diff --git a/src/gfx/generated_image.cpp b/src/gfx/generated_image.cpp
index b7a33528..1efbac25 100644
--- a/src/gfx/generated_image.cpp
+++ b/src/gfx/generated_image.cpp
@@ -228,9 +228,6 @@ Image EnlargeImage::generate(const Options& opt) const {
 	// done
 	return larger;
 }
-ImageCombine EnlargeImage::combine() const {
-	return image->combine();
-}
 bool EnlargeImage::operator == (const GeneratedImage& that) const {
 	const EnlargeImage* that2 = dynamic_cast<const EnlargeImage*>(&that);
 	return that2 && *image      == *that2->image
@@ -242,9 +239,6 @@ bool EnlargeImage::operator == (const GeneratedImage& that) const {
 Image CropImage::generate(const Options& opt) const {
 	return image->generate(opt).Size(wxSize((int)width, (int)height), wxPoint(-(int)offset_x, -(int)offset_y));
 }
-ImageCombine CropImage::combine() const {
-	return image->combine();
-}
 bool CropImage::operator == (const GeneratedImage& that) const {
 	const CropImage* that2 = dynamic_cast<const CropImage*>(&that);
 	return that2 && *image      == *that2->image
@@ -340,9 +334,6 @@ Image DropShadowImage::generate(const Options& opt) const {
 	delete[] shadow;
 	return img;
 }
-ImageCombine DropShadowImage::combine() const {
-	return image->combine();
-}
 bool DropShadowImage::operator == (const GeneratedImage& that) const {
 	const DropShadowImage* that2 = dynamic_cast<const DropShadowImage*>(&that);
 	return that2 && *image   == *that2->image
diff --git a/src/gfx/generated_image.hpp b/src/gfx/generated_image.hpp
index 18ad3b51..c2f9aa8b 100644
--- a/src/gfx/generated_image.hpp
+++ b/src/gfx/generated_image.hpp
@@ -199,41 +199,34 @@ class SaturateImage : public SimpleFilterImage {
 	virtual Image generate(const Options& opt) const;
 	virtual bool operator == (const GeneratedImage& that) const;
   private:
-	GeneratedImageP image;
 	double amount;
 };
 
 // ----------------------------------------------------------------------------- : EnlargeImage
 
 /// Enlarge an image by adding a border around it
-class EnlargeImage : public GeneratedImage {
+class EnlargeImage : public SimpleFilterImage {
   public:
 	inline EnlargeImage(const GeneratedImageP& image, double border_size)
-		: image(image), border_size(fabs(border_size))
+		: SimpleFilterImage(image), border_size(fabs(border_size))
 	{}
 	virtual Image generate(const Options& opt) const;
-	virtual ImageCombine combine() const;
 	virtual bool operator == (const GeneratedImage& that) const;
-	virtual bool local() const { return image->local(); }
   private:
-	GeneratedImageP image;
 	double border_size;
 };
 
 // ----------------------------------------------------------------------------- : CropImage
 
 /// Crop an image at a certain point, to a certain size
-class CropImage : public GeneratedImage {
+class CropImage : public SimpleFilterImage {
   public:
 	inline CropImage(const GeneratedImageP& image, double width, double height, double offset_x, double offset_y)
-		: image(image), width(width), height(height), offset_x(offset_x), offset_y(offset_y)
+		: SimpleFilterImage(image), width(width), height(height), offset_x(offset_x), offset_y(offset_y)
 	{}
 	virtual Image generate(const Options& opt) const;
-	virtual ImageCombine combine() const;
 	virtual bool operator == (const GeneratedImage& that) const;
-	virtual bool local() const { return image->local(); }
   private:
-	GeneratedImageP image;
 	double width, height;
 	double offset_x, offset_y;
 };
@@ -241,18 +234,15 @@ class CropImage : public GeneratedImage {
 // ----------------------------------------------------------------------------- : DropShadowImage
 
 /// Add a drop shadow to an image
-class DropShadowImage : public GeneratedImage {
+class DropShadowImage : public SimpleFilterImage {
   public:
 	inline DropShadowImage(const GeneratedImageP& image, double offset_x, double offset_y, double shadow_alpha, double shadow_blur_radius, Color shadow_color)
-		: image(image), offset_x(offset_x), offset_y(offset_y)
+		: SimpleFilterImage(image), offset_x(offset_x), offset_y(offset_y)
 		, shadow_alpha(shadow_alpha), shadow_blur_radius(shadow_blur_radius), shadow_color(shadow_color)
 	{}
 	virtual Image generate(const Options& opt) const;
-	virtual ImageCombine combine() const;
 	virtual bool operator == (const GeneratedImage& that) const;
-	virtual bool local() const { return image->local(); }
   private:
-	GeneratedImageP image;
 	double offset_x, offset_y;
 	double shadow_alpha;
 	double shadow_blur_radius;
diff --git a/src/gfx/image_effects.cpp b/src/gfx/image_effects.cpp
index 276e6c01..eebdf94d 100644
--- a/src/gfx/image_effects.cpp
+++ b/src/gfx/image_effects.cpp
@@ -13,13 +13,32 @@
 // ----------------------------------------------------------------------------- : Saturation
 
 void saturate(Image& image, double amount) {
-	if (amount == 0) return; // nothing to do
 	Byte* pix = image.GetData();
 	Byte* end = pix + image.GetWidth() * image.GetHeight() * 3;
-	if (amount > 0) {
-		amount = min(amount,0.99);
-		int factor = int(256 * amount);
-		int div    = 768 - 3 * factor;
+	// the formula for saturation is
+	//   rgb' = (rgb - amount * avg) / (1 - amount)
+	// if amount >= 1 then this is some kind of inversion
+	// if amount >  0 then use formula directly
+	// if amount <  0 then de-saturate instead:
+	//   rgb = rgb' + -amount*avg - -amount*rgb'
+	//       = rgb' * (1 - -amount) + -amount*avg
+	// if amount < -1 then we are left with just the average
+	int factor = int(256 * amount);
+	if (factor == 0) {
+		return; // nothing to do
+	} else if (factor == 256) {
+		// super crazy saturation: division by zero
+		// if we take infty to be 255, then it is a >avg test
+		while (pix != end) {
+			int r = pix[0], g = pix[1], b = pix[2];
+			pix[0] = r+r > g+b ? 255 : 0;
+			pix[1] = g+g > b+r ? 255 : 0;
+			pix[2] = b+b > r+g ? 255 : 0;
+			pix += 3;
+		}
+	} else if (factor > 0) {
+		int div = 768 - 3 * factor;
+		assert(div > 0);
 		while (pix != end) {
 			int r = pix[0], g = pix[1], b = pix[2];
 			int avg = factor*(r+g+b);
@@ -28,14 +47,8 @@ void saturate(Image& image, double amount) {
 			pix[2] = col((768*b - avg) / div);
 			pix += 3;
 		}
-	} else if (amount < -0.99) {
-		while (pix != end) {
-			int r = pix[0], g = pix[1], b = pix[2];
-			pix[0] = pix[1] = pix[2] = (r+g+b)/3;
-			pix += 3;
-		}
 	} else {
-		int factor1 = int(256 * -amount);
+		int factor1 = -factor;
 		int factor2 = 768 - 3*factor1;
 		while (pix != end) {
 			int r = pix[0], g = pix[1], b = pix[2];