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The OceanWater sample demonstrates the use of vertex
and pixel shaders to create the effect of a shiny oscillating
ocean surface with multiple waves moving in different
directions. Rendering realistic ocean water is particularly
difficult due to the shape of the surface of the water
and combination of multiple waves, and the reflection
of the clouds, and the sun in the sky.
The vertex and pixel shaders are used to implement
a cubic environment map to provide the reflection of
the sky and clouds, a deforming grid that simulates
the larger waves, two bump maps for smaller waves, and
an indexed Fresnel map to provide the surface color.
The input geometry is a grid of quads with 1 set of
texture coordinates, normals and tangents. A prerendered
sky and clouds texture is used for the cubic environment
map.
The vertex shader is responsible for generating the
combination of sine waves that perturb the position,
and cosine waves that perturb the tangent space vectors
for the vertex. A Taylor series approximation is used
to generate sine and cosine functions within the shader.
Each sine wave has fully adjustable direction, frequency,
speed, and offset, that is configured in the constant
store.
The pixel shader is responsible for producing the bump-mapped
reflective ocean surface. First, the pixel shader averages
the two scrolling normal bump maps to generate a composite
normal. Next, it transforms the tangent space composite
normal into world space and calculates a per-pixel reflection
vector. The reflection vector is used to sample a skybox
cubic environment map. The shader also calculates 2*N.V,
and uses it to sample a Fresnel 1-D texture. This Fresnel
map gives the water a more greenish appearance when
looking straight down into it, and a more bluish appearance
when looking edge on.
The second phase of the pixel shader composites the
water color, from the Fresnel map, the environment map,
and other specular highlights extracted from the environment
map. One trick we use is to square the environment map
color values to make the colors brighter, and to enhance
the contrast for compositing. To get the specular light
sparkles in the water, a specular component is derived
from the green channel of the environment map. The desired
effect is to have the highlights in the water correspond
to bright spots in the sky. To make sure the specular
peaks were only generated from the brightest areas of
the environment map, the specular value extracted from
the green channel was raised to the eighth power. This
has the effect of darkening all but the brightest areas
of the image
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