Understanding Textures, Formats, and Deferred Shading in Computer Graphics - Prof. Timothy, Study notes of Computer Science

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Textures

 Types



Formats

 Samplers

 Access functions

 Deferred Shading

Types

 Signed or unsigned

 (^) Byte  (^) Int  (^) Short Int

 float

 half float (16 bit float)

GL_ARB_half_float_pixel



Packed types (Version >= 1.2)

 GL_UNSIGNED_SHORT_5_6_

 GL_UNSIGNED_SHORT_5_5_5_

 (^) And many more...

sRGB Colorspace

 “standard” RGB

 (^) A perceptually uniform colorspace  (^) Perceived intensity is proportional to sRGB values

 Related to gamma correction

 (^) Monitor doesn't output linear intensities  (^) I: Intensity  (^) C: contrast  (^) V: pixel value  (^) epsilon: black level (brightness)

I = CV

 

sRGB Colorspace

 Gamma correction



sRGB specifies different gamma for different intensity

ranges

 As a result doubling the color component values in an

sRGB texture will result in the perceived intensity doubling.

V

c

= V

1  V (^) c =

12.92 V for V <= 0. 1.055 V 1

−0.055 for V > 0.

GLSL Samplers

 sampler1D, 2D, 3D



samplerCube

 sampler1DShadow

 sampler2DShadow

 (^) Depth textures with comparison

 sampler2DRect



sampler3DRect

 Sampler2DRectShadow

 (^) Bind to GL_TEXTURE_RECTANGLE_ARB  (^) Use unnormalized texture coordinates

Access functions

 texture1D, 2D, 3D



textureCube

 texture1DProj, 2DProj, 3DProj

 texture1DLod, 2DLod, 3DLod, CubeLod

 (^) Arguments (sampler, coord, lod)  (^) Only in vertex shader  (^) Why? Normally LOD (mipmap level) is determined at the fragment processing stage.

 texture1DProjLod, 2DProjLod, 3DProjLod

Depth Texture Access Functions

Commonly used for shadow mapping with

COMPARE_R_TO_TEXTURE



shadow1D, 2D :

 (^) return 0.0 or 1.0 as comparison results  (^) R = coord.p

 shadow1DProj, 2DProj

 (^) return 0.0 or 1.0 as comparison results  (^) R = coord.p/coord.q 

shadow1DLod, 2DLod



shadow1DProjLod, 2DProjLod

Deferred Shading

Deering, Michael; Stephanie Winner, Bic Schediwy, Chris Duffy, Neil Hunt. "The triangle processor and normal vector shader: a VLSI system for high performance graphics". ACM SIGGRAPH Computer Graphics 22 (4): 21–30, 1988. Also see “6800 Leagues Under the Sea” Nvidia deferred shading presentation 

Avoid shading until pixel visibility has been determined.



Render lighting parameters to textures, then compute

fragment colors

 “fat framebuffer” or “geometry buffer” (g-buffer)

 (^) use multiple render targets:  (^) Position (or depth)  (^) Normal  (^) Base (diffuse color)  (^) Material properties

Deferred Shading Setup

 Defining the g-buffer:



What spatially varying parameters will you need to do

shading?

 (^) Normals : Normal mapping, diffuse and specular lighting  (^) Diffuse color  (^) Shininess : specular lighting

 Create framebuffer object and attach textures for g-buffer



Create framebuffer object and attach textures for light

accumulation

G-buffer

 One possible layout:

8 bits per component won't be enough-

Use 16 or 32 bit float per component

3 Shininess Diffuse.r Diffuse.g Diffuse.b Amb. Occ. Position.x Position.y Position.z Emissive Normal.x Normal.y Normal.z

Geometry Phase

 Bind geometry framebuffer object



Draw geometry

 In fragment shader write to g-buffer



Unbind geometry framebuffer object

gl_FragData[0] = vec4( position, 0.0); gl_FragData[1] = vec4( bumpNormal * 0.5 + 0.5, base.a); gl_FragData[2] = vec4( base.rgb, 0.0);

Lighting Phase

 Bind light accumulation framebuffer object



Bind g-buffer textures so we can read from g-buffer

 Use additive blending

 (^) glBlendFunc( GL_ONE, GL_ONE);

 Draw the light volumes (more later)

 In fragment shader

 (^) Look up normal, diffuse color, etc. from textures  (^) Render lit colors 

Unbind light accumulation framebuffer object

The stencil buffer

A logical buffer (usually 1 to 8 bits) which can determine

where drawing happens on a pixel-by-pixel basis.

Stencil operations happen near the end of the pipeline – after

fragment processing and even after alpha testing.

Stencil buffer applications

 Deferred shading light volumes



Shadow volumes (“Stencil shadows”)

 Portals / mirrors

 Screen overlays / UI

 Screen-space transitions : dissolve