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scriptituk / xfade-easing

FFmpeg Xfade easing and extensions: custom expressions; CSS easings; GL Transitions
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css easing ffmpeg gl-transitions glsl slideshow transitions video-processing xfade

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Easing and extensions for FFmpeg Xfade filter

Standard easings • CSS easings • transpiled GLSL transitions • custom expressions

Summary

This project is a port of standard easing equations, CSS easings and many GLSL transitions for use in tandem with easing or alone. The easing expressions can be used for other filters besides xfade.

Summary

There are 2 variants:

  1. custom ffmpeg build with added xfade easing and reverse options
  2. custom expressions for use with standard ffmpeg build

Xfade is a FFmpeg video transition filter with many built-in transitions and an expression evaluator for custom transitions. However the progress rate is linear, starting and stopping abruptly and proceeding at constant speed, therefore transitions lack interest. Easing inserts a progress envelope to smooth transitions in a natural way.

Example usage:

A CLI wrapper script is provided to generate custom expressions, test videos, visual media sequences and more. It also facilitates generic ffmpeg filter easing – see Easing other filters.

The custom ffmpeg variant is fast with a simple API and no restrictions. Installation involves a few patches to a single ffmpeg C source file, with no dependencies. The custom expression variant is convenient but clunky – see Performance – and runs on plain vanilla ffmpeg but with restrictions: it doesn’t support CSS easings, certain transitions, the reverse feature or full colour parameters.

At present extended transitions are limited to ported GLSL transitions but more effects may be added downstream.

Example

wipedown with cubic easing

wipedown cubic

CLI command (for custom ffmpeg use)

ffmpeg -i first.mp4 -i second.mp4 -filter_complex "
    xfade=duration=3:offset=1:easing=cubic-in-out:transition=wipedown
    " output.mp4

Easing mode in-out is the default mode; the above is equivalent to easing=cubic.
The default easing is linear (none).

CLI command (for custom expression use)

ffmpeg -i first.mp4 -i second.mp4 -filter_complex_threads 1 -filter_complex "
    xfade=duration=3:offset=1:transition=custom:expr='
        st(0, if(lt(P, 0.5), 4 * P^3, 1 - 4 * (1-P)^3)) ;
        if(gt(Y, H*(1-ld(0))), A, B)
    '" output.mp4

Here, the expr parameter is shown on two lines for clarity.
The first line is the easing expression $e(P)$ (cubic in-out) which stores its calculated progress value in st(0).
The second line is the transition expression $t(e(P))$ (wipedown) which loads its eased progress value from ld(0) instead of P. The semicolon token combines expressions.

[!CAUTION] ffmpeg option -filter_complex_threads 1 is required because xfade expression variables (the st() & ld() functions) are shared between slice processing jobs and therefore not thread-safe, consequently processing is much slower

Getting the expressions

In this example you can copy the easing expression from file easings-inline.txt and the transition expression from transitions-rgb24-inline.txt or transitions-yuv420p-inline.txt. Those contain inline expressions for CLI use.

Alternatively use the CLI script:

xfade-easing.sh -t wipedown -e cubic -x -

dumps the xfade expr parameter:

'st(0,if(lt(P,0.5),4*P^3,1-4*(1-P)^3));if(gt(Y,H*(1-ld(0))),A,B)'

Using a script

Some expressions are very long, so using a filtergraph script keeps things manageable and readable.

For this same example you can copy the easing expression from file easings-script.txt and the transition expression from transitions-rgb24-script.txt or transitions-yuv420p-script.txt. Those contain multiline expressions for script use (but the inline expressions work too).

Alternatively use xfade-easing.sh with expansion specifiers expr='%n%X' (see Usage):

xfade-easing.sh -t wipedown -e cubic -s "xfade=offset=10:duration=5:transition=custom:expr='%n%X'" -x script.txt

writes the complete xfade filter description to file script.txt:

xfade=offset=10:duration=5:transition=custom:expr='
st(0, if(lt(P, 0.5), 4 * P^3, 1 - 4 * (1-P)^3))
;
if(gt(Y, H * (1 - ld(0))), A, B)'

and the command becomes

ffmpeg -i first.mp4 -i second.mp4 -filter_complex_threads 1 -/filter_complex script.txt output.mp4`

[!NOTE] this option syntax has changed and is not documented:

  • for ffmpeg version 7+ use -/filter_complex filename
  • for earlier versions use -filter_complex_script filename

Custom FFmpeg

Building ffmpeg

This is easy:

  1. check the Compilation Guide and generic instructions for any prerequisites, e.g. macOS requires Xcode
  2. get the ffmpeg source tree:
    use latest stable release at Download Source Code then extract the .xz archive:
    tar -xJf ffmpeg-x.x.x.tar.xz or use xz/gunzip/etc.
  3. cd ffmpeg and patch libavfilter/vf_xfade.c:
  4. download xfade-easing.h to libavfilter/
  5. run ./configure with any --prefix and other options (drawtext requires --enable-libfreetype --enable-libharfbuzz --enable-libfontconfig); to replicate an existing configuration run ffmpeg -hide_banner -buildconf and copy-paste the options
  6. install required library packages:
    use a package management tool AptGet/MacPorts/Homebrew/etc. (if you install ffmpeg itself then its dependencies will also get installed ready for the custom build); export PATH,LD_LIBRARY_PATH,LDFLAGS environment variables to find the package components
  7. run make, it takes a while
    the C99 code mixes declarations and statements so may produce profuse compiler warnings
    the fix for ld: warning: text-based stub file are out of sync warnings is here
  8. if required run make install or use ffmpeg from the root source directory
  9. test using ffmpeg -hide_banner --help filter=xfade: there should be easing and reverse options under xfade AVOptions

For simplicity, xfade-easing is implemented as static functions in the header file xfade-easing.h and included into vf_xfade.c at an optimal place. As those functions have no external linkage and completely implement an interface that is only visible to the vf_xfade.c compilation unit, this approach is justified IMO, even if unusual, and obviates changing the Makefile. Implementation within header files is not uncommon.

Testing

The custom FFmpeg version has been built and tested on Mac with Clang and Ubuntu Linux with GCC. It uses C99 features so expect some warnings. Any advice on building for Windows would be appreciated.

Custom expressions

Pre-generated easing and transition expressions are in the expr/ subdirectory for mix and match use. The CLI script can produce combined expressions in any syntax using expansion specifiers (like printf).

Inline, for -filter_complex

This format is condensed into a single line stripped of whitespace.

Example: elastic out easing (leaves progress in st(0))

st(0,cos(20*(1-P)*PI/3)/2^(10*(1-P)))

Script, for -/filter_complex

This format is best for expressions that are too unwieldy for inline ffmpeg commands.

Example: gl_rotate_scale_fade transition (expects progress in ld(0) (cf. rotate_scale_fade.glsl)

st(1, 0.5);
st(2, 0.5);
st(3, 1);
st(4, 8);
st(5, X / W - ld(1));
st(6, 1 - Y / H - ld(2));
st(7, hypot(ld(5), ld(6)));
st(5, ld(5) / ld(7));
st(6, ld(6) / ld(7));
st(0, 1 - P);
st(8, 2 * abs(ld(0) - 0.5));
st(8, ld(7) / (ld(4) * (1 - ld(8)) + ld(8)));
st(3, 2 * PI * ld(3) * ld(0));
st(4, sin(ld(3)));
st(3, cos(ld(3)));
st(7, ld(5) * ld(3) - ld(6) * ld(4));
st(6, ld(5) * ld(4) + ld(6) * ld(3));
st(1, ld(1) + ld(7) * ld(8));
st(2, ld(2) + ld(6) * ld(8));
if(between(ld(1), 0, 1) * between(ld(2), 0, 1),
 st(1, ld(1) * W);
 st(2, (1 - ld(2)) * H);
 st(3, ifnot(PLANE, a0(ld(1),ld(2)), ifnot(1-PLANE, a1(ld(1),ld(2)), ifnot(2-PLANE, a2(ld(1),ld(2)), a3(ld(1),ld(2))))));
 st(4, ifnot(PLANE, b0(ld(1),ld(2)), ifnot(1-PLANE, b1(ld(1),ld(2)), ifnot(2-PLANE, b2(ld(1),ld(2)), b3(ld(1),ld(2))))));
 ld(3) * (1 - ld(0)) + ld(4) * ld(0),
 st(1, 0.15);
 gte(ld(1),0) * max(ld(1), eq(PLANE,3)) * 255
)

Uneased, for transitions without easing

These use P directly for progress instead of ld(0). They are especially useful for non-xfade transitions where custom expressions are always needed.

Example: gl_WaterDrop transition (cf. WaterDrop.glsl)

st(1, 30);
st(2, 30);
st(3, 1 - ld(0));
st(4, X / W - 0.5);
st(5, 0.5 - Y / H);
st(6, hypot(ld(4), ld(5)));
st(7, if(lte(ld(6), ld(3)),
 st(1, sin(ld(6) * ld(1) - ld(3) * ld(2)));
 st(4, ld(4) * ld(1));
 st(5, ld(5) * ld(1));
 st(4, X + ld(4) * W);
 st(5, Y - ld(5) * H);
 ifnot(PLANE, a0(ld(4),ld(5)), ifnot(1-PLANE, a1(ld(4),ld(5)), ifnot(2-PLANE, a2(ld(4),ld(5)), a3(ld(4),ld(5))))),
 A
));
ld(7) * (1 - ld(3)) + B * ld(3)

Generic, for easing other filters

These ease ld(0) instead od P - see Easing other filters.

Easing

Standard easings (Robert Penner)

This implementation derives from Michael Pohoreski’s single argument version of Robert Penner’s easing functions, further optimised by me for the peculiarities of xfade.

standard easings

Supplementary easings

The squareroot and cuberoot easings focus more on the middle regions and less on the extremes, opposite to quadratic and cubic respectively:

supplementary easings

All standard and supplementary easings

Here are all the above easings superimposed using the Desmos Graphing Calculator:

all easings

CSS easings

The custom ffmpeg variant supports CSS Easing Functions Level 2 which are too complex for custom expressions:

Linear easing

The new CSS linear() function can approximate any progress contour by interpolating between adjacent points, documented at W3C here. There’s a CSS Linear() Generator online by its pioneer Jake Archibald to convert easings expressed in JavaScript or SVG to linear().

linear easing

Cubic Bézier easing

There are 4 fixed CSS smoothing curves and a general cubic-bezier() easing function documented at W3C here. See also the CSS Cubic Bezier Generator to craft your own. The implementation used here is transpiled from Apple’s open-source Webkit.

cubic-bezier easing

Step easing

The CSS steps() staircase function is for transitions that jump a constant amount, documented at W3C here.

step easing

Reversed CSS easing

Standard easings have ease-in and ease-out modes but not CSS easings. See Reversing xfade effects to achieve this with the custom ffmpeg build.

Overshoots

The elastic and back easings overshoot and undershoot, causing many transitions to clip and others to show colour distortion. Therefore they are quite useless for xfade (but see Easing other filters). CSS easings linear() and cubic-bezier() can also overshoot.

Rendering expressions can only access the two frames of data available. A wrapping overshoot strategy might work for simple horizontal/vertical effects whereby fetching X & Y pixel data is intercepted but at present eased progress outside the range 0 to 1 yields unpredictable results.

Easing other filters

The easing expressions are useful for filters other than xfade, e.g. blend, drawtext, geq, overlay, rotate, zoompan, etc. – anywhere an ffmpeg expr is used to calculate filter options.

For this purpose the CLI script includes text expansion codes %g & %G to generate generic easing expressions for the value in ld(0) (instead of P for xfade), leaving the result in ld(0). You can also copy generic easing expressions from file generic-easings-inline.txt for inline -filter_complex use, or generic-easings-script.txt for -/filter_complex scripts.

To ease other filters, store a normalised input value in st(0,…), append the easing expression, then scale the eased result left in ld(0).

Example: zoompan filter with elastic-out zooming

zoompan elastic-out easing

Here’s the zoom option expression for the zoompan filter:

zoom='st(0, clip((time - 1) / 3, 0, 1));
        st(0, 1 - cos(20 * ld(0) * PI / 3) / 2^(10 * ld(0)));
      lerp(1, 3, ld(0))'

The first line stores a 3 second duration delayed by 1 second normalised to a value between 0 and 1.
The last line scales the result to zoom between 1x and 3x.
The middle line performs elastic-out easing, obtained from generic-easings-script.txt, or
xfade-easing.sh -e elastic-out -s %G -x -

The zoompan filter can produce impressive Ken Burns effects when zoom, x, y are all dynamic. Adding easing can take the illusion of motion even further.

Example: zoompan with back zooming and drawtext with squareroot scrolling

zoompan back + drawtext squareroot easing

The initial zoom here is 1.2x to accommodate the 10% undershoot that back easing produces. So the zoompan zoom expression, with back expr from generic-easings-inline.txt, is:

z='st(0, clip((time - 1) / 3, 0, 1));
     st(0,if(lt(ld(0),0.5),2*ld(0)*ld(0)*(2*ld(0)*3.59491-2.59491),1-2*(1-ld(0))^2*(4.59491-2*ld(0)*3.59491)));
   lerp(1.2, 3.1, ld(0))'

And the drawtext y expression with squareroot easing is:

y='st(0, clip((t - 1) / 3, 0, 1));
     st(0, if(lt(ld(0), 0.5), sqrt(ld(0) / 2), 1 - sqrt((1-ld(0)) / 2)));
   lerp(line_h - 10, h - line_h * 2 - 5, ld(0))'

Transitions

Xfade transitions

The custom ffmpeg variant eases the built-in xfade transitions; these are provided for custom expression use. They are converted from C-code in vf_xfade.c to custom expressions for use with easing. Omitted transitions are distance and hblur which perform aggregation, so cannot be computed efficiently on a per plane-pixel basis.

XFade gallery

Here are the xfade transitions processed using custom expressions instead of the built-in transitions (for testing), without easing – see also the FFmpeg Wiki Xfade page:

Xfade gallery

GLSL transitions

The open collection of GL Transitions initiative lead by Gaëtan Renaudeau (gre) “aims to establish an universal collection of transitions that various softwares can use” released under a Free License.

Other GLSL transition sources were found on shadertoy and the Vegas Forum. All GLSL transitions adapted to the GL Transition Specification are in glsl/.

Many of the transitions at gl-transitions and elsewhere have been here transpiled into native C transitions (for custom ffmpeg variant) and custom expressions (for custom expression variant) for use with or without easing. The list shows the names, authors, and customisation parameters and defaults:

GLSL gallery

Here are the ported GLSL transitions with default parameters and no easing. See also the GL Transitions Gallery which lacks many recent contributor transitions plus even more stacking up as Pull requests – which is why I have not added my bundle.

GL gallery

With easing

GLSL transitions can also be eased, although easing is integral with some:

Example: Swirl transition with bounce easing

Swirl bounce

Customisation parameters

Many GLSL transitions accept parameters to customise the transition effect. The parameters and default values are shown above.

Example: two pinwheel speeds: 'gl_pinwheel(0.5)' and 'gl_pinwheel(10)'

gl_pinwheel(10)

Parameters are appended to the transition name as CSVs within parenthesis.

For the custom ffmpeg variant the parameters may be name=value pairs in any order, e.g. gl_WaterDrop(speed=20,amplitude=50), or they may be indexed values, as follows.

For the custom expression variant the parameters must be indexed values only but empty values assume defaults, e.g. gl_GridFlip(5,3,,0.1,,1) arguments are size.x=5,size.y=3,dividerWidth=0.1,background=1 with default values for other parameters.

Custom expressions can also be amended directly: parameters are specified using store functions st(p,v) where p is the parameter number and v its value. So for gl_pinwheel with a speed value 10, change the first line of its expr below to st(1, 10);.

st(1, 2);
st(2, 1 - ld(0));
st(1, atan2(0.5 - Y / H, X / W - 0.5) + ld(2) * ld(1));
st(1, mod(ld(1), PI / 4));
if(lte(ld(2), ld(1)), A, B)

Similarly, gl_directionalwarp takes 3 parameters: smoothness, direction.x, direction.y (from xfade-easing.sh -L) and its expr starts with 3 corresponding st() (store) functions which may be changed from their default values:

st(1, 0.1);
st(2, -1);
st(3, 1);
st(4, hypot(ld(2), ld(3)));
etc.

Altered GLSL transition parameters

Example: gl_InvertedPageCurl 30° with uncurl (useful for sheet music with repeats)
'gl_InvertedPageCurl(30,0.15,0)' and 'gl_InvertedPageCurl(30,0.15,1)' concatenated

gl_InvertedPageCurl(30)

Porting

GLSL shader code runs on the GPU in real time. However GL Transition and Xfade APIs are broadly similar and non-complex algorithms are easily ported using vector resolution.

context GL Transitions Xfade filter notes
progress uniform float progress
moves from 0 to 1		 P
moves from 1 to 0		 progress ≡ 1 - P
ratio uniform float ratio W / H GL width and height are normalised
coordinates vec2 uv
uv.y == 0 is bottom
uv == vec2(1.0) is top-right		 X, Y
Y == 0 is top
(X,Y) == (W,H) is bottom-right		 uv.x ≡ X / W
uv.y ≡ 1 - Y / H
texture vec4 getFromColor(vec2 uv)
vec4 getToColor(vec2 uv)		 a0(x,y) to a3(x,y)
or A for first input
b0(x,y) to b3(x,y)
or B for second input		 vec4 transition(vec2 uv) {...} runs for every pixel position
xfade expr is evaluated for every texture component (plane) and pixel position
plane data normalised RGBA GBRA or YUVA unsigned integer xfade bit depth depends on pixel format

To make the transpiled code easier to follow, original variable names from the GLSL and xfade source code are replicated in xfade-easing.sh and xfade-easing.h. The script uses pseudo functions to emulate real functions, expanding them inline later.

Example: porting transition gl_randomsquares

randomsquares.glsl:

uniform ivec2 size; // = ivec2(10, 10)
uniform float smoothness; // = 0.5

float rand (vec2 co) {
    return fract(sin(dot(co.xy ,vec2(12.9898,78.233))) * 43758.5453);
}

vec4 transition(vec2 p) {
    float r = rand(floor(vec2(size) * p));
    float m = smoothstep(0.0, -smoothness, r - (progress * (1.0 + smoothness)));
    return mix(getFromColor(p), getToColor(p), m);
}

xfade-easing.sh (custom expression variant):

gl_randomsquares) # (case)
    _make "st(1, ${a[0]-10});" # size.x
    _make "st(2, ${a[1]-10});" # size.y
    _make "st(3, ${a[2]-0.5});" # smoothness
    _make 'st(1, floor(ld(1) * X / W));'
    _make 'st(2, floor(ld(2) * (1 - Y / H)));'
    _make 'st(4, frand(ld(1), ld(2), 4));' # r
    _make 'st(4, ld(4) - ((1 - P) * (1 + ld(3))));'
    _make 'st(4, smoothstep(0, -ld(3), ld(4), 4));' # m
    _make 'mix(A, B, ld(4))'
    ;;

Here, frand(), smoothstep() and mix() are pseudo functions. Customizable parameters are generally stored first. _make is just an expression string builder function.

xfade-easing.h (custom ffmpeg variant):

static vec4 gl_randomsquares(const XTransition *e)
{
    INIT_BEGIN
    ARG2(ivec2, size, 10, 10)
    ARG1(float, smoothness, 0.5)
    INIT_END
    float r = frand2(floor2(mul2(vec2i(size), e->p)));
    float m = smoothstep(0, -smoothness, r - e->progress * (1 + smoothness));
    return mix4(e->a, e->b, m);
}

Here, vec4 and ivec2 simulate GLSL vector types and XTransition encapsulates all data pertaining to a transition:

typedef struct { // modelled on GL Transition Specification v1
    float progress; // transition progress, moves from 0.0 to 1.0 (cf. P)
    float ratio; // viewport width / height (cf. W / H)
    vec2 p; // pixel position in slice, .y==0 is bottom (cf. X, Y)
    vec4 a, b; // plane data at p (cf. A, B)
    ...
} XTransition;

Curls and Rolls

Transition gl_InvertedPageCurl

This is transpiled from the InvertedPageCurl GL transition which originated from the WebVfx WebGL pagecurl shader which is itself based on code by Calyptus Life AB which seems no longer available. The Hewlett-Packard accreditation by Sergey Kosarevsky is obscure but preserved here.

Transition gl_SimplePageCurl

This is adapted from the elegant simple page curl effect by Andrew Hung who also provides an excellent shader breakdown to demystify the deformation effect.
It is more versatile than gl_InvertedPageCurl and takes the following parameters:

The main differences from gl_InvertedPageCurl are:

Example: using gl_SimplePageCurl to emulate gl_InvertedPageCurl
'gl_InvertedPageCurl(30)' vs 'gl_SimplePageCurl(24.8,0.159,1,0,1,0.8,0.1)'
these parameters factor in the disregarded aspect ratio (5:4 here), 1/2π radius, roll effect, greyscale overleaf, and shadowing.

gl_PageCurl

There is barely any noticeable difference, which confirms Mr Hung’s observation that complex mathematics is unjustified: scalar product projections and simple trigonometry are sufficient.

Example: gl_SimplePageCurl with various angle and roll options (Abstract and Renaissance art by Kandinsky and Titian)

gl_SimplePageCurl

A by-product effect is a wipe transition in any direction, achieved by setting radius=0,roll=1.

Example: 'gl_SimplePageCurl(160,0,1)' 160° wipe

gl_SimplePageCurl-wipe

Transition gl_SimpleBookCurl

(custom ffmpeg only)

This is adapted from gl_SimplePageCurl to clamp the curl to the virtual ‘spine’ at the horizontal centre, then to flatten the radius to zero, using built-in easing to appear more realistic.
It takes the following parameters:

Example: gl_SimpleBookCurl with various angle and radius values paging forwards and backwards, rotated and overlaid onto a desk texture (credit: Ethical Corporation Magazine)

gl_SimpleBookCurl

Random transitions

The CLI script can generate random GLSL-style transitions using the pseudo transition name gl_random which shuffles all the available transition names then cycles through them. Random transitions are particularly useful for slideshows. As the random transition is initially unknown, the expression options will not work in this case.

For the custom expression variant which cannot take named parameters, customisation parameters are ignored and resort to defaults.

For the custom ffmpeg variant which can take named parameters, a common background colour or transparency can be set using background, e.g.:
'gl_random(background=black)' results in a black background (the default)
'gl_random(background=white)' results in a white background
'gl_random(background=gray@0.5)' results in a semi-transparent grey background
'gl_random(background=-1)' results in a transparent background
and for even more randomness:
'gl_random(background=random)' results in a random background colour picked by ffmpeg
These only affect transitions that take a background parameter.

Example: (7 random transitions)
xfade-easing.sh -X -t gl_random -v left.mp4 gl*.png (left)
xfade-easing.sh -t 'gl_random(background=random)' -v right.mp4 gl*.png (right)

gl_random)

Colour considerations

Pixel format

Transitions that affect colour components work differently for RGB than non-RGB colour spaces and for different bit depths. For the custom expression variant, xfade-easing.sh emulates vf_xfade.c function config_output() logic, deducing the RGB signal type AV_PIX_FMT_FLAG_RGB from the -f option format name (rgb/bgr/etc. see pixdesc.c) and the bit depth from ffmpeg -pix_fmts data. It can then set the black, white and mid plane values correctly. See How does FFmpeg identify color spaces? for details.

The expression files in expr/ cater for RGB and YUV formats with 8-bit component depth. For faster processing of greyscale media use xfade-easing.sh -f gray. Greyscale is not RGB therefore it is processed like a luma plane.

If in doubt, check with ffmpeg -pix_fmts or use the xfade-easing.sh -f option.

Colour parameters

These conventions are adopted:

Consequently a value of exactly 1 is rendered white but 2 (RGB #000002) is nearly black. If you really want R=0,G=0,B=1 then specify the alpha component, which is fully opaque by default, i.e. #000001FF.

The custom expression variant does not support colour so only transparent and grey are supported, values -1 to +1.
e.g. gl_swap(0.4, 0.2, 3, 0.67) for 67% grey background.

The custom ffmpeg variant supports the full Color syntax including named colours and variable alpha,
e.g. gl_Stripe_Wipe(color1=DeepSkyBlue, color2=ffd700)

Colour value examples:

Avoid decimal numbers, e.g. 255 is not blue but opaque black (RGB #000000FF).

Example: gl_StarWipe transition with Lime green (#00FF00) border colour
gl_StarWipe(borderThickness=0.1, borderColor=Lime)

star

Transparency

The expression files in expr/ cater for RGBA and YUVA formats with 4 planes.

For lossless intermediate video content with alpha channel support use the xfade-easing.sh -v -f options with an alpha format, e.g. rgba/yuva420p, and .mkv filename extension.

For lossy video with alpha use an alpha format and the .webm extension.

For animated GIFs with transparency use a non-alpha format and the -g option to specify the transparent colour, and the .gif extension. These require gifsicle. Empirically, using an alpha format with the ffmpeg palettegen filter and reserve_transparent option does not produce faultless transparent animated GIFs, whereas post-processing the opaque image is reliable if the transparent colour is unique. This needs further investigation.

To specify alpha in transition parameters, see Colour parameters.

Example: overlaid transparent gl_RotateScaleVanish transition with quadratic-in easing

xfade-easing.sh -f rgba -e quadratic-in -t 'gl_RotateScaleVanish(FadeInSecond=0,ReverseEffect=1,trkMat=1)' -v alpha.mkv -z 250x skaro.png tardis.png
ffmpeg -i gallifrey.png -i alpha.mkv -filter_complex '[0]scale=250:-2[b]; [b][1]overlay' drwho.mp4

alpha

This demonstrates the additional trkMat option which tracks the Tardis alpha value to expose Skaro behind, then Gallifrey’s Citadel when the transition ends, both planets being opaque images.
(trkMat is only availble in the custom ffmpeg variant)

See also the example under Transition gl_SimpleBookCurl.

Reversing xfade effects

(custom ffmpeg only)

The generic xfade reverse option reverses the transition and/or easing effects. It takes a single digit:

It is necessary to swap the inputs during the reversed transition to match the inputs before and after the transition, i.e. during the offset time and after the offset+duration time. This is tricky but not impossible to implement in the xfade-easing.sh script for custom expressions. The script has a -b option to pass reverse values through to xfade for the custom ffmpeg variant.

Most standard xfade transitions have reversed equivalents, e.g. wipeleft and wiperight, but few GLSL transitions do. Unlike standard easings, CSS easings also have no mirror-image reversal mode.

Example: using the same CSS Linear coefficients as above
easing: 'linear(0, 0.5 30%, 0.2 60% 80%, 1)' transition: gl_FanUp reverse: 0–3

reverse effect

There is no gl_FanDown transition but reversing gl_FanUp provides one.

Custom expression performance

FFmpeg expr strings initially get parsed into an expression tree of AVExpr nodes in libavutil/eval.c. That expression is then executed for every pixel in each plane, which obviously incurs a performance hit, considerably exacerbated by disabling slice threading in order to use the st() and ld() functions. So custom transition expressions are not fast.

The following times are based on empirical timings scaled by benchmark scores (the Geekbench Mac Benchmark Chart). They are rough estimates in seconds to process a 3-second transition of HD720 (1280x720) 3-plane media (rgb24) through a null muxer at Mac benchmark midpoints. For greyscale (single plane), subtract two thirds. For an alpha plane, add a third. Mac model performance varies enormously so the Mac vintage dates are only approximate. Windows performance has not been measured.

benchmark →
transition ↓		 2335‑3120
M1,M2,M3 Macs		 1150‑1655
2017‑19 Macs		 700‑1150
2013‑16 Macs		 195‑700
2008‑12 Macs
fade wipeleft wipeup 2 4 6 12
wiperight wipedown 3 6 9 18
wipetl wipetr wipebl wipebr 5 10 14 30
squeezeh squeezev gl_CornerVanish 8 16 24 48
fadefast fadeslow rectcrop 10 20 30 60
coverup dissolve gl_Diamond 14 28 42 84
slideleft smoothup coverleft coverright revealleft revealright coverdown revealup revealdown gl_DoubleDiamond gl_FanUp gl_randomNoisex 16 32 48 100
slideright slideup slidedown smoothleft circlecrop vertopen diagtl gl_FanIn gl_pinwheel 18 36 54 110
smoothdown smoothright vertclose horzopen horzclose gl_FanOut 20 40 60 120
diagbl diagbr diagtr radial gl_CrossOut 22 42 66 133
gl_BookFlip gl_heart gl_polar_function 24 46 72 147
hlslice hrslice vdslice vuslice 28 54 84 171
circleclose circleopen gl_angular gl_Slides 30 60 88 180
gl_PolkaDotsCurtain hlwind hrwind vuwind vdwind 34 66 100 210
fadeblack fadewhite gl_WaterDrop 38 76 114 228
pixelize zoomin gl_Bounce gl_cannabisleaf gl_randomsquares gl_windowblinds 42 84 126 260
gl_Dreamy gl_Flower 48 95 140 300
fadegrays gl_rotateTransition 54 105 160 336
gl_crosswarp gl_DirectionalScaled gl_ripple gl_Rolls 57 114 168 340
gl_doorway gl_StarWipe 63 126 189 380
gl_RotateScaleVanish gl_Swirl 69 133 200 420
gl_InvertedPageCurl gl_squareswire 80 160 240 480
gl_CrazyParametricFun gl_crosshatch gl_static_wipe 84 168 252 520
gl_directionalwarp gl_rotate_scale_fade 95 189 280 580
gl_cube gl_hexagonalize gl_Mosaic gl_swap 108 209 320 660
gl_perlin 126 252 380 760
gl_SimplePageCurl 168 336 500 1020
gl_kaleidoscope 247 480 720 1500
gl_powerKaleido 1000 1960 2960 6100

The slowest supported transition gl_powerKaleido is clearly impractical for most purposes! The most complex transition is gl_InvertedPageCurl which involved considerable refactoring; it omits anti-aliasing for simplicity.

See xfade-easing.h for the C code transpiled from GLSL that helped to optimize the custom expressions. See the files in glsl/ refactored from other GLSL transition sources that were used for intermediate testing in the GL Transition Editor.

Using the custom ffmpeg build on M2 Macs, the slowest transition takes just 4 seconds for the same task. However gl_Exponential_Swish with blurring can take 3 minutes! While much slower than a GPU, CPU processing is at least tolerable. Unlike built-in xfade transitions the custom ffmpeg C code in xfade-easing.h deploys a single pixel iterator for all extended transition functions which in turn operate on all planes at once. And it does not require -filter_complex_threads 1.

Performance-wise, custom expressions are slower by a factor of 41 (median), 47 (mean) with a huge standard deviation of 26!

Other faster ways to use GL Transitions with FFmpeg are:

CLI script

xfade-easing.sh is a Bash 4 shell wrapper for ffmpeg. It can:

Usage

FFmpeg XFade easing and extensions version 3.1.1 by Raymond Luckhurst, https://scriptit.uk
Wrapper script to render eased XFade/GLSL transitions natively or with custom expressions.
Generates easing and transition expressions for xfade and for easing other filters.
Also creates easing graphs, demo videos, presentations and slideshows.
See https://github.com/scriptituk/xfade-easing
Usage: xfade-easing.sh [options] [image/video inputs]
Options:
    -t transition name and arguments, if any (default: fade); use -L for list
       args in parenthesis as CSV, e.g.: gl_perlin(5,0.1) (both variants)
       or key=value pairs, e.g.: gl_perlin(smoothness=0.1, scale=5) (custom ffmpeg only)
       use gl_random to cycle through shuffled transitions ported from GLSL
    -e easing function and arguments, if any (default: linear)
       CSS args in parenthesis as CSV, e.g.: cubic-bezier(0.17,0.67,0.83,0.67)
    -b reverse transition and/or easing effect (custom ffmpeg only) (default: 0)
       1 reverses the inputs and transition effect; 2 reverses the easing; 3 reverses both
    -x expr output filename (default: no expr), accepts expansions, - for stdout
    -a append to expr output file
    -s expr output format string with text expansion (default: '%x')
       %f expands to pixel format, %F to format in upper case
       %e expands to the easing name
       %t expands to the transition name
       %E, %T upper case expansions of %e, %t
       %c expands to the CSS easing arguments
       %a expands to the GL transition arguments; %A to the default arguments (if any)
       %x expands to the generated expr, condensed, intended for inline filterchains
       %X uncondensed version of %x, intended for -/filter_complex script files
       %p expands to the progress easing expression, condensed, for inline filterchains
       %g expands to the generic easing expression (for other filters), condensed
       %z expands to the eased transition expression only, condensed
          for the uneased transition expression only, omit -e option and use %x or %X
       %P, %G, %Z, uncondensed versions of %p, %g, %z, for -/filter_complex script files
       %n inserts a newline
    -p easing plot filename (default: no plot), accepts expansions
       formats: gif, jpg, png, svg, pdf, eps, html <canvas>, from file extension
    -m multiple easings to plot on one graph (default: the -e easing)
       CSV easings with optional legend prefix, e.g. in=cubic-in,out=cubic-out,in-out=cubic
    -q plot title (default: easing name, or Easings for multiple plots)
    -c canvas size for easing plot (default: 640x480, scaled to inches for PDF/EPS)
       format: WxH; omitting W or H keeps aspect ratio, e.g. -z x300 scales W
    -v video output filename (default: no video), accepts expansions
       formats: animated gif, mkv (FFV1 lossless), mp4 (x264), webm, from file extension
       if - then format is the null muxer (no output)
       if -f format has alpha then mkv and webm generate transparent video output
       for gifs see -g; if gifsicle is available then gifs will be optimised
    -r video framerate (default: 25fps)
    -f pixel format (default: rgb24): use ffmpeg -pix_fmts for list
    -g gif transparent colour, requires gifsicle and a non-alpha format (default: none)
    -z video size (default: input 1 size)
       format: WxH; omitting W or H keeps aspect ratio, e.g. -z 400x scales H
    -d video transition duration (default: 3s, minimum: 0) (see note after -l)
    -i time between video transitions (default: 1s, minimum: 0) (see note after -l)
    -l video length (default: 5s)
       note: options -d, -i, -l are interdependent: l=ni+(n-1)d for n inputs
       given -t & -l, d is calculated; else given -l, t is calculated; else l is calculated
    -j allow input videos to play within transitions (default: no)
       normally videos only play during the -i time but this sets them playing throughout
    -n show effect name on video as text (requires the libfreetype library)
    -u video text font size multiplier (default: 1.0)
    -k video stack orientation,gap,colour,padding (default: ,0,white,0), e.g. h,2,red,1
       stacks uneased and eased videos horizontally (h), vertically (v) or auto (a)
       auto selects the orientation that displays easing to best effect
       also stacks transitions with default and custom parameters, eased or not
       videos are only stacked if they are different (nonlinear-eased or customised)
       unstacked videos can be padded using orientation=1, e.g. 1,0,blue,5
    -L list all transitions and easings
    -H show this usage text
    -V show this script version
    -X use custom expressions, not the xfade API that supports xfade-easing natively
       by default native support is detected automatically using ffmpeg --help filter=xfade
       the native API adds easing and reverse options and runs much faster
       e.g. xfade=duration=4:offset=1:easing=quintic-out:transition=wiperight
       e.g. xfade=duration=5:offset=2:easing='cubic-bezier(.17,.67,.83,.67)' \
            :transition='gl_swap(depth=5,reflection=0.7,perspective=0.6)' (see repo README)
    -I set ffmpeg loglevel to info for -v (default: warning)
    -D dump debug messages to stderr and set ffmpeg loglevel to debug for -v
    -P log xfade progress percentage using custom expression print() function (implies -I)
    -T temporary file directory (default: /tmp)
    -K keep temporary files if temporary directory is not /tmp
Notes:
    1. point the shebang path to a bash4 location (defaults to MacPorts install)
    2. this script requires Bash 4 (2009), ffmpeg, ffprobe, gawk, gsed, seq
       also gnuplot for plots, gifsicle for transparent animated gifs
    3. use ffmpeg option -filter_complex_threads 1 (slower) because xfade expression
       vars used by st() & ld() are shared across slices, therefore not thread-safe
       (the custom ffmpeg build works without -filter_complex_threads 1)
    4. CSS easings are supported in the custom ffmpeg build but not as custom expressions
    4. certain xfade transitions are not implemented as custom expressions because
       they perform aggregation (distance, hblur)
    5. many GLSL transitions are also ported, some of which take customisation parameters
       to override defaults append parameters in parenthesis (see -t option)
    6. certain GLSL transitions are only available in the custom ffmpeg build
    7. many transitions do not lend themselves well to easing, others have built-in easing
       easings that overshoot (back & elastic) may cause weird effects

Generating expr code

Expr code is generated using the -x option and customised with the -s,-a,-f options.

Generating test plots

Plots are generated using the -p option and customised with the -m,-q,-c options.

Plot data is logged using the print function of the ffmpeg expression evaluator for the first plane and first pixel as xfade progress P goes from 1 to 0 at 100fps.

bounce plot

The plots above in Standard easings show test plots for all standard easings and all three modes (in, out and in-out).

Generating demo videos

Videos are generated using the -v option and customised with the -b,-r,-f,-g,-z,-d,-i,-l,-j,-n,-u,-k options.

[!NOTE] all transition effect demos on this page are animated GIFs regardless of the commands shown

Timing

Input media is serialised according to the expression $L=NI+(N-1)D$ where L is the total video length (option -l); I is the individual display time (option -i); D is the transition duration (option -d); N is the number of inputs. Transition offsets are spaced accordingly. Depending on option -j and the input media length, pre and post padding is added by frame cloning to ensure enough frames are available for transition processing. See Usage for the precedence of options -l, -i, -d.

Examples

See also