Vertex Buffer Objects, Frame Buffer Objects and Geometry shaders

The modern use of “shader” was introduced to the public by Pixar with their “RenderMan Interface Specification, Version 3.0” originally published in May, 1988.

Image

As graphics processing units evolved, major graphics software libraries such as OpenGL and Direct3D began to support shaders. The first shader-capable GPUs originally only supported pixel shading, but vertex shaders were then introduced when developers realized the power of shaders and sought to take advantage of its potential. Geometry shaders were only fairly recently introduced with Direct3D 10 and OpenGL 3.2, but are currently supported only by high-end video cards.

Geometry in a complete three dimensional scene is lit according to the defined locations of light sources, reflection, and other surface properties. Some hardware implementations of the graphics pipeline compute lighting only at the vertices of the polygons being rendered.

Image

The lighting values between vertices are then interpolated during rasterization. Per-fragment or per-pixel lighting, as well as other effects, can be done on modern graphics hardware as a post-rasterization process by means of a shader program. Modern graphics hardware also supports per-vertex shading through the use of vertex shaders.

Shaders are simple programs that describe the traits of either a vertex or a pixel. Vertex shaders describe the traits such as position, texture coordinates and colors of a vertex, while pixel shaders describe color, z-depth and the alpha value of a fragment. A vertex shader is called for each vertex in a primitive often after tessellation; thus one vertex in, one updated vertex out. Each vertex is then rendered as a series of pixels onto a surface that will be transported to the screen.

Shaders replace a section of video hardware often referred to as the Fixed Function Pipeline (FFP) – so-called because it performs lighting and texture mapping in a hard-coded manner. Shaders provide a programmable alternative to this hard-coded approach for the convenience of the programmers seeking to manage their code better.

Image

The CPU sends instructions (compiled shading language programs) and geometry data to the graphics processing unit, located on the graphics card. In the vertex shader, the geometry is transformed.If a geometry shader is in the graphic processing unit and active, some changes of the geometries in the scene are performed. If a tessellation shader is activated in the graphic processing unit and active, the geometries in the scene can be subdivided.

The calculated geometry is triangulated as the triangles are broken down into fragment quads (one fragment quad is a 2 × 2 fragment primitive). Fragment quads are modified according to the pixel shader, then the depth test is executed, fragments that pass will get written to the screen and might get blended into the frame buffer. The graphic pipeline uses these steps in order to transform three dimensional (and/or two dimensional data into useful two dimensional data for displaying. In general, this is a large pixel matrix or “frame buffer”.

Image

Vertex shaders are passed through once for each vertex given to the graphics processor. The purpose is to transform each vertex’s 3D position in virtual space to the 2D coordinate at which it appears on the screen (as well as a depth value for the Z-buffer).

Image

Vertex shaders are capable of altering properties such as position, color, and texture coordinates, but cannot create new vertices like geometry shaders can. The output of the vertex shader goes to the next stage in the pipeline, which is either a geometry shader if present, or the pixel shader and rasterizer otherwise. Vertex shaders can enable powerful control over the details of position, movement, lighting, and color in any scene involving 3D models.

Geometry shaders are a relatively new type of shader, introduced in Direct3D 10 and OpenGL 3.2; formerly available in OpenGL 2.0+ with the use of extensions. This type of shader can generate new graphics primitives, such as points, lines, and triangles, from those primitives that were sent to the beginning of the graphics pipeline.

Image

Geometry shader programs are executed after vertex shaders. They take as input a whole primitive, possibly with adjacency information. For example, when operating on triangles, the three vertices are the geometry shader’s input. The shader can then emit zero or more primitives, which are rasterized and their fragments ultimately passed to a pixel shader.

Typical uses of a geometry shader include point sprite generation, geometry tessellation in which you cover a surface with a pattern of flat shapes so that there are no overlaps or gaps, shadow volume extrusion where the edges forming the silhouette are extruded away from the light to construct the faces of the shadow volume, and single pass rendering to a cube map. A typical real world example of the benefits of geometry shaders would be automatic mesh complexity modification. A series of line strips representing control points for a curve are passed to the geometry shader and depending on the complexity required the shader can automatically generate extra lines each of which provides a better approximation of a curve.

Image

Pixel shaders, which are also known as fragment shaders, compute color and other attributes of each fragment. Pixel shaders range from always outputting the same color, to applying a lighting value, to performing bump mapping, specular highlights, shadow mapping, translucency and other amazing feats of rendering as shown here.

Image

They can alter the depth of the fragment for Z-buffering, or output more than one color if multiple render targets are active. In 3D graphics, a pixel shader alone cannot produce very complex effects, because it operates only on a single fragment, without knowledge of a scene’s geometry. However, pixel shaders do detect and acknowledge the screen coordinate being drawn, and can sample the screen and nearby pixels if the contents of the entire screen are passed as a texture to the shader. This technique can enable a wide variety of 2D postprocessing effects, such as blur, or edge detection/enhancement for cartoon/cel shading. Pixel shaders may also be applied in intermediate stages to any two-dimensional images in the pipeline, whereas vertex shaders always require a 3D model. For example, a fragment shader is the only type of shader that can act as a postprocessor or filter for a video stream after it has been rasterized.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s