Compose methods

Here are some confused and confusing notes about compose methods.

• Calculation of alpha, when there is no mask (third image)
• SVG standard
• Scripts

convert dest src -compose OP -composite result

Result size etc will be of destination.

"-gravity" and "-geometry" position src with respect to dest.

If a mask is given, mask's alpha is ignored; treated as greyscale. Limits area modified (black in mask = no modification, ie result = dest. Mask is aligned with destination.

DP. = Duff-Porter

M. = Mathematical. With default channel (with sync), colours obey the operation but alpha is screened. With no sync, alpha channel obey the operation.

A. = Associative (ie if dest and src can be swapped, result is the same).

aOver. = alpha calculation as for Over.

Atop	DP. aDst. As SrcAtop
Blend	aPlus. Interpolation. Use with "-define compose:args={src_percent}[,{dest_percent}]". dest_percent defaults to 100-src_percent.
Blur	aDst. Use with -define compose:args='{Xscale}[x{Yscale}[+{angle}]]'.
Bumpmap	M. aBumpMap. result = dest * grey(src)
ChangeMask	aChangeMask. result.alpha = (src==dest) ? 0 : 1, where "==" is defined by fuzz.
Clear	DP. aClear. result := transparent.
ColorBurn	aOver. Blackout, but white stays white. result = 1 - ( (1-dest) / src).
ColorDodge	aOver. Whiteout, but black stays black. result = dest / (1-src)
Colorize	aHue. Hue followed by Saturate.
Copy	aCopy. result := dest. Background outside src is unchanged.
CopyBlack	aDst. result.black = src.black, if they exist.
CopyBlue	aDst.
CopyCyan	aDst. Synonym for CopyRed.
CopyGreen	aDst.
CopyMagenta	aDst. Synonym for CopyGreen.
CopyOpacity	aSrc. result.alpha = src_has_alpha ? src.alpha : intensity(src)
CopyRed	aDst.
CopyYellow	aDst. Synonym for CopyBlue.
Darken	M. aOver. src < dest ? src : dest; individual channels
DarkenIntensity	M. aDarkenIntensity. src < dest ? src : dest; test is intensity; copies all channels
Divide	M. aOver. As DivideDst.
DivideDst	M. aOver. result = src/dest
DivideSrc	M. aOver. result = dest/src
Difference	M. aOver. result = abs (src - dest)
Displace	aDisplace.
Dissolve	aOver. As Over, but modifies alpha, from "-define compose:args={src_percent}[,{dst_percent}]". dest_percent defaults to 100. If 100 < src_percent <= 200, actually sets dest_percent = 200-src_percent; src_percent = 100.
Distort	aDistort.
Dst	DP. aDst. No-op. Result := dest.
DstAtop	DP. aSrc.
DstIn	DP. aIn. result.alpha = dest.alpha * src.alpha
DstOut	DP. aDstOut. result.alpha = dest.alpha * (1-src.alpha)
DstOver	DP. aOver.
Exclusion	M. aOver. result = src + dest - 2*src*dest
HardLight	aOver. src <= 0.5 ? 2*src*dest : 1 - 2*(1-src)*(1-dest). Like overlay, but src/dest swapped.
HardMix	aOver. src <= 1-dest ? 0 : 1. Like overlay, but src/dest swapped.
Hue	aHue. result.hue = src.hue.
In	DP. aIn. As SrcIn
Lighten	M. aOver. src > dest ? src : dest; individual channels
LightenIntensity	M. aLightenIntensity. src > dest ? src : dest; test is intensity; copies all channels
LinearBurn	aOver. Blackout. result = src + dest - 1.
LinearDodge	aOver. Whiteout. result = src + dest, but alpha blending is as for Over.
LinearLight	aOver. Strong shading, creating whiteout/blackout. result = 2*src + dest - 1.
Luminize	aHue. result.luminance = src.luminance.
Mathematics	aOver. result = A*src*dest + B*src + C*dest + D. Use with "-define compose:args='A,B,C,D'".
Minus	M. aOver. As MinusDst.
MinusDst	M. aOver. result = src - dest
MinusSrc	M. aOver. result = dest - src
Modulate	aDst. Use with "-define compose:args={brightness}[,{saturation}]". result.lightness = src.lightness * brightness. saturation (default 100) generally desaturates.
ModulusAdd	M. aOver. result = dest + src; wrapped.
ModulusSubtract	M. aOver. result = src - dest; wrapped
Multiply	M. aOver. result = dest * src. Darkens.
None	aDst.
Out	aOut. As SrcOut
Overlay	aOver. dest <= 0.5 ? 2*src*dest : 1 - 2*(1-src)*(1-dest). Thus dark dest darkens image; light dest lightens it. Dest black/white stays black/white; otherwise colour and tone influenced by src. Removes (?) transparency.
Over	DP. aOver. As SrcOver
PegtopLight	aOver. Similar to SoftLight but simpler. result = 2*src*dest + src^2*(1 - 2*dest)
PinLight	aOver. Weird.
Plus	M. aPlus. result = dest + src; clipped. Alpha is added.
Replace	aCopy.
Saturate	aHue. result.saturation = src.saturation.
Screen	M. aOver. result = 1 - (1-src)*(1-dest). Lightens.
SoftLight	aOver. Similar to HardLight, but softer. src<0.5 ? (2*src-1)*(dest-dest2)+dest : (2*src-1)*(dest0.5-dest)+dest.
Src	DP. aSrc. result := dest. Background outside src becomes transparent black.
SrcAtop	DP. aDst. Like Over but dest alpha is unchanged.
SrcIn	DP. aIn.
SrcOut	aOut.
SrcOver	aOver. DP.
VividLight	aOver. Refinement of LinearLight, avoiding shading at extremes. result = src <= 0.5 ? 1-(1-dest)/(2*src) : dest/(2*(1-src)).
Xor	DP. aXor. Overlays, then result.alpha = dest.alpha XOR src.alpha

"compose:outside-overlay=false" prevents Duff-Porter composition clearing the destination image outside the overlaid area.

See also SVG Compositing Specification.

Also see Blending Modes of Photoshop & Co..

See testCompose.bat.

Calculation of alpha, when there is no mask (third image)

"Outside" means the part of dest that isn't overlapped by src. "Inside" means the image where dest and src overlap.

aClear: result.a = 0

aDst: result.a = dest.a

aIn: Outside: result.a = 0. Inside: result.a = dest.a * src.a

aOver: Outside: result.a = dest.a. Inside: result.a = 1 - ((1 - dest.a) * (1 - src.a)) = 1 - (1 - src.a - dest.a + src.a * dest.a) = scr.a + dest.a - src.a * dest.a

aOut: Outside: result.a = 0. Inside: result.a = src.a * (1 - dest.a)

aSrc: Outside: result.a = 0. Inside: result.a = src.a

aPlus: Outside: result.a = dest.a. Inside: result.a = dest.a + src.a

aXor: Outside: result.a = dest.a. Inside: result.a = dest.a XOR src.a

SVG standard

From SVG Compositing Specification:

Dca' = f(Sc, Dc) * Sa * Da  + Y * Sca * (1-Da)  + Z * Dca * (1-Sa)
Da'  =         X * Sa * Da  + Y * Sa * (1-Da)   + Z * Da * (1-Sa)

Where:

Dca Premultiplied (by the corresponding alpha value) destination color component
Sca Premultiplied source color component
Da Destination opacity component
Sa Source opacity component
Da Destination opacity component

Scripts

testCompose.bat:

if "%IM%"=="" call imagepath

setlocal enabledelayedexpansion
set I=%IM%

set SIZE=-size 80x80
set SIZE2=-size 100x100

%I%convert %SIZE% xc:red gradient: -compose CopyOpacity -composite tc_red.png

%I%convert %SIZE2% xc:green gradient: -rotate 90 -compose CopyOpacity -composite tc_green.png


rem %I%convert %SIZE% xc:red tc_red.png

rem %I%convert %SIZE2% xc:green -rotate 90 tc_green.png



set COMPLIST=%TEMP%\compList.txt
%I%convert -list compose >%COMPLIST%

echo Divide>>%COMPLIST%
echo Minus>>%COMPLIST%
cSort /i%COMPLIST% /o%COMPLIST%

rem goto skipImages
type %COMPLIST%

for /F %%L in (%COMPLIST%) do (
  %I%convert tc_green.png tc_red.png -gravity Center -compose %%L -composite ^
  -write tca_%%L.png ^
  -alpha Extract ^
  tca_%%L_a.png
)
:skipImages

goto skipCompare
del test_comp.lis

goto skipListComp
for /F %%L in (%COMPLIST%) do (
  %I%compare -metric RMSE ^
  tca_%%L_a.png ^
  tca_Over_a.png ^
  NULL:

for /F "tokens=1,2 usebackq delims=() " %%R IN (`%I%compare ^
  -metric RMSE ^
  tca_%%L_a.png tca_Copy_a.png ^
  NULL: 2^>^&1`) do (
echo answer: %%R %%S
set COMP_INT=%%R
set COMP_FLT=%%S
echo %%R,%%L>>test_comp.lis
)
)

cSort /itest_comp.lis /otest_comp.lis /k0,1


:skipListComp
for /F %%L in (%COMPLIST%) do (
  %I%compare -metric RMSE ^
  tca_%%L_a.png ^
  tca_Over_a.png ^
  NULL:

for /F "tokens=1,2 usebackq delims=() " %%R IN (`%I%compare -metric RMSE tca_%%L_a.png tca_Out_a.png NULL: 2^>^&1`) do (
echo answer: %%R %%S
set COMP_INT=%%R
set COMP_FLT=%%S
echo %%R,%%L>>test_comp.lis
)
)

cSort /itest_comp.lis /otest_comp.lis /k0,1

:skipCompare

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Page version v1.0 19-Apr-2014.

Page created 29-Nov-2017 13:59:56.

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