## Tuesday, September 28, 2010

### some CG terms that you might get confused

1. Algorithm
ach algorithm is a list of well-defined instructions for completing a task.
http://en.wikipedia.org/wiki/Algorithm

2. Ray Casting (not to get confused with Ray Tracing)
Ray casting can refer to:

* the general problem of determining the first object intersected by a ray,
* a technique for hidden surface removal based on finding the first intersection of a ray cast from the eye through each pixel of an image,
* a non-recursive variant of ray tracing that only casts primary rays, or
* a direct volume rendering method, also called volume ray casting.
http://en.wikipedia.org/wiki/Ray_casting

3. Ray Tracing
is a technique for generating an image by tracing the path of light through pixels in an image plane and simulating the effects of its encounters with virtual objects. The technique is capable of producing a very high degree of visual realism, usually higher than that of typical scanline rendering methods,
http://en.wikipedia.org/wiki/Ray_tracing_%28graphics%29

4. Scanline Rendering
in 3D computer graphics, that works on a row-by-row basis rather than a polygon-by-polygon or pixel-by-pixel basis.
http://en.wikipedia.org/wiki/Scanline_rendering

## Wednesday, September 22, 2010

### Something you don't know about Blending Mode, aka "matte-free" compositing

a. The blending operations below (1-4) are "SYMMETRICAL", which means you can swap the 2 input image without getting different results.

1. SCREEN MODE:
-double up the exposure (it shouldn't block the background)
-the Weighted Screen Operation (reduced the opacity of background, less opacity was seen, as the usual Screen operation might make the image looked pale and disaturated)
-good for lense flare, light glow, laser, light saber compositing

2. MULTIPLY:
-each image retain their "identity", a more punchy result is given

3. MAXIMUM (LIGHTEN):
-compare 2 image on a pixel by pixel basis. Whichever pixel is the maximum becomes the output
-good for fire (life footage) compositing

4. MINIMUM (DARKEN):
-the opposite of MAXIMUM

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b. COMPLEX BLENDING MODE:

SOME BASIC MATH AND EQUATIONS:

1. "A" (blend layer) over "B" (base layer)

"take complement of the base layer (B) and then multiply that by the blend layer (A)"
thus, the equation will be:

(1-B) x A

if "A" as "input 1", "B" as "input 2", the mythical math node as 3 channel equations will look something like this:

R channel out = (1- r1) x r1
G channel out = (1- g1) x g2
B channel out = (1- b1) x b2

2. "pseudo-code" is used here. you will have to convert the equations to the syntax for your package.
"*" for multiply
"if/then" for square root

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1. OVERLAY:
- changes its behavior depending on its pixel value of the base layer color. if base layer color pixel is less then 0.5, it will do a Multiply operation; if more then 0.5, it switches to a Screen operation.

If (B <>
2 x A x B

else
1 - 2 x (1 - A) x (1 - B)

2. HARD LIGHT:
- changes its behavior depending on its pixel value of the blend layer color. if blend layer color pixel is less then 0.5, it will do a Multiply operation; if more then 0.5, it switches to a Screen operation.

If (A<0.5)>
2 x A x B

else
1 - 2 x (1 - A) x (1 - B)

3. SOFT LIGHT
- also changes its behavior depending on its pixel value of the blend layer color. Its visual result is similar to OVERLAY operation but the calculation is much more complicated:

If (A<0.5)>
2 x A x B + (B² x (1 - 2 x A)

else
sqrt(B) x (2 x A - 1) x 2 x B x (1 - A)

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more run down of other Photoshop blending modes, visit:

UNKNOWN KEY WORD:
-Banding Effect

QUOTED FROM
DIGITAL COMPOSITING FOR FILM AND VIDEO, BY STEVE WRIGHT