diff --git a/content/core/compositing.md b/content/core/compositing.md new file mode 100644 index 0000000000000000000000000000000000000000..50b03b769540fbc8718eeb4a08c7abc4a7afdcdb --- /dev/null +++ b/content/core/compositing.md @@ -0,0 +1,286 @@ ++++ +title = "Compositing and layer modes in GIMP" +date = "2022-09-15" +abbrev = "Compositing" +author = "The GIMP Development Team" +weight = 0 ++++ + +This document describes the process of compositing layers and the layer modes +in GIMP. + +## License + +This is free documentation; you can modify and/or redistribute +it according to the terms of the GNU General Public License +as published by the Free Software Foundation, either version +2 of the license, or (at your option) any later version. + +## About this document + +This document was originally written by Henning Makholm and part of the +XCF file format specification. Because the topics here are more general +in the context of GIMP they have been moved into a separate document. + +## 9. COMPOSITING AND LAYER MODES + +This section describes the "flattening" process that GIMP applies +when a multi-layer image is displayed in the editor or exported to +other image file formats. It is present for reference only; an XCF +consumer may well elect to do something different with pixel data from +the layers than flattening them. + +Most XCF consumers will need to react to the layer mode property of +each layer; such a reaction must be informed by knowledge of how the +different layer modes affect the flattening process. In some +applications it might be acceptable for an XCF consumer to refuse +processing images with layer modes other than "Normal", but such an +application will probably not be considered fully XCF capable by its +users. + +In this section we consider primary color (or grayscale) intensities +and alpha values for pixels to be real numbers ranging from 0.0 to +1.0. This makes many of the formulas easier; the reader is asked to +keep in mind that a (linear) conversion from the integral 0..255 scale +of the actual XCF scale is implied whenever data from the XCF file is +mentioned. + +Any practical implementation of the computations specified below may +suffer rounding errors; this specification do not detail how these are +to be handled. GIMP itself rounds values to an integral number of +255ths at many points in the computation. This specification does not +specify exactly which these points are, and authors of XCF renderers +that aim to reproduce the effects of GIMP's flattening down to the +least significant bits are referred to studying its source code. + +In the description below, the variable letter "a" is used for alpha +values. The variable letter "r", "g", "b" are used for primary +intensities, "y" is used for grayscale intensities, and "i" is used +for color map indexed. The letter "c" is used for the complete +color information for a pixel; depending on the color mode of the +image that is either an `(r,g,b)` triple, `a y`, or `a c`. + +The flattening process works independently for each pixel in the +canvas area. The description of some layer modes in the GIMP manual +may give the impression that they involve filters that let pixels +influence neighbor pixels, but that is not true. + +This description does not attempt to preserve the color information +for completely transparent pixels in a layer. If an application uses +this color information, it should document explicitly how it behaves +when transparent pixels from several different layers cover the same +point of the canvas. + +### Flattening overview + +This is how to compute the flattened result for a single pixel +position (in theory, that is - efficient implementations will of +course follow this procedure or an equivalent one for many pixels in +parallel): + +1. Initialize a "working pixel" `(a1,c1)` to be completely transparent + (that is, `a1=0.0` and the value of `c1` is immaterial). + +2. Do the following for each visible layer in the image, starting with + the one that comes LAST in the master layer list: + + 3. Ignore the layer if it is the floating selection, or if it + does not overlap the pixel position in question. + + 4. Let `(a2,c2)` be the pixel data for the layer at the pixel + position in question. If the layer does not have an alpha + channel, then set `a1` to `1.0`. + + 5. If the layer is the one that the floating selection is attached + to and the floating selection overlaps the pixel position in + question, then do the following: + + 6. Let `(a3,c3)` be the pixel data for the floating selection + layer at the pixel position in question. + + 7. If there is a selection channel, then let `x` be its value + at the pixel position in question, and set `a3` to `a3*x`. + + 8. Let m3 be the layer mode of the floating selection. + + 9. Set `(a2,c2)` to `COMPOSITE(a2,c2, a3,c3,m3)`. + The `COMPOSITE` function is defined below. + + 10. If the layer has a layer mask and it is enabled, then let x be + the value of the layer mask at the pixel position in question, + and set `a2` to `a2*x`. + + 11. Let `m2` be the layer mode of the layer. + + 12. If the layer is the bottommost visible layer (i.e., if it is + the last visible layer in the master layer list) and m2 is not + "Normal" or "Dissolve", then set m2 to "Normal". + + 13. Set `(a1,c1)` to `COMPOSITE(a1,c1, a2,c2,m2)`. + The `COMPOSITE` function is defined below. + +14. If the flattened image is to be shown against a background of color `c0`, + then actually visible pixel is `COMPOSITE(1.0, c0, a1, c1, Normal)`. + + Note that unless all layers have mode Normal, it would give the + wrong result to start by initializing `(a1,c1)` to `(1.0,c0)`. + +### Helper functions + +The following auxiliary functions are used in the definition of +`COMPOSITE` below: + +``` + MIN(x1,...,xn) is the least value of x1...xn + + MAX(x1,...,xn) is the largest value of `x1...xn + + MID(x1,...,xn) = (MIN(x1,...,xn)+MAX(x1,...,xn))/2 + + CLAMP(x) = if x < 0 then 0.0 else if x > 1 then 1.0 else x + + BLEND(a1,x1, a2,x2) = (1-k)*x1 + k*x2 + where k = a2/(1-(1-a1)*(1-a2)) +``` + +### Layer modes + +This and the following sections define the COMPOSITE function used in +the general flattening algorithm. + +"Normal" mode for RGB or grayscale images is the usual mode of +compositing in computer graphics with alpha channels. In indexed +mode, the alpha value gets rounded to either 1.0 or 0.0 such that +no colors outside the color map get produced: + +``` + COMPOSITE(a1,y1, a2,y2,Normal) + = ( 1-(1-a1)*(1-a2), BLEND(a1,y1, a2,y2) ) + + COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,Normal) + = ( 1-(1-a1)*(1-a2), BLEND(a1,r1, a2,r2), + BLEND(a1,g1, a2,g2), + BLEND(a1,b1, a2,b2) ) + + COMPOSITE(a1,i1, a2,i2,Normal) = if a2 > 0.5 then (1.0,i2) else (a1,i1) +``` + +"Dissolve" mode corresponds to randomly dithering the alpha channel to +the set `{0.0, 1.0}`: + +``` + COMPOSITE(a1,c1, a2,c2,Dissolve) = chose pseudo-randomly between + (1.0,c2) with probability a2 + (a1,c1) with probability 1-a2 +``` + +These two modes are the only ones that make sense for all of the RGB, +grayscale and indexed color models. In the indexed color model, all +layer modes except Dissolve are treated as Normal. + +Most layer modes belong to the following group, which makes sense for +RGB and grayscale images, but not for indexed ones: + +``` + COMPOSITE(a1,y2, a2,y2,m) + = ( a1, BLEND(a1,y1, MIN(a1,a2),f(y1,y2, m)) ) + + COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,m) + = ( a1, BLEND(a1,r2, MIN(a1,a2),f(r1,r2, m)), + BLEND(a1,g1, MIN(a1,a2),f(g1,g2, m)), + BLEND(a1,b1, MIN(a1,a2),f(b1,g2, m)) ) +``` + +when `3 <= m <= 10 or 15 <= m <= 21`. + +The following table defines `f(x1,x2,m)`: + +``` + Multiply: f(x1,x2, 3) = x1*x2 + Screen: f(x1,x2, 4) = 1-(1-x1)*(1-x2) + Overlay: f(x1,x2, 5) = (1-x2)*x1^2 + x2*(1-(1-x2)^2) + Difference: f(x1,x2, 6) = if x1 > x2 then x1-x2 else x2-x1 + Addition: f(x1,x2, 7) = CLAMP(x1+x2) + Subtract: f(x1,x2, 8) = CLAMP(x1-x2) + Darken Only: f(x1,x2, 9) = MIN(x1,x2) + Lighten Only: f(x1,x2, 10) = MAX(x1,x2) + Divide: f(x1,x2, 15) = CLAMP(x1/x2) + Dodge: f(x1,x2, 16) = CLAMP(x1/(1-x2)) + Burn f(x1,x2, 17) = CLAMP(1-(1-x1)/x2) + Hard Light: f(x1,x2, 18) = if x2 < 0.5 then 2*x1*x2 else 1-2*(1-x1)(1-x2) + Soft Light: f(x1,x2, 19) = (1-x2)*x1^2 + x2*(1-(1-x2)^2) + Grain Extract: f(x1,x2, 20) = CLAMP(x1-x2+0.5) + Grain Merge: f(x1,x2, 21) = CLAMP(x1+x2-0.5) +``` + +Note that the "Overlay" and "Soft Light" modes have identical effects. +In the "Divide", "Dodge", and "Burn" modes, division by zero should +be considered to produce a number so large that CLAMP(x/0) = 1 unless +x=0, in which case CLAMP(0/0) = 0. + +The remaining four layer modes only make sense in the RGB color model; +if the color mode of the image is grayscale or indexed they will be +interpreted as Normal. + +``` + COMPOSITE(a1,r1,g1,b1, a2,r2,g2,b2,m) + = ( a1, BLEND(a1,r2, MIN(a1,a2),r0), + BLEND(a1,g1, MIN(a1,a2),g0), + BLEND(a1,b1, MIN(a1,a2),b0) ) + where (r0,g0,b0) = h(r1,g1,b1, r2,g2,b2, m) +``` + +when `11 <= m <= 14`. + +For defining these modes, we say that + +``` +(r,g,b) has the _hue_ of (r',g',b') + if r' = g' = b' and r >= g = b + or there exist p and q such that p>=0 and r=p*r'+q and b=p*b'+q and g=p*g'+q + +(r,g,b) has the _value_ of (r',g',b') + if MAX(r,g,b) = MAX(r',g',b') + +(r,g,b) has the _HSV-saturation_ of (r',g',b') + if r' = g' = b' = 0 and r = g = b + or MIN(r,g,b) = MAX(r,g,b)*MIN(r',g',b')/MAX(r',g',b') + +(r,g,b) has the _luminosity_ of (r',g',b') + if MID(r,g,b) = MID(r',g',b') + +(r,g,b) has the _HSL-saturation_ of (r',g',b') + if r' = g' = b' and r = g = b + or MAX(r,g,b)-MIN(r,g,b) = MIN(MID(r,g,b),1-MID(r,g,b)) * + (MAX(r',g',b')-MIN(r',g',b'))/MIN(MID(r',g',b'),1-MID(r',g',b')) + +Mode 11: Hue (H of HSV) + + h(r1,g1,b1, r2,g2,b2, 11) is + if r2=g2=b2 then (r1,g1,b1) unchanged + otherwise: the color that has + the hue of (r1,g2,b2) + the value of (r1,g1,b1) + the HSV-saturation of (r1,g1,b1) + +Mode 12: Saturation (S of HSV) + + h(r1,g1,b1, r2,g2,b2, 12) is the color that has + the hue of (r1,g1,b1) + the value of (r1,g1,b1) + the HSV-saturation of (r2,g2,b2) + +Mode 13: Color (H and S of HSL) + + h(r1,g1,b1, r2,g2,b2, 13) is the color that has + the hue of (r2,g2,b2) + the luminosity of (r1,g1,b1) + the HSL-saturation of (r2,g2,b2) + +Mode 14: Value (V of HSV) + + h(r1,g1,b1, r2,g2,b2, 14) is the color that has + the hue of (r1,g1,b1) + the value of (r2,g2,b2) + the HSV-saturation of (r1,g1,b1) +``` diff --git a/content/core/exif_handling.md b/content/core/exif_handling.md new file mode 100644 index 0000000000000000000000000000000000000000..491a053ce7d336460897975f24cc1c6ab1d475f1 --- /dev/null +++ b/content/core/exif_handling.md @@ -0,0 +1,93 @@ ++++ +title = "How GIMP should handle EXIF data" +date = "2005-02-01" +abbrev = "EXIF" +author = "Bill Skaggs" +weight = 0 ++++ + +This is a summary of how an image editing program is supposed to +handle each of the EXIF fields, according to the EXIF specs. Note +that this expresses my understanding based on a quick reading, and +should not be taken as gospel. For details on the contents of each of +these fields, consult the formal EXIF specifications, available from +[http://www.exif.org/specifications.html](http://www.exif.org/specifications.html). + +(Note: according to the EXIF specs, an EXIF jpeg file must have a name +that is ASCII, in 8.3 format, with extension .JPG, but of course we +are not going to enforce this.) + +Fields that should be used to set up the GIMP image when an EXIF file +is loaded, either by using them to configure the image (e.g., +orientation), or by placing them in parasites that can be accessed by +non-exif-aware functions. + +* Orientation +* XResolution +* YResolution +* ResolutionUnit +* ImageDescription +* Artist +* Copyright +* Colorspace +* ComponentsConfiguration +* UserComment +* SubjectArea +* SubjectLocation +* ImageUniqueID +* PixelXDimension +* PixelYDimension + +Fields that should be modified by GIMP when an EXIF jpeg file is +saved. A letter "M" indicates fields whose presence is mandatory +according to the spec. + +* Orientation +* XResolution (M) +* YResolution (M) +* ResolutionUnit (M) +* Software +* DateTime +* ImageDescription +* Artist +* Colorspace (M) +* PixelXDimension (M) +* PixelYDimension (M) +* ComponentsConfiguration (M) +* UserComment +* SubsecTime +* SubjectArea +* SubjectLocation +* FileSource +* ImageUniqueID +* thumbnail Compression (M) +* thumbnail XResolution (M) +* thumbnail YResolution (M) +* thumbnail JPEGInterchangeFormat (M) +* thumbnail JPEGInterchangeFormatLength (M) + +Fields that should be used and saved related to color management: + +* TransferFunction +* WhitePoint +* PrimaryChromaticity +* YCbCrCoefficients +* ReferenceBlackWhite + +Fields that should be deleted if they exist when saving a file as +jpeg, because they only apply to uncompressed (TIFF) data. This +applies both to the main image and to the thumbnail, if there is one. + +* ImageWidth +* ImageLength +* BitsPerSample +* Compression +* SamplesPerPixel +* PhotometricInterpretation +* StripOffsets +* PlanarConfiguration +* YCbCrSubSampling + + +Any field not mentioned here should be passed through unchanged. +