Shaders, the 3D photoshop Part 2

In my previous blogpost, I went over the many algorithms used in both 2D and 3D computer graphics. I talked about how they are essentially the same. We’ll use a screen shot from my game Under the Radar that I editted in photoshop, before and after respectively.

Image

Image

Drop shadowing in photoshop is the same as shadow mapping in which it checks if a point is visible from the light or not. If a point is visible from the light then it’s obviously not in shadow, otherwise it is. The basic shadow mapping algorithm can be described as short as this:

– Render the scene from the lights view and store the depths as shadow map

– Render the scene from the camera and compare the depths, if the current fragments depth is greater than the shadow depth then the fragment is in shadow

In some instances, drop shadows are used to make objects stand out of the background with a an outline, in shaders this is done with sobel edge filters.

The Sobel operator performs a 2-D spatial gradient measurement on an image and so emphasizes regions of high spatial frequency that correspond to edges. Typically it is used to find the approximate absolute gradient magnitude at each point in an input grayscale image.

In theory at least, the operator consists of a pair of 3×3 convolution kernels. One kernel is simply the other rotated by 90°. This is very similar to the Roberts Cross operator.

These kernels are designed to respond maximally to edges running vertically and horizontally relative to the pixel grid, one kernel for each of the two perpendicular orientations. The kernels can be applied separately to the input image, to produce separate measurements of the gradient component in each orientation (call these Gx and Gy). These can then be combined together to find the absolute magnitude of the gradient at each point and the orientation of that gradient.

In photoshop, filters are added to images to randomize the noise and alter the look. The equivalent in shaders is known as normal mapping. Normal maps are images that store the direction of normals directly in the RGB values of the image. They are much more accurate, as rather than only simulating the pixel being away from the face along a line, they can simulate that pixel being moved at any direction, in an arbitrary way. The drawbacks to normal maps are that unlike bump maps, which can easily be painted by hand, normal maps usually have to be generated in some way, often from higher resolution geometry than the geometry you’re applying the map to.

Normal maps in Blender store a normal as follows:

  • Red maps from (0-255) to X (-1.0 – 1.0)
  • Green maps from (0-255) to Y (-1.0 – 1.0)
  • Blue maps from (0-255) to Z (0.0 – 1.0)

Since normals all point towards a viewer, negative Z-values are not stored, also map blue colors (128-255) to (0.0 – 1.0). The latter convention is used in “Doom 3” as an example.

Those are a majority of effects that shaders use that are similar to photoshop effects, there’s also color adjustment which can be done in color to HSL shaders along with other sorts of effects.

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