Alpha compositing: support grey+alpha src and non-alpha dst

README.md

@@ -417,10 +417,6 @@ If the background contains an alpha value then WebP and PNG format output images
 
 Merge alpha transparency channel, if any, with `background`.
 
-#### Experimental: overlayWith(filename)
-
-**Experimental:** Composite image with transparent overlay. Both input and overlay image must be RGBA and their dimensions must match.
-
 #### rotate([angle])
 
 Rotate the output image by either an explicit angle or auto-orient based on the EXIF `Orientation` tag.
@@ -498,6 +494,14 @@ The output image will still be web-friendly sRGB and contain three (identical) c
 
 Enhance output image contrast by stretching its luminance to cover the full dynamic range. This typically reduces performance by 30%.
 
+#### overlayWith(filename)
+
+_Experimental_
+
+Alpha composite `filename` over the processed (resized, extracted) image. The dimensions of the two images must match.
+
+* `filename` is a String containing the filename of an image with an alpha channel.
+
 ### Output options
 
 #### jpeg()

src/common.h

@@ -1,6 +1,8 @@
 #ifndef SRC_COMMON_H_
 #define SRC_COMMON_H_
 
+#include <string>
+
 namespace sharp {
 
   enum class ImageType {

src/operations.cc

@@ -1,38 +1,55 @@
 #include <vips/vips.h>
 
+#include "common.h"
 #include "operations.h"
 
 namespace sharp {
 
   /*
-    Composite images `src` and `dst` with premultiplied alpha channel and output
-    image with premultiplied alpha.
+    Alpha composite src over dst
+    Assumes alpha channels are already premultiplied and will be unpremultiplied after
    */
-  int Composite(VipsObject *context, VipsImage *srcPremultiplied, VipsImage *dstPremultiplied, VipsImage **outPremultiplied) {
-    if (srcPremultiplied->Bands != 4 || dstPremultiplied->Bands != 4)
-      return -1;
-
-    // Extract RGB bands:
-    VipsImage *srcRGBPremultiplied;
-    if (vips_extract_band(srcPremultiplied, &srcRGBPremultiplied, 0, "n", srcPremultiplied->Bands - 1, NULL))
-      return -1;
-    vips_object_local(context, srcRGBPremultiplied);
+  int Composite(VipsObject *context, VipsImage *src, VipsImage *dst, VipsImage **out) {
+    using sharp::HasAlpha;
 
-    VipsImage *dstRGBPremultiplied;
-    if (vips_extract_band(dstPremultiplied, &dstRGBPremultiplied, 0, "n", dstPremultiplied->Bands - 1, NULL))
+    // Split src into non-alpha and alpha
+    VipsImage *srcWithoutAlpha;
+    if (vips_extract_band(src, &srcWithoutAlpha, 0, "n", src->Bands - 1, NULL))
       return -1;
-    vips_object_local(context, dstRGBPremultiplied);
-
-    // Extract alpha bands:
+    vips_object_local(context, srcWithoutAlpha);
     VipsImage *srcAlpha;
-    if (vips_extract_band(srcPremultiplied, &srcAlpha, srcPremultiplied->Bands - 1, "n", 1, NULL))
+    if (vips_extract_band(src, &srcAlpha, src->Bands - 1, "n", 1, NULL))
       return -1;
     vips_object_local(context, srcAlpha);
 
+    // Split dst into non-alpha and alpha channels
+    VipsImage *dstWithoutAlpha;
     VipsImage *dstAlpha;
-    if (vips_extract_band(dstPremultiplied, &dstAlpha, dstPremultiplied->Bands - 1, "n", 1, NULL))
-      return -1;
-    vips_object_local(context, dstAlpha);
+    if (HasAlpha(dst)) {
+      // Non-alpha: extract all-but-last channel
+      if (vips_extract_band(dst, &dstWithoutAlpha, 0, "n", dst->Bands - 1, NULL)) {
+        return -1;
+      }
+      vips_object_local(context, dstWithoutAlpha);
+      // Alpha: Extract last channel
+      if (vips_extract_band(dst, &dstAlpha, dst->Bands - 1, "n", 1, NULL)) {
+        return -1;
+      }
+      vips_object_local(context, dstAlpha);
+    } else {
+      // Non-alpha: Copy reference
+      dstWithoutAlpha = dst;
+      // Alpha: Use blank, opaque (0xFF) image
+      VipsImage *black;
+      if (vips_black(&black, dst->Xsize, dst->Ysize, NULL)) {
+        return -1;
+      }
+      vips_object_local(context, black);
+      if (vips_invert(black, &dstAlpha, NULL)) {
+        return -1;
+      }
+      vips_object_local(context, dstAlpha);
+    }
 
     // Compute normalized input alpha channels:
     VipsImage *srcAlphaNormalized;
@@ -85,12 +102,12 @@ namespace sharp {
     // externally.
     //
     VipsImage *t2;
-    if (vips_multiply(dstRGBPremultiplied, t0, &t2, NULL))
+    if (vips_multiply(dstWithoutAlpha, t0, &t2, NULL))
       return -1;
     vips_object_local(context, t2);
 
     VipsImage *outRGBPremultiplied;
-    if (vips_add(srcRGBPremultiplied, t2, &outRGBPremultiplied, NULL))
+    if (vips_add(srcWithoutAlpha, t2, &outRGBPremultiplied, NULL))
       return -1;
     vips_object_local(context, outRGBPremultiplied);
 
@@ -101,24 +118,15 @@ namespace sharp {
     vips_object_local(context, outAlpha);
 
     // Combine RGB and alpha channel into output image:
-    VipsImage *joined;
-    if (vips_bandjoin2(outRGBPremultiplied, outAlpha, &joined, NULL))
-      return -1;
-
-    // Return a reference to the composited output image
-    *outPremultiplied = joined;
-    return 0;
+    return vips_bandjoin2(outRGBPremultiplied, outAlpha, out, NULL);
   }
 
   /*
    * Premultiply alpha channel of `image`.
    */
   int Premultiply(VipsObject *context, VipsImage *image, VipsImage **out) {
-    VipsImage *imagePremultiplied;
-
 #if (VIPS_MAJOR_VERSION >= 9 || (VIPS_MAJOR_VERSION >= 8 && VIPS_MINOR_VERSION >= 1))
-    if (vips_premultiply(image, &imagePremultiplied, NULL))
-      return -1;
+    return vips_premultiply(image, out, NULL);
 #else
     VipsImage *imageRGB;
     if (vips_extract_band(image, &imageRGB, 0, "n", image->Bands - 1, NULL))
@@ -140,24 +148,16 @@ namespace sharp {
       return -1;
     vips_object_local(context, imageRGBPremultiplied);
 
-    if (vips_bandjoin2(imageRGBPremultiplied, imageAlpha, &imagePremultiplied, NULL))
-      return -1;
+    return vips_bandjoin2(imageRGBPremultiplied, imageAlpha, out, NULL);
 #endif
-
-    // Return a reference to the premultiplied output image
-    *out = imagePremultiplied;
-    return 0;
   }
 
   /*
    * Unpremultiply alpha channel of `image`.
    */
   int Unpremultiply(VipsObject *context, VipsImage *image, VipsImage **out) {
-    VipsImage *imageUnpremultiplied;
-
 #if (VIPS_MAJOR_VERSION >= 9 || (VIPS_MAJOR_VERSION >= 8 && VIPS_MINOR_VERSION >= 1))
-    if (vips_unpremultiply(image, &imageUnpremultiplied, NULL))
-      return -1;
+    return vips_unpremultiply(image, out, NULL);
 #else
     VipsImage *imageRGBPremultipliedTransformed;
     if (vips_extract_band(image, &imageRGBPremultipliedTransformed, 0, "n", image->Bands - 1, NULL))
@@ -179,13 +179,8 @@ namespace sharp {
       return -1;
     vips_object_local(context, imageRGBUnpremultipliedTransformed);
 
-    if (vips_bandjoin2(imageRGBUnpremultipliedTransformed, imageAlphaTransformed, &imageUnpremultiplied, NULL))
-      return -1;
+    return vips_bandjoin2(imageRGBUnpremultipliedTransformed, imageAlphaTransformed, out, NULL);
 #endif
-
-    // Return a reference to the unpremultiplied output image
-    *out = imageUnpremultiplied;
-    return 0;
   }
 
 }  // namespace sharp

src/pipeline.cc

@@ -484,23 +484,22 @@ class PipelineWorker : public NanAsyncWorker {
       }
     }
 
-    // Premultiply image alpha channel before all transformations to avoid
-    // dark fringing around bright pixels
-    // See: http://entropymine.com/imageworsener/resizealpha/
     bool shouldAffineTransform = xresidual != 0.0 || yresidual != 0.0;
     bool shouldBlur = baton->blurSigma != 0.0;
     bool shouldSharpen = baton->sharpenRadius != 0;
     bool shouldTransform = shouldAffineTransform || shouldBlur || shouldSharpen;
     bool hasOverlay = !baton->overlayPath.empty();
     bool shouldPremultiplyAlpha = HasAlpha(image) && image->Bands == 4 && (shouldTransform || hasOverlay);
 
+    // Premultiply image alpha channel before all transformations to avoid
+    // dark fringing around bright pixels
+    // See: http://entropymine.com/imageworsener/resizealpha/
     if (shouldPremultiplyAlpha) {
       VipsImage *imagePremultiplied;
       if (Premultiply(hook, image, &imagePremultiplied)) {
         (baton->err).append("Failed to premultiply alpha channel.");
         return Error();
       }
-
       vips_object_local(hook, imagePremultiplied);
       image = imagePremultiplied;
     }
@@ -820,41 +819,34 @@ class PipelineWorker : public NanAsyncWorker {
       if (overlayImageType != ImageType::UNKNOWN) {
         overlayImage = InitImage(baton->overlayPath.c_str(), baton->accessMethod);
         if (overlayImage == NULL) {
-          (baton->err).append("Overlay input file has corrupt header");
-          overlayImageType = ImageType::UNKNOWN;
+          (baton->err).append("Overlay image has corrupt header");
+          return Error();
         } else {
           vips_object_local(hook, overlayImage);
         }
       } else {
-        (baton->err).append("Overlay input file is of an unsupported image format");
-      }
-
-      if (overlayImage == NULL || overlayImageType == ImageType::UNKNOWN) {
+        (baton->err).append("Overlay image is of an unsupported image format");
         return Error();
       }
-
       if (!HasAlpha(overlayImage)) {
-        (baton->err).append("Overlay input must have an alpha channel");
+        (baton->err).append("Overlay image must have an alpha channel");
         return Error();
       }
-
-      if (!HasAlpha(image)) {
-        (baton->err).append("Input image must have an alpha channel");
-        return Error();
-      }
-
-      if (overlayImage->Bands != 4) {
-        (baton->err).append("Overlay input image must have 4 channels");
+      if (overlayImage->Xsize != image->Xsize && overlayImage->Ysize != image->Ysize) {
+        (baton->err).append("Overlay image must have same dimensions as resized image");
         return Error();
       }
 
-      if (image->Bands != 4) {
-        (baton->err).append("Input image must have 4 channels");
+      // Ensure overlay is sRGB
+      VipsImage *overlayImageRGB;
+      if (vips_colourspace(overlayImage, &overlayImageRGB, VIPS_INTERPRETATION_sRGB, NULL)) {
         return Error();
       }
+      vips_object_local(hook, overlayImageRGB);
 
+      // Premultiply overlay
       VipsImage *overlayImagePremultiplied;
-      if (Premultiply(hook, overlayImage, &overlayImagePremultiplied)) {
+      if (Premultiply(hook, overlayImageRGB, &overlayImagePremultiplied)) {
         (baton->err).append("Failed to premultiply alpha channel of overlay image.");
         return Error();
       }
@@ -883,14 +875,14 @@ class PipelineWorker : public NanAsyncWorker {
 
     // Convert image to sRGB, if not already
     if (image->Type != VIPS_INTERPRETATION_sRGB) {
-      // Switch intrepretation to sRGB
+      // Switch interpretation to sRGB
       VipsImage *rgb;
       if (vips_colourspace(image, &rgb, VIPS_INTERPRETATION_sRGB, NULL)) {
         return Error();
       }
       vips_object_local(hook, rgb);
       image = rgb;
-      // Tranform colours from embedded profile to sRGB profile
+      // Transform colours from embedded profile to sRGB profile
       if (baton->withMetadata && HasProfile(image)) {
         VipsImage *profiled;
         if (vips_icc_transform(image, &profiled, srgbProfile.c_str(), "embedded", TRUE, NULL)) {

test/fixtures/index.js

@@ -62,6 +62,7 @@ module.exports = {
   inputPngAlphaPremultiplicationLarge: getPath('alpha-premultiply-2048x1536-paper.png'),
 
   inputWebP: getPath('4.webp'), // http://www.gstatic.com/webp/gallery/4.webp
+  inputWebPWithTransparency: getPath('5_webp_a.webp'), // http://www.gstatic.com/webp/gallery3/5_webp_a.webp
   inputTiff: getPath('G31D.TIF'), // http://www.fileformat.info/format/tiff/sample/e6c9a6e5253348f4aef6d17b534360ab/index.htm
   inputGif: getPath('Crash_test.gif'), // http://upload.wikimedia.org/wikipedia/commons/e/e3/Crash_test.gif
   inputSvg: getPath('Wikimedia-logo.svg'), // http://commons.wikimedia.org/wiki/File:Wikimedia-logo.svg