15 Filter Effects

15.1 Introduction

A model for adding declarative raster-based rendering effects to a 2D graphics environment is presented. As a result, the expressiveness of the traditional 2D rendering model is greatly enhanced, while still preserving the device independence, scalability, and high level geometric description of the underlying graphics.

15.2 Background

On the Web, many graphics are presented as bitmap images in gif, jpg, or png format. Among the many disadvantages of this approach is the general difficulty of keeping the raster data in sync with the rest of the Web site. Many times, a web site designer must resort to a bitmap editor to simply change the title of a button. As the Web gets more dynamic, we desire a way to describe the "piece parts" of a site in a more flexible format. This chapter describes SVG's declarative filter effects model, which when combined with the 2D power of SVG can describe much of the common artwork on the web in such a way that client-side generation and alteration can be performed easily.

15.3 Basic Model

The filter effects model consists of a set of filtering operations (called "processing nodes" in the descriptions below) on one or more graphic primitives. Each processing node takes a set of graphics primitives as input and generates a new set of graphics primitives as output. After all processing nodes have completed, the resulting set of graphics primitives are returned and then rendered to the target media.

For example, a simple filter could replace one graphic by two -- by adding a black copy of original offset to create a drop shadow. In effect, there are now two layers of graphics, both with the same original set of graphics primitives. In this example, the bottommost shadow layer could be blurred and become a raster layer, while the topmost layer could remain as higher-order graphics primitives (e.g., text or vector objects). Ultimately, the two layers are composited together and rendered into the background.

Filter effects introduce an additional step into the traditional 2D graphics pipeline. Consider the traditional 2D graphics pipeline:

Traditional 2D graphics pipeline

Vector graphics primitives are specified abstractly and rendered onto the output device through a geometric transformation called the current transformation matrix, or CTM. The CTM allows the vector graphics code to be specified in a device independent coordinate system. At rendering time, the CTM accounts for any differences in resolution or orientation of the input vector description space and the device coordinate system. According to the "painter's model", areas on the device which are outside of the vector graphic shapes remain unchanged from their previous contents (in this case the droplet pattern).

Consider now, altering this pipeline slightly to allow intermediate filtering operations performed on the graphics primitives, producing a modified set of graphics primitives. The modified set of graphics primitives are rendered onto the output device in a second pass (??? This picture needs to be updated to show vector effects in addition to raster effects):

Rendering via Continuous Tone
Intermediate Image

Many of the filter effects processing nodes operate on raster images only. If a given processing nodes requires an image as input, then all graphics elements are converted into an intermediate continuous tone raster image (a process called rasterization), which is then passed to the given processing node for processing. For those cases where rasterization is necessary, we introduce a new transformation matrix called the Effect Transformation Matrix, or ETM. Whenever it is necessary to perform raster effects, the graphics primitives will need to be rendered (i.e., rasterized) via the ETM onto the intermediate continuous tone image, using the ETM as the user space to image space transformation matrix. Ultimately, this intermediate continuous tone image will be rendered onto the output device using the standard 2D imaging path via a modified transform, CTM', such that the net effect of ETM followed by CTM' is equivalent to the original CTM. It is important to note that the filter effects intermediate continuous tone image contains coverage information so that non rendered parts of the original graphic are transparent in the intermediate image and remain unaffected on the output device, as required by the painter's model. A physical analog to this process is to imagine rendering the vector primitives onto a sheet of clear acetate and then transforming and overlaying the acetate sheet onto the output device. The resulting imaging model remains as device-independent as the original one, except we are now using the 2D imaging model itself to generate images to render.

So far, we really haven't added any new expressiveness to the imaging model. What we have done is reformulated the traditional 2D rendering model to allow an intermediate phase. However, now we can extend this further by allowing the application of vector and raster filter effects operations on the intermediate graphics primitives, still without sacrificing device independence. After applying the filter effects operations, the resulting set of graphics primitives are then rendered onto the device in the same way as above. (??? This picture needs to be updated to show vector effects in addition to raster effects).

Rendering via Continuous Tone
Intermediate Step with Filter Effects Processing

In the picture above, the intermediate set of graphics primitives was processed in two ways. First a simple bump map lighting calculation was applied to add a 3D look, then another copy of the original layer was offset, blurred and colored black to form a shadow. The resulting transparent layers were then rendered via the painter's model onto the output device.

15.4 Defining and Invoking a Filter Effect

Filter effects are defined by a <filter> element with an associated ID. Filter effects are applied to elements which have a filter: property which reference a <filter> element. Here is an example:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG June 1999//EN" 
  "http://www.w3.org/Graphics/SVG/svg-19990625.dtd">
<svg width="4in" height="3in">
  <defs>
    <filter id="CoolTextEffect">
      <!-- Definition of filter goes here -->
    </filter>
  </defs>
  <text style="filter:url(#CoolTextEffect)">Text with a cool effect</text>
</svg>

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When applied to grouping elements such as <g>, the filter: property applies to the contents of the group as a whole. The effect should be as if the group's children did not render to the screen directly but instead just added their resulting graphics primitives to the group's graphics display list (GDL), which is then passed to the filter for processing. After the group filter is processed, then the result of the filter should be rendered to the target device (or passed onto a parent grouping element for further processing in cases such as when the parent has its own group filter).

The <filter> element consists of a sequence of processing nodes which take a set of graphics primitives as input, apply filter effects operations on the graphics primitives, and produce a modified set of graphics primitives as output. The processing nodes are executed in sequential order. Here is an example:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG June 1999//EN" 
  "http://www.w3.org/Graphics/SVG/svg-19990625.dtd">
  <filter id="Shadow">
    <REGaussianBlur     in="SourceAlpha" 
                        radius="3"
                        nodeid="blurredAlpha" />
    <REOffset           in="blurredAlpha" 
                        dx="2" dy="1"
                        nodeid="offsetBlurredAlpha" /> 
    <REDiffuseLighting  in="blurredAlpha" 
                        diffuse-constant=".5"
                        nodeid="bumpMapDiffuse" />
    <REComposite        in="bumpMapDiffuse" in2="SourcePaint"
                        operator="arithmetic" k1="1"
                        nodeid="litPaint" />  
    <RESpecularLighting in="blurredAlpha" 
                        specular-constant=".5"
                        specular-exponent="10"
                        light-color="distant-light"
                        nodeid="bumpMapSpecular" />  
    <REComposite        in="litPaint" in2="bumpMapSpecular"
                        operator="arithmetic" k2="1" k3="1"
                        nodeid="litPaint" />  
    <REComposite        in="litPaint"
                        in2="SourceAlpha" 
                        mode="AinB"
                        nodeid="litPaint" /> 
    <REMerge>
      <REMergeNode in="litPaint" />
      <REMergeNode in="offsetBlurredAlpha" />
    </REMerge>
  </filter>

  <text style="font-size:36; fill:red; filter:url(#Shadow)"
        x="10" y="250">Shadowed Text</text>
</svg>

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For most processing nodes, the in (and sometimes in2) attribute identifies the graphics which serve as input and the nodeid attribute gives a name for the resulting output. The in and in2 attributes can point to either:

A built-in keyword. In the example above, the <REGaussianBlur> processing node specifies keyword SourceGraphic, which indicates that the original set of graphics primitives available at the start of the filter should be used as input to the processing node.
A reference to an earlier nodeid. In the example above, the <REOffset> processing node refers to the most recent processing node whose nodeid is blurredAlpha. (In this case, that would be the <REGaussianBlur> processing node.)

The default value for in is the output generated from the previous processing node. In those cases where the output from a given processing node is used as input only by the next processing node, it is not necessary to specify the nodeid on the previous processing node or the in attribute on the next processing node. In the example above, there are a few cases (show highlighted) where nodeid and in did not have to be provided.

Filters do not use XML IDs for nodeids; instead, nodeids can be any arbitrary attribute string value. nodeids only are meaningful within a given <filter> element and thus have only local scope. If a nodeid appears multiple times within a given <filter> element, then a reference to that nodeid will use the closest preceding processing node with the given nodeid. Forward references to nodeids are invalid.

15.5 Filter Effects Region

A <filter> element can define a region on the canvas on which a given filter effect applies and can provide a resolution for any intermediate continuous tone images used in to process any raster-based processing nodes. The <filter> element has the following attributes:

filter-units={ userspace | object-bbox }. Defines the coordinate system for attributes x, y, width, height. If filter-units="userspace" (the default), x, y, width, height represent values in the current user coordinate system in place at the time when the <filter> element is defined. If filter-units="object-bbox", then x, y, width, height represent values in an abstract coordinate system where (0,0) is the (minx,miny) in user space of the tight bounding box of the object referencing the filter, and (1,1) is the (maxx,maxy) corner of the bounding box. (Note: the bounding box represents the maximum extent of the shape of the object in X and Y with respect to the user coordinate system of the object exclusive of stroke-width.)
x, y, width, height, which indicate the rectangle for the offscreen buffer, where the values are either relative to the current user coordinate system (if filter-units="userspace") or relative to the current object (if filter-units="target-object"). Note that the clipping path used to render any graphics within the filter will consists of the intersection of the current clipping path associated with the given object and the rectangle defined by x, y, width, height. The default values for x, y, width, height are 0%, 0%, 100% and 100%, respectively.
filter-res, which has the form x-pixels [y-pixels]) and indicates the width/height of the intermediate images in pixels. If not provided, then a reasonable default resolution appropriate for the target device will be used. (For displays, an appropriate display resolution, preferably the current display's pixel resolution, should be the default. For printing, an appropriate common printer resolution, such as 400dpi, should be the default.)

For performance reasons on display devices, it is recommended that the filter effect region is designed to match pixel-for-pixel with the background pixmap.

It is often necessary to provide padding space because the filter effect might impact bits slightly outside the tight-fitting bounding box on a given object. For these purposes, it is possible to provide negative percentage values for x, y and percentages values greater than 100% for width, height. For example, x="-10%" y="-10%" width="110%" height="110%".

15.6 Parameter Substitution

It is possible to define a generalized template filter and then invoke it multiple times with slightly different parameters using the filter effect's parameter substitution features. Filter parameters are defined by defining a given attribute value within a processing node to be:

the string "{$"
immediately followed by a valid XML name
immediately followed by the string "}"

To illustrate:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG June 1999//EN" 
  "http://www.w3.org/Graphics/SVG/svg-19990625.dtd">
<svg width="4in" height="3in">
  <defs>
    <filter id="UseParams">
      <REImageTurbulence base-frequency="{$freq}" num-octaves="{$num}" />
    </filter>
  </defs>
  <rect x="70" y="10" width="40" height="40"
        style="fill:red; 
               filter:url(#UseParams);
               filter-params:freq(20) num(10)" />
</svg>

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15.7 Common Attributes

The following two attributes are available for all processing nodes (the exception is REMerge and REImage, which do not have an in attribute):

Common Attributes

nodeid Assigned name for this node. If supplied, then the GDL resulting after processing the given node is saved away and can be referenced as input to a subsequent processing node.

in If supplied, then indicates that this processing node should use either the output of previous node as input or use some standard keyword to specify alternate input. Available keywords include:

SourceGraphic. This built-in input represents the graphics elements that were the original input into the <filter> element. For raster effects processing nodes, the graphics elements will be rasterized into an initially clear RGBA raster in image space. Pixels left untouched by the original graphic will be left clear. The image is specified to be rendered in linear RGBA pixels. The alpha channel of this image captures any anti-aliasing specified by SVG. (Since the raster is linear, the alpha channel of this image will represent the exact percent coverage of each pixel.)
SourceAlpha. Same as SourceGraphic except only the alpha channel is specified. The color channels of this image are implicitly black and are unaffected by any image processing operations. Again, pixels unpainted by the SourceGraphic will be 0. The SourceAlpha image will also reflects any Opacity settings in the SourceGraphic. If this option is used, then some implementations may need to rasterize the graphics elements in order to extract the alpha channel.
BackgroundImage This built-in input represents an image snapshot of the rendering surface under the filter region at the time that the <filter> element was invoked.
BackgroundAlpha Same as BackgroundImage except only the alpha channel is specified.
FillPaint This image represents the color data specified by the current SVG rendering state, transformed to image space. The FillPaint image has conceptually infinite extent in image space. (Since it is usually either just a constant color, or a tile). Frequently this image is opaque everywhere, but it might not be if the "paint" itself has alpha, as in the case of an alpha gradient or transparent pattern. For the simple case where the source graphic represents a simple filled object, it is guaranteed that: SourceGraphic = In(FillPaint,SourceAlpha) where In(A,B) represents the resulting image of Porter-Duff compositing operation A in B (see below).
StrokePaint Similar to FillPaint, except for the stroke color as specified in SVG. Again for the simple case where the source graphic represents stroked path, it is guaranteed that: SourceGraphic = In(StrokePaint,SourceAlpha) where In(A,B) represents the resulting image of Porter-Duff compositing operation A in B (see below).

15.8 Filter Processing Nodes

The following is a catalog of the individual processing nodes. Unless otherwise stated, all image filters operate on linear premultiplied RGBA samples. Filters which work more naturally on non premultiplied data (ColorMatrix and ComponentTransfer) will temporarily undo and redo premultiplication as specified. All raster effect filtering operations take 1 to N input RGBA images, additional attributes as parameters, and produce a single output RGBA image.

NodeType	ColorMatrix
Processing Node-Specific Attributes	type, string (one of: matrix, saturate, hue-rotate, luminance-to-alpha) values For matrix: space-separated list of 20 element color transform (a00 a01 a02 a03 a04 a10 a11 ... a34). For example, the identity matrix could be expressed as: type="matrix" values="1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0" For saturate: one real number (0 to 1) For hue-rotate: one real number (degrees) Not applicable for luminance-to-alpha
Description	This filter performs \| R' \| \| a00 a01 a02 a03 a04 \| \| R \| \| G' \| \| a10 a11 a12 a13 a14 \| \| G \| \| B' \| = \| a20 a21 a22 a23 a24 \| * \| B \| \| A' \| \| a30 a31 a32 a33 a34 \| \| A \| \| 1 \| \| 0 0 0 0 1 \| \| 1 \| for every pixel. The RGBA and R'G'B'A' values are automatically non-premultiplied temporarily for this operation. The following shortcut definitions are provide for compactness. The following tables show the mapping from the shorthand form to the corresponding longhand (i.e., matrix with 20 values) form: saturate value (0..100) s = value/100 \| R' \| \|0.213+0.787s 0.715-0.715s 0.072-0.072s 0 0 \| \| R \| \| G' \| \|0.213-0.213s 0.715+0.285s 0.072-0.072s 0 0 \| \| G \| \| B' \| = \|0.213-0.213s 0.715-0.715s 0.072+0.928s 0 0 \| * \| B \| \| A' \| \| 0 0 0 1 0 \| \| A \| \| 1 \| \| 0 0 0 0 1 \| \| 1 \| hue-rotate value (0..360) \| R' \| \| a00 a01 a02 0 0 \| \| R \| \| G' \| \| a10 a11 a12 0 0 \| \| G \| \| B' \| = \| a20 a21 a22 0 0 \| * \| B \| \| A' \| \| 0 0 0 1 0 \| \| A \| \| 1 \| \| 0 0 0 0 1 \| \| 1 \| where the terms a00, a01, etc. are calculated as follows: \| a11 a12 a13 \| [+0.213 +0.715 +0.072] \| a21 a22 a13 \| = [+0.213 +0.715 +0.072] + \| a11 a12 a13 \| [+0.213 +0.715 +0.072] [+0.787 -0.715 -0.072] cos(hue-rotate value) * [-0.212 +0.285 -0.072] + [-0.213 -0.715 +0.928] [-0.213 -0.715+0.928] sin(hue-rotate value) * [+0.143 +0.140-0.283] [-0.787 +0.715+0.072] Thus, the upper left term of the hue matrix turns out to be: .213 + cos(hue-rotate value).787 - sin(hue-rotate value).213 luminance-to-alpha \| R' \| \| 0 0 0 0 0 \| \| R \| \| G' \| \| 0 0 0 0 0 \| \| G \| \| B' \| = \| 0 0 0 0 0 \| * \| B \| \| A' \| \| 0.299 0.587 0.114 0 0 \| \| A \| \| 1 \| \| 0 0 0 0 1 \| \| 1 \|
Implementation issues	These matrices often perform an identity mapping in the alpha channel. If that is the case, an implementation can avoid the costly undoing & redoing of the premultiplication for all pixels with A = 1.

NodeType	ComponentTransfer
Processing Node-Specific Attributes	None.
Processing Node-Specific Sub-Elements	Each <ComponentTransfer> element needs to have exactly one each of the following sub-elements, each of which is an empty element: <Fr>, transfer function for red component <Fg>, transfer function for green component <Fb>, transfer function for blue component <Fa>, transfer function for alpha component Each of these sub-elements (i.e., <Fr>, <Fg>, <Fb>, <Fa>) can have the following attributes: Common parameters to all transfer modes: type, string (one of: identity, table, linear, gamma) Parameters specific to particular transfer modes: For table: table-values, list of real number values v0,v1,...vn. For linear: slope, real number value giving slope of linear equation. intercept, real number value giving Y-intercept of linear equation. For gamma (see descriptiong below for descriptions): amplitude, real number value. exponent, real number value. offset, real number value.
Description	This filter performs component-wise remapping of data as follows: R' = Fr( R ) G' = Fr( G ) B' = Fr( B ) A' = Fr( A ) for every pixel. The RGBA and R'G'B'A' values are automatically non-premultiplied temporarily for this operation. When type="table", the transfer function consists of a linearly interpolated lookup table. k/N <= C < (k+1)/N => C' = vk + (C - k/N)N (vk+1 - vk) When type="linear", the transfer function consists of a linear function describes by the following equation: C' = slopeC + offset When type="gamma", the transfer function consists of the following equation: C' = amplitudepow(C, exponent) + offset
Comments	This filter allows operations like brightness adjustment, contrast adjustment, color balance or thresholding. We might want to consider some predefined transfer functions such as identity, gamma, sRGB transfer, sine-wave, etc.
Implementation issues	Similar to the ColorMatrix filter, the undoing and redoing of the premultiplication can be avoided if Fa is the identity transform and A = 1.

NodeType

REBlend

Processing Node-Specific Attributes

mode, One of the image blending modes (see table below). Default is: normal

in2, The second image ("B" in the formulas) for the compositing operation.

Description

This filter composites two objects together using commonly used high-end imaging software blending modes. Performs the combination of the two input images pixel-wise in image space.

Implementation Notes

The compositing formula, expressed using premultiplied colors:

qr = qb*(1-qa) + qa*(1-qb) + qa*qb
cr = (1-qa)*cb + (1-qb)*ca + qa*qb*(Blend(ca/qa,cb/qb))
where: 
  qr = Result opacity 
  cr = Result color (RGB) - premultiplied 
  qa = Opacity value at a given pixel for image A 
  qb = Opacity value at a given pixel for image B 
  ca = Color (RGB) at a given pixel for image A - premultiplied 
  cb = Color (RGB) at a given pixel for image B - premultiplied 
  Blend = Image compositing function, depending on the compositing mode

The following table provides the list of available image blending modes:

Image Blending Mode	Blend() function
normal	Ca
multiply	Ca*Cb
screen	1-(1-Ca)*(1-Cb)
darken	(to be provided later)
lighten	(to be provided later)

NodeType	REColor
Processing Node-Specific Attributes	color, RGBA color (floating point?)
Description	Creates an image with infinite extent filled with color

NodeType	REComposite
Processing Node-Specific Attributes	operator, one of (over, in, out, atop, xor, arithmetic). Default is: over. arithmetic-constants, k1,k2,k3,k4 in2, The second image ("B" in the formulas) for the compositing operation.
Description	This filter performs the combination of the two input images pixel-wise in image space. over, in, atop, out, xor use the Porter-Duff compositing operations. For these operations, the extent of the resulting image can be affected. In other words, even if two images do not overlap in image space, the extent for over will essentially include the union of the extents of the two input images. arithmetic evaluates k1i1i2 + k2i1 + k3i2 + k4, using componentwise arithmetic with the result clamped between [0..1].
Comments	arithmetic are useful for combining the output from the DiffuseLighting and SpecularLighting filters with texture data. arithmetic is also useful for implementing dissolve.

NodeType	REDiffuseLighting
Processing Node-Specific Attributes	result-scale (Multiplicative scale for the result. This allows the result of the DiffuseLighting nodeto represent values greater than 1) surface-scale height of surface when Ain = 1. diffuse-constant kd in Phong lighting model. Range 0.0 to 1.0 light-color RGB
Processing Node-Specific Sub-Elements	One of <distant-light azimuth= elevation= > <point-light x= y= z= > <spot-light x= y= z= points-at-x= points-at-y= points-at-z= specular-exponent=>
Description	Light an image using the alpha channel as a bump map. The resulting image is an RGBA opaque image based on the light color with alpha = 1.0 everywhere. The lighting caculation follows the standard diffuse component of the Phong lighting model. The resulting image depends on the light color, light position and surface geometry of the input bump map. Color or texture is mean to be applied via a multiply (mul) composite operation. Dr = (kd * N.L * Lr) / result-scale Dg = (kd * N.L * Lg) / result-scale Db = (kd * N.L * Lb) / result-scale Da = 1.0 / result-scale where kd = diffuse lighting constant N = surface normal unit vector, a function of x and y L = unit vector pointing from surface to light, a function of x and y in the point and spot light cases Lr,Lg,Lb = RGB components of light, a function of x and y in the spot light case result-scale = overall scaling factor N is a function of x and y and depends on the surface gradient as follows: The surface described by the input alpha image Ain (x,y) is: Z (x,y) = surface-scale * Ain (x,y) Surface normal is calculated using the Sobel gradient 3x3 filter: Nx (x,y)= - surface-scale * 1/4(( I(x+1,y-1) + 2I(x+1,y) + I(x+1,y+1)) - (I(x-1,y-1) + 2I(x-1,y) + I(x-1,y+1))) Ny (x,y)= - surface-scale 1/4(( I(x-1,y+1) + 2I(x,y+1) + I(x+1,y+1)) - (I(x-1,y-1) + 2I(x,y-1) + I(x+1,y-1))) Nz (x,y) = 1.0 N = (Nx, Ny, Nz) / Norm((Nx,Ny,Nz)) L, the unit vector from the image sample to the light is calculated as follows: For Infinite light sources it is constant: Lx = cos(azimuth)cos(elevation) Ly = -sin(azimuth)cos(elevation) Lz = sin(elevation) For Point and spot lights it is a function of position: Lx = Lightx - x Ly = Lighty - y Lz = Lightz - Z(x,y) L = (Lx, Ly, Lz) / Norm(Lx, Ly, Lz) where Lightx, Lighty, and Lightz are the input light position. Lr,Lg,Lb, the light color vector is a function of position in the spot light case only: Lr = Lightrpow((-L.S),specular-exponent) Lg = Lightgpow((-L.S),specular-exponent) Lb = Lightbpow((-L.S),specular-exponent) where S is the unit vector pointing from the light to the point (points-at-x, points-at-y, points-at-z) in the x-y plane: Sx = points-at-x - Lightx Sy = points-at-y - Lighty Sz = points-at-z - Lightz S = (Sx, Sy, Sz) / Norm(Sx, Sy, Sz) If L.S is positive no light is present. (Lr = Lg = Lb = 0)
Comments	This filter produces a light map, which can be combined with a texture image using the multiply term of the arithmetic <Composite> compositing method. Multiple light sources can be simulated by adding several of these light maps together before applying it to the texture image.

NodeType	REDisplacementMap
Processing Node-Specific Attributes	scale x-channel-selector one of R,G,B or A. y-channel-selector one of R,G,B or A in2, The second image ("B" in the formulas) for the compositing operation.
Description	Uses Input2 to spatially displace Input1, (similar to the Photoshop displacement filter). This is the transformation to be performed: P'(x,y) <- P( x + scale * ((XC(x,y) - .5), y + scale * (YC(x,y) - .5)) where P(x,y) is the source image, Input1, and P'(x,y) is the destination. XC(x,y) and YC(x,y) are the component values of the designated by the x-channel-selector and y-channel-selector. For example, to use the R component of Image2 to control displacement in x and the G component of Image2 to control displacement in y, set x-channel-selector to "R" and y-channel-selector to "G".
Comments	The displacement map defines the inverse of the mapping performed.
Implementation issues	This filter can have arbitrary non-localized effect on the input which might require substantial buffering in the processing pipeline. However with this formulation, any intermediate buffering needs can be determined by scale which represents the maximum displacement in either x or y.

NodeType	REGaussianBlur
Processing Node-Specific Attributes	std-deviation.
Description	Perform gaussian blur on the input image. The Gaussian blur kernel is an appoximation of the normalized convolution: H(x) = exp(-x2/ (2s2)) / sqrt(2* pis2) where 's' is the standard deviation specified by std-deviation. This can be implemented as a separable convolution. For larger values of 's' (s >= 2.0), an approximation can be used: Three successive box-blurs build a piece-wise quadratic convolution kernel, which approximates the gaussian kernel to within roughly 3%. let d = floor(s 3sqrt(2pi)/4 + 0.5) ... if d is odd, use three box-blurs of size 'd', centered on the output pixel. ... if d is even, two box-blurs of size 'd' (the first one centered one pixel to the left, the second one centered one pixel to the right of the output pixel one box blur of size 'd+1' centered on the output pixel.
Implementation Issues	Frequently this operation will take place on alpha-only images, such as that produced by the built-in input, SourceAlpha. The implementation may notice this and optimize the single channel case. If the input has infinite extent and is constant, this operation has no effect. If the input has infinite extent and is a tile, the filter is evaluated with periodic boundary conditions.

NodeType

REImage

Processing Node-Specific Attributes

href, reference to external image data.
transform, supplemental transformation specification

Description

Refers to an external image which is loaded or rendered into an RGBA raster. If imaging-matrix is not specified, the image takes on its natural width and height and is positioned at 0,0 in image space.

The imageref could refer to an external image, or just be a reference to another piece of SVG. This node produces an image similar to the builtin image source SourceGraphic except from an external source.

NodeType	REMerge
Processing Node-Specific Attributes	none
Processing Node-Specific Sub-Elements	Each <REMerge> element can have any number of <REMergeNode> subelements, each of which has an in attribute.
Description	Composites input image layers on top of each other using the over operator with Input1 on the bottom and the last specified input, InputN, on top.
Comments	Many effects produce a number of intermediate layers in order to create the final output image. This filter allows us to collapse those into a single image. Although this could be done by using n-1 Composite-filters, it is more convenient to have this common operation available in this form, and offers the implementation some additional flexibility (see below).
Implementation issues	The canonical implementation of REMerge is to render the entire effect into one RGBA layer, and then render the resulting layer on the output device. In certain cases (in particular if the output device itself is a continuous tone device), and since merging is associative, it may be a sufficient approximation to evaluate the effect one layer at a time and render each layer individually onto the output device bottom to top.

NodeType	REMorphology
Processing Node-Specific Attributes	operator,one of erode or dilate. radius, extent of operation
Description	This filter is intended to have a similar effect as the min/max filter in Photoshop and the width layer attribute in ImageStyler. It is useful for "fattening" or "thinning" an alpha channel, The dilation (or erosion) kernel is a square of side 2*radius + 1.
Implementation issues	Frequently this operation will take place on alpha-only images, such as that produced by the built-in input, SourceAlpha. In that case, the implementation might want to optimize the single channel case. If the input has infinite extent and is constant, this operation has no effect. If the input has infinite extent and is a tile, the filter is evaluated with periodic boundary conditions.

NodeType	REOffset
Processing Node-Specific Attributes	dx,dy
Description	Offsets an image relative to its current position in the image space by the specified vector.
Comments	This is important for effects like drop shadow etc.

NodeType	RESpecularLighting
Processing Node-Specific Attributes	surface-scale height of surface when Ain = 1. specular-constant ks in Phong lighting model. Range 0.0 to 1.0 specular-exponent exponent for specular term, larger is more "shiny". Range 1.0 to 128.0. light-color RGB One of distant-light azimuth, elevation point-light (x,y,z) spot-light (x,y,z, points-at-x, points-at-y, points-at-z, specular-exponent)
Description	Light an image using the alpha channel as a bump map. The resulting image is an RGBA image based on the light color. The lighting caculation follows the standard specular component of the Phong lighting model. The resulting image depends on the light color, light position and surface geometry of the input bump map. The result of the lighting calculation is added. We assume that the viewer is at infinity the z direction (i.e the unit vector in the eye direction is (0,0,1) everywhere. Sr = ks * pow(N.H, specular-exponent) * Lr Sg = ks * pow(N.H, specular-exponent) * Lg Sb = ks * pow(N.H, specular-exponent) * Lb Sa = max(Sr, Sg, Sb) where ks = specular lighting constant N = surface normal unit vector, a function of x and y H = "halfway" unit vectorbetween eye unit vector and light unit vector Lr,Lg,Lb = RGB components of light See DiffuseLighting for definition of N and (Lr, Lg, Lb). The definition of H reflects our assumption of the constant eye vector E = (0,0,1): H = (L + E) / Norm(L+E) where L is the light unit vector. Unlike the DiffuseLighting, the SpecularLighting filter produces a non-opaque image. This is due to the fact that specular result (Sr,Sg,Sb,Sa) is meant to be added to the textured image. The alpha channel of the result is the max of the color components, so that where the specular light is zero, no additional coverage is added to the image and a fully white highlight will add opacity.
Comments	This filter produces an image which contains the specular reflection part of the lighting calculation. Such a map is intended to be combined with a texture using the add term of the arithmetic Composite method. Multiple light sources can be simulated by adding several of these light maps before applying it to the texture image.
Implementation issues	The DiffuseLighting and SpecularLighting filters will often be applied together. An implementation may detect this and calculate both maps in one pass, instead of two.

NodeType	RETile
Processing Node-Specific Attributes	none
Description	Creates an image with infinite extent by replicating source image in image space.

NodeType	RETurbulence
Processing Node-Specific Attributes	base-frequency num-octaves type, one offractal-noise or turbulence.
Description	Adds noise to an image using the Perlin turbulence-function. It is possible to create bandwidth-limited noise by synthesizing only one octave. For a detailed description the of the Perlin turbulence-function, see "Texturing and Modeling", Ebert et al, AP Professional, 1994. If the input image is infinite in extent, as is the case with a constant color or a tile, the resulting image will have maximal size in image space.
Comments	This filter allows the synthesis of artificial textures like clouds or marble.
Implementation issues	It might be useful to provide an actual implementation for the turbulence function, so that consistent results are achievable.

NodeType

VEAdjustGraphics

Processing Node-Specific Attributes

transform a supplemental transformation (e.g., a scale, a rotate or a translate) on the current filter effects coordinate system

style override CSS properties that applies to all objects in the current GDL

Description

Add a supplemental transformation (e.g., a scale, a rotate or a translate) on the current coordinate system or supply override CSS properties (e.g., a different fill or stroke property) that will apply to all objects in the current GDL

NodeType	VEPathTurbulence
Processing Node-Specific Attributes	base-frequency num-octaves
Description	Adds noise to a vector graphics using 1-dimensional Perlin turbulence-function.
Implementation issues	It might be useful to provide an actual implementation for the turbulence function, so that consistent results are achievable.

15 Filter Effects

15.1 Introduction

15.2 Background

15.3 Basic Model

Traditional 2D graphics pipeline

Rendering via Continuous Tone Intermediate Image

Rendering via Continuous Tone Intermediate Step with Filter Effects Processing

15.4 Defining and Invoking a Filter Effect

15.5 Filter Effects Region

15.6 Parameter Substitution

15.7 Common Attributes

15.8 Filter Processing Nodes

ColorMatrix

ComponentTransfer

REBlend

REColor

REComposite

REDiffuseLighting

REDisplacementMap

REGaussianBlur

REImage

REMerge

REMorphology

REOffset

RESpecularLighting

RETile

RETurbulence

VEAdjustGraphics

VEPathTurbulence

Rendering via Continuous Tone
Intermediate Image

Rendering via Continuous Tone
Intermediate Step with Filter Effects Processing