Add trim option to provide a specific background colour (#3332)

Co-authored-by: Mart Jansink <mart@cinemait.nl>

src/operations.cc

@@ -262,26 +262,42 @@ namespace sharp {
   /*
     Trim an image
   */
-  VImage Trim(VImage image, double const threshold) {
+  VImage Trim(VImage image, std::vector<double> background, double threshold) {
     if (image.width() < 3 && image.height() < 3) {
       throw VError("Image to trim must be at least 3x3 pixels");
     }
-    // Top-left pixel provides the background colour
-    VImage background = image.extract_area(0, 0, 1, 1);
-    if (HasAlpha(background)) {
-      background = background.flatten();
+
+    // Scale up 8-bit values to match 16-bit input image
+    double multiplier = sharp::Is16Bit(image.interpretation()) ? 256.0 : 1.0;
+    threshold *= multiplier;
+
+    std::vector<double> backgroundAlpha(1);
+    if (background.size() == 0) {
+      // Top-left pixel provides the default background colour if none is given
+      background = image.extract_area(0, 0, 1, 1)(0, 0);
+      multiplier = 1.0;
     }
+    if (background.size() == 4) {
+      // Just discard the alpha because flattening the background colour with
+      // itself (effectively what find_trim() does) gives the same result
+      backgroundAlpha[0] = background[3] * multiplier;
+    }
+    background = {
+      background[0] * multiplier,
+      background[1] * multiplier,
+      background[2] * multiplier
+    };
+
     int left, top, width, height;
     left = image.find_trim(&top, &width, &height, VImage::option()
-      ->set("background", background(0, 0))
+      ->set("background", background)
       ->set("threshold", threshold));
     if (HasAlpha(image)) {
       // Search alpha channel (A)
       int leftA, topA, widthA, heightA;
       VImage alpha = image[image.bands() - 1];
-      VImage backgroundAlpha = alpha.extract_area(0, 0, 1, 1);
       leftA = alpha.find_trim(&topA, &widthA, &heightA, VImage::option()
-        ->set("background", backgroundAlpha(0, 0))
+        ->set("background", backgroundAlpha)
         ->set("threshold", threshold));
       if (widthA > 0 && heightA > 0) {
         if (width > 0 && height > 0) {

src/operations.h

@@ -85,7 +85,7 @@ namespace sharp {
   /*
     Trim an image
   */
-  VImage Trim(VImage image, double const threshold);
+  VImage Trim(VImage image, std::vector<double> background, double const threshold);
 
   /*
    * Linear adjustment (a * in + b)

src/pipeline.cc

@@ -118,7 +118,7 @@ class PipelineWorker : public Napi::AsyncWorker {
       // Trim
       if (baton->trimThreshold > 0.0) {
         MultiPageUnsupported(nPages, "Trim");
-        image = sharp::Trim(image, baton->trimThreshold);
+        image = sharp::Trim(image, baton->trimBackground, baton->trimThreshold);
         baton->trimOffsetLeft = image.xoffset();
         baton->trimOffsetTop = image.yoffset();
       }
@@ -1451,6 +1451,7 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
   baton->sharpenY3 = sharp::AttrAsDouble(options, "sharpenY3");
   baton->threshold = sharp::AttrAsInt32(options, "threshold");
   baton->thresholdGrayscale = sharp::AttrAsBool(options, "thresholdGrayscale");
+  baton->trimBackground = sharp::AttrAsVectorOfDouble(options, "trimBackground");
   baton->trimThreshold = sharp::AttrAsDouble(options, "trimThreshold");
   baton->gamma = sharp::AttrAsDouble(options, "gamma");
   baton->gammaOut = sharp::AttrAsDouble(options, "gammaOut");

src/pipeline.h

@@ -97,6 +97,7 @@ struct PipelineBaton {
   double sharpenY3;
   int threshold;
   bool thresholdGrayscale;
+  std::vector<double> trimBackground;
   double trimThreshold;
   int trimOffsetLeft;
   int trimOffsetTop;
@@ -248,6 +249,7 @@ struct PipelineBaton {
     sharpenY3(20.0),
     threshold(0),
     thresholdGrayscale(true),
+    trimBackground{},
     trimThreshold(0.0),
     trimOffsetLeft(0),
     trimOffsetTop(0),