Cutout Tutorial

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Creating a cutout using a black and white image or alpha channel

Transcript of Cutout Tutorial

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Creating a cutout using a black and white image or alpha channel

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One technique used in many CGI projects are projection cards. Which is basically mapping an image onto a low resolution plane. Usually in conjunction with a transparency map to get edge detail.

The advantages of using “cards” is the immense detail you get from the color map. In the example given of the forest you can see that four sided planes are con-verted into a large number of trees. The edge has details that would result in a very high resolution mesh if modelled. Also of interest is the highly detailed shadow that results from the cards. In essence we are getting a whole forest that would take hours and hours to create in full 3D in minutes.

GETTING STARTED FOR MENTAL RAY1. First we will set up the scene. We need to create the planes. Lets create two “tree” planes and one “ground” plane.

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2. Create a spotlight with ray trace shadows turned on.

3. Aim the light at the planes and ground. Test render the scene.

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4. Next create an MIA_X mental ray material. Add the Autumn002.jpg image to the di�use color channel. Add the MIA_X material the left plane. If we do a test render the image will show up on the plane with a shadow but it is not cut out. We need to use a black and white image to cut it out.

5. Open up the MIA_X material Advanced section >Cutout Opacity. Create a Maya Software “Luminance” node.

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The Luminance node can be found in the Color Utili-ties section of the Maya Software nodes.

In the Luminance node Value Channel add the Autumn002b.jpg �le. The node map should look like the image below.

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The plane now has the texture added. You will notice that the texture is solid. The transparency will only show up at render time.

Hit render to see the result. Notice that the tree is cut out and the shadow is also shaped like the silhouette of the tree image.

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MAYA SOFTWARE SHADERS1. We will continue with the same scene but will be using maya shaders. For this purpose we will use a phong shader. Go to the Maya Tab in the hypershade. Create a phong shader.

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In the perspective camera window the texture will use the alpha channel and the color channel for hardware rendering.

Test render the image. You will notice that it renders the alpha and the shadow but has two artefacts.

Highlights from the specular channel.

Ray trace shadow problems.

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The specular issues arise from the fact that specular and re�ection don’t rec-ognize the transparency channel as a cutout.

So we will turn Specular Color to blackand change the Re�ective Channel to 0

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The Ray trace shadows can be �xed by changing the shadow attenuation from .5 to 0.

The Luminance node can be found in the Color Utili-ties section of the Maya Software nodes.

In the Luminance node Value Channel add the Autumn002b.jpg �le. The node map should look like the image below.

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Render the image out. The specular, re�ective and shadow artefact are now �xed.

If your cutout does not need specular or re�ection then using a lambert shader is much simpler.

WITH ARTEFACTS FIXED

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The �nal render using both the MIA_X and the Phong shaders.