CSS 2D Transforms Module Level 3

Abstract

CSS 2D Transforms allows elements rendered by CSS to be transformed in two-dimensional space.

Status of this document

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This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

The list of changes made to this specification is available.

Table of contents

• 1. Introduction
• 2. The ‘transform’ Property
• 3. The ‘transform-origin’ Property
• 4. The Transformation Functions
• 5. Transform Values and Lists
• 6. Transitions and animations between transform values
• 7. Matrix decomposition for animation
• 8. DOM Interfaces
  • 8.1. CSSMatrix
• 9. References
  • Normative references
  • Other references
• Property index
• Index

1. Introduction

This section is not normative.

The CSS visual formatting model describes a coordinate system within which each element is positioned. Positions and sizes in this coordinate space can be thought of as being expressed in pixels, starting in the upper left corner of the parent with positive values proceeding to the right and down.

This coordinate space can be modified with the ‘transform’ property. Using transform, elements can be translated, rotated and scaled in two dimensional space. The coordinate space behaves as described in the coordinate system transformations section of the SVG 1.1 specification. This is a coordinate system with two axes: the X axis increases horizontally to the right; the Y axis increases vertically downwards.

Specifying a value other than ‘none’ for the ‘transform’ property establishes a new local coordinate system at the element that it is applied to. Transformations are cumulative. That is, elements establish their local coordinate system within the coordinate system of their parent. In this way, a ‘transform’ property effectively accumulates all the ‘transform’ properties of its ancestors. The accumulation of these transforms defines a current transformation matrix (CTM) for the element.

The transform property does not affect the flow of the content surrounding the transformed element. However, the value of the overflow area takes into account transformed elements. This behavior is similar to what happens when elements are translated via relative positioning. Therefore, if the value of the ‘overflow’ property is ‘scroll’ or ‘auto’, scrollbars will appear as needed to see content that is transformed outside the visible area.

Any value other than ‘none’ for the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants.

2. The ‘transform’ Property

A two-dimensional transformation is applied to an element through the ‘transform’ property. This property contains a list of transform functions. The final transformation value for an element is obtained by performing a matrix concatenation of each entry in the list. The set of transform functions is similar to those allowed by SVG.

3. The ‘transform-origin’ Property

The ‘transform-origin’ property establishes the origin of transformation for an element. This property is applied by first translating the element by the negated value of the property, then applying the element's transform, then translating by the property value. This effectively moves the desired transformation origin of the element to (0,0) in the local coordinate system, then applies the element's transform, then moves the element back to its original position.

If only one value is specified, the second value is assumed to be ‘center’. If at least one value is not a keyword, then the first value represents the horizontal position and the second represents the vertical position. Negative <percentage> and <length> values are allowed.

4. The Transformation Functions

The value of the transform property is a list of <transform-functions> applied in the order provided. The individual transform functions are separated by whitespace. The set of allowed transform functions is given below. In this list the type <translation-value> is defined as a <length> or <percentage> value, and the <angle> type is defined by CSS Values and Units.

matrix(<number>, <number>, <number>, <number>, <number>, <number>)

specifies a 2D transformation in the form of a transformation matrix of six values. matrix(a,b,c,d,e,f) is equivalent to applying the transformation matrix [a b c d e f].

translate(<translation-value>[, <translation-value>])

specifies a 2D translation by the vector [tx, ty], where tx is the first translation-value parameter and ty is the optional second translation-value parameter. If <ty> is not provided, ty has zero as a value.

translateX(<translation-value>)

specifies a translation by the given amount in the X direction.

translateY(<translation-value>)

specifies a translation by the given amount in the Y direction.

scale(<number>[, <number>])

specifies a 2D scale operation by the [sx,sy] scaling vector described by the 2 parameters. If the second parameter is not provided, it is takes a value equal to the first.

scaleX(<number>)

specifies a scale operation using the [sx,1] scaling vector, where sx is given as the parameter.

scaleY(<number>)

specifies a scale operation using the [1,sy] scaling vector, where sy is given as the parameter.

rotate(<angle>)

specifies a 2D rotation by the angle specified in the parameter about the origin of the element, as defined by the transform-origin property.

skewX(<angle>)

specifies a skew transformation along the X axis by the given angle.

skewY(<angle>)

specifies a skew transformation along the Y axis by the given angle.

skew(<angle> [, <angle>])

specifies a skew transformation along the X and Y axes. The first angle parameter specifies the skew on the X axis. The second angle parameter specifies the skew on the Y axis. If the second parameter is not given then a value of 0 is used for the Y angle (ie. no skew on the Y axis).

5. Transform Values and Lists

The <translation-value> values are defined as [<percentage> | <length>]. All other value types are described as CSS types. If a list of transforms is provided, then the net effect is as if each transform had been specified separately in the order provided. For example,

  <div style="transform:translate(-10px,-20px) scale(2) rotate(45deg) translate(5px,10px)"/>
  

is functionally equivalent to:

  <div style="transform:translate(-10px,-20px)">
    <div style="transform:scale(2)">
      <div style="transform:rotate(45deg)">
        <div style="transform:translate(5px,10px)">
        </div>
      </div>
    </div>
  </div>
  

  div {
      transform: translate(100px, 100px);
  }
  

Move the element by 100 pixels in both the X and Y directions.

  div {
      height: 100px; width: 100px;
      transform: translate(80px, 80px) scale(1.5, 1.5) rotate(45deg);
  }
  

Move the element by 80 pixels in both the X and Y directions, then scale the element by 150%, then rotate it 45 degrees clockwise about the Z axis. Note that the scale and rotate operate about the center of the element, since the element has the default transform-origin of 50% 50%.

6. Transitions and animations between transform values

When animating or transitioning the value of a transform property the rules described below are applied. The ‘from’ transform is the transform at the start of the transition or current keyframe. The ‘end’ transform is the transform at the end of the transition or current keyframe.

• If the ‘from’ and ‘to’ transforms are both single functions of the same type:
  • For translate, translateX, translateY, scale, scaleX, scaleY, rotate, skew, skewX and skewY functions:
    • the individual components of the function are interpolated numerically.
  • For matrix:
    • the matrix is decomposed using the method described by unmatrix into separate translation, scale, rotation and skew matrices, then each decomposed matrix is interpolated numerically, and finally combined in order to produce a resulting 3x2 matrix.
• If both the ‘from’ and ‘to’ transforms are "none":
  • There is no interpolation necessary
• If one of the ‘from’ or ‘to’ transforms is "none":
  • The ‘none’ is replaced by an equivalent identity function list for the corresponding transform function list.

    For example, if the ‘from’ transform is "scale(2)" and the ‘to’ transform is "none" then the value "scale(1)" will be used as the ‘to’ value, and animation will proceed using the rule above. Similarly, if the ‘from’ transform is "none" and the ‘to’ transform is "scale(2) rotate(50deg)" then the animation will execute as if the ‘from’ value is "scale(1) rotate(0)".

    The identity functions are translate(0), translateX(0), translateY(0), scale(1), scaleX(1), scaleY(1), rotate(0), rotateX(0), rotateY(0), skewX(0), skewY(0), skew(0, 0) and matrix(1, 0, 0, 1, 0, 0).

• If both the ‘from’ and ‘to’ transforms have the same number of transform functions and corresponding functions in each transform list are of the same type:
  • Each transform function is animated with its corresponding destination function in isolation using the rules described above. The individual values are then applied as a list to produce resulting transform value.
• Otherwise:
  • The transform function lists are each converted into the equivalent matrix value and animation proceeds using the rule for a single function above.

In some cases, an animation might cause a transformation matrix to be singular or non-invertible. For example, an animation in which scale moves from 1 to -1. At the time when the matrix is in such a state, the transformed element is not rendered.

7. Matrix decomposition for animation

When interpolating between 2 matrices, each is decomposed into the corresponding translation, rotation, scale, skew, and perspective values. Not all matrices can be accurately described by these values. Those that can't are decomposed into the most accurate representation possible, using the technique below. This technique is taken from The "unmatrix" method in "Graphics Gems II, edited by Jim Arvo". The pseudocode below works on a 4x4 homogeneous matrix. A 3x2 2D matrix is therefore first converted to 4x4 homogeneous form.

  Input: matrix       ; a 4x4 matrix
  Output: translation ; a 3 component vector
          rotation    ; Euler angles, represented as a 3 component vector
          scale       ; a 3 component vector
          skew        ; skew factors XY,XZ,YZ represented as a 3 component vector
          perspective ; a 4 component vector
  Returns false if the matrix cannot be decomposed, true if it can

    Supporting functions (point is a 3 component vector, matrix is a 4x4 matrix):
      float  determinant(matrix)          returns the 4x4 determinant of the matrix
      matrix inverse(matrix)              returns the inverse of the passed matrix
      matrix transpose(matrix)            returns the transpose of the passed matrix
      point  multVecMatrix(point, matrix) multiplies the passed point by the passed matrix 
                                          and returns the transformed point
      float  length(point)                returns the length of the passed vector
      point  normalize(point)             normalizes the length of the passed point to 1
      float  dot(point, point)            returns the dot product of the passed points
      float  cos(float)                   returns the cosine of the passed angle in radians
      float  asin(float)                  returns the arcsine in radians of the passed value
      float  atan2(float y, float x)      returns the principal value of the arc tangent of 
                                          y/x, using the signs of both arguments to determine 
                                          the quadrant of the return value

    Decomposition also makes use of the following function:
      point combine(point a, point b, float ascl, float bscl)
          result[0] = (ascl * a[0]) + (bscl * b[0])
          result[1] = (ascl * a[1]) + (bscl * b[1])
          result[2] = (ascl * a[2]) + (bscl * b[2])
          return result


    // Normalize the matrix.
    if (matrix[3][3] == 0)
        return false

    for (i = 0; i < 4; i++)
        for (j = 0; j < 4; j++)
            matrix[i][j] /= matrix[3][3]

    // perspectiveMatrix is used to solve for perspective, but it also provides
    // an easy way to test for singularity of the upper 3x3 component.
    perspectiveMatrix = matrix

    for (i = 0; i < 3; i++)
        perspectiveMatrix[i][3] = 0

    perspectiveMatrix[3][3] = 1

    if (determinant(perspectiveMatrix) == 0)
        return false

    // First, isolate perspective.
    if (matrix[0][3] != 0 || matrix[1][3] != 0 || matrix[2][3] != 0)
        // rightHandSide is the right hand side of the equation.
        rightHandSide[0] = matrix[0][3];
        rightHandSide[1] = matrix[1][3];
        rightHandSide[2] = matrix[2][3];
        rightHandSide[3] = matrix[3][3];

        // Solve the equation by inverting perspectiveMatrix and multiplying
        // rightHandSide by the inverse.
        inversePerspectiveMatrix = inverse(perspectiveMatrix)
        transposedInversePerspectiveMatrix = transposeMatrix4(inversePerspectiveMatrix)
        perspective = multVecMatrix(rightHandSide, transposedInversePerspectiveMatrix)

         // Clear the perspective partition
        matrix[0][3] = matrix[1][3] = matrix[2][3] = 0
        matrix[3][3] = 1
    else
        // No perspective.
        perspective[0] = perspective[1] = perspective[2] = 0
        perspective[3] = 1

    // Next take care of translation
    translate[0] = matrix[3][0]
    matrix[3][0] = 0
    translate[1] = matrix[3][1]
    matrix[3][1] = 0
    translate[2] = matrix[3][2]
    matrix[3][2] = 0

    // Now get scale and shear. 'row' is a 3 element array of 3 component vectors
    for (i = 0; i < 3; i++)
        row[i][0] = matrix[i][0]
        row[i][1] = matrix[i][1]
        row[i][2] = matrix[i][2]

    // Compute X scale factor and normalize first row.
    scale[0] = length(row[0])
    row[0] = normalize(row[0])

    // Compute XY shear factor and make 2nd row orthogonal to 1st.
    skew[0] = dot(row[0], row[1])
    row[1] = combine(row[1], row[0], 1.0, -skew[0])

    // Now, compute Y scale and normalize 2nd row.
    scale[1] = length(row[1])
    row[1] = normalize(row[1])
    skew[0] /= scale[1];

    // Compute XZ and YZ shears, orthogonalize 3rd row
    skew[1] = dot(row[0], row[2])
    row[2] = combine(row[2], row[0], 1.0, -skew[1])
    skew[2] = dot(row[1], row[2])
    row[2] = combine(row[2], row[1], 1.0, -skew[2])

    // Next, get Z scale and normalize 3rd row.
    scale[2] = length(row[2])
    row[2] = normalize(row[2])
    skew[1] /= scale[2]
    skew[2] /= scale[2]

    // At this point, the matrix (in rows) is orthonormal.
    // Check for a coordinate system flip.  If the determinant
    // is -1, then negate the matrix and the scaling factors.
    pdum3 = cross(row[1], row[2])
    if (dot(row[0], pdum3) < 0)
        for (i = 0; i < 3; i++) {
            scale[0] *= -1;
            row[i][0] *= -1
            row[i][1] *= -1
            row[i][2] *= -1

    // Now, get the rotations ou
    rotate[1] = asin(-row[0][2]);
    if (cos(rotate[1]) != 0)
       rotate[0] = atan2(row[1][2], row[2][2]);
       rotate[2] = atan2(row[0][1], row[0][0]);
    else
       rotate[0] = atan2(-row[2][0], row[1][1]);
       rotate[2] = 0;

    return true;
    

Each component of each returned value is linearly interpolated with the corresponding component of the other matrix. The resulting components are then recomposed into a final matrix as though combining the following transform functions:

        matrix3d(1,0,0,0, 0,1,0,0, 0,0,1,0, perspective[0], perspective[1], perspective[2], perspective[3])
        translate3d(translation[0], translation[1], translation[2])
        rotateX(rotation[0]) rotateY(rotation[1]) rotateZ(rotation[2])
        matrix3d(1,0,0,0, 0,1,0,0, 0,skew[2],1,0, 0,0,0,1)
        matrix3d(1,0,0,0, 0,1,0,0, skew[1],0,1,0, 0,0,0,1)
        matrix3d(1,0,0,0, skew[0],1,0,0, 0,0,1,0, 0,0,0,1)
        scale3d(scale[0], scale[1], scale[2])
      

8. DOM Interfaces

This section describes the interfaces and functionality added to the DOM to support runtime access to the functionality described above.

8.1. CSSMatrix

Interface CSSMatrix

The CSSMatrix interface represents a 4x4 homogeneous matrix.

IDL Definition

  interface CSSMatrix {
      attribute float a;
      attribute float b;
      attribute float c;
      attribute float d;
      attribute float e;
      attribute float f;

      void        setMatrixValue(in DOMString string) raises(DOMException);
      CSSMatrix   multiply(in CSSMatrix secondMatrix);
      CSSMatrix   multiplyLeft(in CSSMatrix secondMatrix);
      CSSMatrix   inverse() raises(DOMException);
      CSSMatrix   translate(in float x, in float y);
      CSSMatrix   scale(in float scaleX, in float scaleY);
      CSSMatrix   skew(in float angleX, in float angleY);
      CSSMatrix   rotate(in float angle);
  };

Attributes

a-f of type float

Each of these attributes represents one of the values in the 3x2 matrix.

Methods

setMatrixValue

The setMatrixValue method replaces the existing matrix with one computed from parsing the passed string as though it had been assigned to the transform property in a CSS style rule.

Parameters

string of type DOMString

The string to parse.

No Return Value

Exceptions

DOMException SYNTAX_ERR

Thrown when the provided string can not be parsed into a CSSMatrix.

multiply

The multiply method returns a new CSSMatrix which is the result of this matrix multiplied by the passed matrix, with the passed matrix to the right. This matrix is not modified.

Parameters

secondMatrix of type CSSMatrix

The matrix to multiply.

Return Value

CSSMatrix

The result matrix.

No Exceptions

multiplyLeft

The multiplyLeft method returns a new CSSMatrix which is the result of this matrix multiplied by the passed matrix, with the passed matrix to the left. This matrix is not modified.

Parameters

secondMatrix of type CSSMatrix

The matrix to multiply.

Return Value

CSSMatrix

The result matrix.

No Exceptions

inverse

The inverse method returns a new matrix which is the inverse of this matrix. This matrix is not modified.

No Parameters

Return Value

CSSMatrix

The inverted matrix.

Exceptions

DOMException NOT_SUPPORTED_ERR

Thrown when the CSSMatrix can not be inverted.

translate

The translate method returns a new matrix which is this matrix post multiplied by a translation matrix containing the passed values. This matrix is not modified.

Parameters

x of type float

The X component of the translation value.

y of type float

The Y component of the translation value.

Return Value

CSSMatrix

The result matrix.

No Exceptions

scale

The scale method returns a new matrix which is this matrix post multiplied by a scale matrix containing the passed values. If the y component is undefined, the x component value is used in its place. This matrix is not modified.

Parameters

scaleX of type float

The X component of the scale value.

scaleY of type float

The (optional) Y component of the scale value.

Return Value

CSSMatrix

The result matrix.

No Exceptions

rotate

The rotate method returns a new matrix which is this matrix post multiplied by a rotation matrix. The rotation value is in degrees. This matrix is not modified.

Parameters

angle of type float

The angle of rotation.

Return Value

CSSMatrix

The result matrix.

No Exceptions

skew

The skew method returns a new matrix which is this matrix post multiplied by a skew matrix. The rotation value is in degrees. This matrix is not modified.

Parameters

angleX of type float

The angle of skew along the X axis.

angleY of type float

The angle of skew along the Y axis.

Return Value

CSSMatrix

The result matrix.

No Exceptions

In addition to the interface listed above, the getComputedStyle method of the Window object has been updated. The transform property of the style object returned by getComputedStyle contains a DOMString of the form "matrix(a, b, c, d, e, f)" representing the 3x2 matrix that is the result of applying the individual functions listed in the transform property.

9. References

Normative references

Other references

Property index

Index

• transform, 2.
• transform-origin, 3.