Assignment 7: Alpha Transparency

Requirements

• Create a new partially transparent Effect
• In other words, an Entity with a Material using this Effect should be visible, but you should be able to also see objects behind it. (Something like a window or a glass ball, for example.)
• BLENDOP is ADD
• SRCBLEND is SRCALPHA
• DESTBLEND is INVSRCALPHA
• How transparent the object is should be determined by a uniform float parameter between 1 (fully opaque) and 0 (completely transparent). This way different materials can be more or less transparent and still use the same Effect.
• Create a new Effect that uses binary alpha
• In other words, every fragment that the Effect outputs should either be completely opaque or completely transparent.
• Choose a threshold value between 0 and 1
• You can use the HLSL clip() function to discard the fragment if it is less than the threshold value:
• clip( alpha - threshold );
• (clip(x) will discard the fragment if x is less than 0)
• After calling clip() you should probably set the alpha value to 1
• Create a new Effect that is additive
• Whatever color the fragment shader outputs should be added to what is already stored in the image's corresponding pixel
• BLENDOP is ADD
• SRCBLEND is ONE
• DESTBLEND is ONE
• Your scene must have at least one Entity that uses a binary alpha Material
• For this to be effective it must get its alpha from a texture
• Your scene must have at least one Entity that uses an additive Material
• Your scene must have at least three Entities that use a partially transparent Material, and those Entities must have some overlap on screen to prove that they are sorted correctly back-to-front
• In other words, when I start your program there should be one partially transparent object closest to the camera, and then I must be able to see another partially transparent object through the closest one, and then I should be able to see another partially transparent object through both of them
• I should be able to change the order that these partially transparent Entities are declared in your Scene file and the sorting should still work correctly
• You must separate the Entities that get drawn into two different "buckets":
• Opaque
• This should contain every Entity that is drawn with materials that have either no alpha or binary alpha
• (In other words, the result of any fragments drawn in this bucket only depend on the current fragment shader; whatever value the pixel had before will have no influence on the current value)
• This bucket must use the same sorting algorithm you created for Assignment 05/06 (Effect->Material->Entity)
• Alpha blending must not be enabled while this bucket is drawing
• Translucent
• This should contain every Entity that is drawn with materials where ALPHABLENDENABLE is true
• (In other words, the result of each fragment drawn in this bucket depends not only on the current fragment shader, but also on whatever previous value the pixel had)
• The Entities in this bucket must be sorted back-to-front based on the distance from the camera
• The sorting should be done on a per-Entity basis
• You can use the squared distance from the Entity to the camera to sort, since it is cheaper than computing the actual distance: x^2 + y^2 + z^2
• You don't need to do any secondary sorting besides distance (the chances of two objects being the same distance from the camera are pretty small)
• The Opaque bucket must be drawn first, and the Translucent bucket is drawn next
• You must create new PIX events for each bucket
• All of your events from Assignment 05/06 should now be under "Opaque"
• All of the new alpha blend Entities should be under "Translucent". Since the Translucent draw calls are sorted by distance you won't need any sub-events other than "Entity"
• Your writeup must include two screenshots from PIX:
• One where all I see are the collapsed buckets. So, it should look something like this:
• Frame
• Clear
• BeginScene
• Opaque
• Translucent
• EndScene
• Present
• One where the buckets are expanded

Details

Enabling Alpha

• You will need to change Direct3D render states
• Look up the documentation for IDirect3DDevice9::SetRenderState()
• Enable alpha blending by setting ALPHABLENDENABLE to true
• Set the other relevant render states (the source, destination, and operation) based on what the current Effect specifies
• All of the render states to set should be data-driven, and must come from your Effect file.
• There are two potential strategies for storing render states in a file
• You may choose to actually store key/values that match the Direct3D states exactly. Then when you switch to a new Effect you can just iterate through them and set them as is
• If you do this you need to make sure you have a good strategy to ensure that you are always in a known state. You would probably need to store an ALPHABLENDENABLE for every Effect, for example, and have it set to either true or false, or else as soon as you set it to true it would never get turned off again
• You may choose to store the render states in your file in more of a high-level representation, and make your renderer code smart enough to know what to do
• For example, you might just have a single "alpha" state, which could be either "none", "blend", or "additive". Then, the C++ renderer code would be responsible for knowing what states to set for each of those.
• Note that alpha blending should not be enabled for binary alpha Effects, since we are handling it as part of the shader

Additive Materials

• Generally you want a texture with a black background (since adding zero to anything doesn't affect it), and then some sort of special effect that will be added to the scene. I found this link with some free textures that might give you some ideas (be sure to read and respect the license if you use one)

Binary Alpha Textures

• Something like a leaf works well for binary alpha. I found this link with some free textures (be sure to read and respect the license if you use one)