image plane's alpha is black, it should use the black for the alpha where there is only image plane displayed. Advantage: all in one pass; disadvantage: i don't think it actually works right, even though this is what you are "supposed" to do. also, don't know if it will work with z-depth.

3. Turn off the image plane, and turn on the composite flag when you render. set the threshold to zero. this will amp up the semi-transparent parts of your image without affecting the matte, which is good for compositing platforms that don't un-pre-multiply the image before layering (like inferno and henry). advantages: super easy, and all-in-one; disadvantages: may not help depending on what compositing platform you are using. also i've seen this screw up sometimes, but i can't say for sure if it was a maya problem or an inferno problem.

4. a super cheat: Turn the matte opacity down on all your object materials. essentially, now you can get an "inverted" matte (i.e., white on the background, and black where your object is), which is easy enough to invert later. advantages: you can set it up for one pass, and not have to deal with maya's weird image planes; disadvantages: kind of a pain to set up for every mateial, and you will have to experiment with different settings for the matte opacity settings to see which works correctly.

5. don't use an image plane. instead make a poly plane, fit it to the render size exactly, assign a file texture to it, and use the matte opacity on that material to make it render a color, but a black alpha. advantages: very flexible and will probably get the alpha right in one pass. disavantages: if you have to line up your image plane exactly, it can be hard, and you will probably see your image softened. however since it sounds like you're going to composite later anyway, this may not be an issue.

these are starting points. you will have to experiment with each to get it to work right for your situation. i've used all of them at some point, with varying degrees of success.

i hope this helps
-robin

Robin
robin@uberware.net