A Non-Photorealistic Fragment Shader in OpenGL 2.0

A Non-Photorealistic Fragment Shader in OpenGL 2.0

Bert FreudenbergBert Freudenberg

Institut für Simulation und GraphikInstitut für Simulation und GraphikUniversity of Magdeburg, GermanyUniversity of Magdeburg, Germany

Bert Freudenberg, University of Magdeburg

OutlineOutline• OpenGL 2.0 proposal OpenGL 2.0 proposal

• Vertex and fragment shadersVertex and fragment shaders

• Non-photorealistic shadingNon-photorealistic shading

• Anti-aliasing in a shaderAnti-aliasing in a shader

• LightingLighting

• Adding noiseAdding noise

• ConclusionConclusion

Bert Freudenberg, University of Magdeburg

OpenGL 2.0 ProposalOpenGL 2.0 Proposal

Shading LanguageShading Language•High-level language for

•vertex shaders

•fragment shaders

•even more

•Experimentally implemented as GL2 extension

•available on 3Dlabs Wildcat VP

Bert Freudenberg, University of Magdeburg

Base for our shaderBase for our shaderScott F. Johnston’s “Mock Media”Scott F. Johnston’s “Mock Media”

•From “Advanced RenderMan: Beyond the Companion” (SIGGRAPH ’98 Course #11)

•RenderMan surface shader for woodprint-like appearance

•Shading by lines of varying width

Bert Freudenberg, University of Magdeburg

Vertex ShaderVertex Shader

• Uses constant and per-vertex data to set up Uses constant and per-vertex data to set up attributes attributes varyingvarying across the primitive across the primitive

• Our shader:Our shader:•one surface parameter

•screen-space position

Bert Freudenberg, University of Magdeburg

Vertex ShaderVertex Shadervarying float v; void main(void){ v = gl_MultiTexCoord0.s; gl_Position = gl_ModelViewProjectionMatrix *

gl_Vertex;}

Bert Freudenberg, University of Magdeburg

Fragment ShaderFragment Shader

• Gets interpolated Gets interpolated varyingvarying parametersparameters

• Similar to RenderMan surface shaderSimilar to RenderMan surface shader

• Evaluates some function to output Evaluates some function to output colorcolor at a at a certain point in parameter spacecertain point in parameter space

• Our shader: Our shader: •output black or white to create lines

Bert Freudenberg, University of Magdeburg

Parameter Parameter varying float v;varying float v;

v = texcoord0.s;v = texcoord0.s;

• Assigned in vertex Assigned in vertex shadershader

• Interpolated value in Interpolated value in fragment shaderfragment shader

Bert Freudenberg, University of Magdeburg

Sawtooth waveSawtooth wavesawtooth = sawtooth =

fract( v * 16. );fract( v * 16. );

Bert Freudenberg, University of Magdeburg

Triangle waveTriangle wavetriangle = triangle =

abs(2. * sawtooth - abs(2. * sawtooth - 1.);1.);

Bert Freudenberg, University of Magdeburg

Square waveSquare wavesquare = square =

step(0.5, triangle);step(0.5, triangle);

• Aliasing!Aliasing!

Bert Freudenberg, University of Magdeburg

Anti-aliasingAnti-aliasing

• FSAA will not helpFSAA will not help• just raises resolution

• Need to remove higher frequenciesNeed to remove higher frequencies•manually

• Step function has unlimited frequencyStep function has unlimited frequency•use smooth step instead

Bert Freudenberg, University of Magdeburg

Constant width smoothstep()Constant width smoothstep()

square =square =smoothstep(0.4, 0.6, smoothstep(0.4, 0.6, triangle);triangle);

• Buggy, hence:Buggy, hence:edge0 = 0.4; edge1 = 0.6;t = clamp((triangle -

edge0) / (edge1 - edge0), 0., 1.);

square = t * t * (3. - 2. * t);

• Aliasing + blurringAliasing + blurring

Bert Freudenberg, University of Magdeburg

DerivativesDerivatives• dPdx(v), dPdy(v)dPdx(v), dPdy(v)

• Derivative of Derivative of parameter v in screen parameter v in screen x and yx and y

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Length of derivativeLength of derivativedp = length(vec2(dPdx, dp = length(vec2(dPdx,

dPdy));dPdy));

Bert Freudenberg, University of Magdeburg

Adaptive filter widthAdaptive filter widthfilterstep()filterstep()

float edge = 0.5;float edge = 0.5;

float w = 64. * dp;float w = 64. * dp;

float square = float square = clamp((triangle + 0.5 clamp((triangle + 0.5 * w - edge) / w, 0., 1.);* w - edge) / w, 0., 1.);

Bert Freudenberg, University of Magdeburg

Raising frequencyRaising frequency

• No more individual No more individual lineslines

• Too dense in certain Too dense in certain regionsregions

• Adjust frequency Adjust frequency based on screen based on screen space densityspace density

Bert Freudenberg, University of Magdeburg

Partitioning by derivativePartitioning by derivativeilogdp = floor(log2(dp));ilogdp = floor(log2(dp));

• No log2() yetNo log2() yet• Texture as lookup tableTexture as lookup table

vec3 tex0 = texture3(0, dp * 8.);

float logdp = (tex0.r - 0.5) * 256. + tex0.g;

Bert Freudenberg, University of Magdeburg

Adjusting frequencyAdjusting frequencyexp2(floor(log2(dp))) * exp2(floor(log2(dp))) *

f;f;

• frequency doubles in frequency doubles in discrete stepsdiscrete steps

• also works for also works for distance!distance!

Bert Freudenberg, University of Magdeburg

Tapering endsTapering ends• linearly interpolate to linearly interpolate to

double frequencydouble frequency

t = fract(log2(dp));triangle = abs((1. + t) *

triangle - t);

Bert Freudenberg, University of Magdeburg

LightingLightingVertex ShaderVertex Shader

pos = vec3(gl_ModelViewMatrix * gl_Vertex);

tnorm = normalize(gl_NormalMatrix * gl_Normal);

vec3 lightVec = normalize(LightPosition - pos);

lightIntensity = max(dot(lightVec, tnorm), 0.0);

Bert Freudenberg, University of Magdeburg

LightingLighting• Line width Line width

dependent on dependent on lightinglighting

• Adjust threshold by Adjust threshold by light intensitylight intensity

square = step(lightIntensity, triangle);

Bert Freudenberg, University of Magdeburg

NoiseNoise• No noise() yetNo noise() yet

3D tilable noise tex = 3D tilable noise tex = (F(x,y,z)*(t-x)*(t-y)*(t-z)+F(x-t,y,z)*(x)*(t-y)*(t-z)+F(x-t,y-t,z)*(x)*(y)*(t-z)+F(x,y-t,z)*(t-x)*(y)*(t-z)+F(x,y,z-t)*(t-x)*(t-y)*(z)+F(x-t,y,z-t)*(x)*(t-y)*(z)+F(x-t,y-t,z-t)*(x)*(y)*(z)+F(x,y-t,z-t)*(t-x)*(y)*(z))/(t*t*t);

Bert Freudenberg, University of Magdeburg

Noisy widthNoisy width• Bias thresholdBias threshold

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Noisy wigglesNoisy wiggles

Bert Freudenberg, University of Magdeburg

ConclusionConclusion

• Learn about RenderMan shadersLearn about RenderMan shaders

• Translate to OpenGL 2.0 Translate to OpenGL 2.0 •almost straight-forward

• Anti-aliasing is an issueAnti-aliasing is an issue•use derivatives

• Non-photorealistic Rendering is cool!Non-photorealistic Rendering is cool!