AlphaMask combines alphas instead of overwriting

src/gfx/blend_image.cpp

@@ -87,6 +87,24 @@ void set_alpha(Image& img, const Image& img_alpha) {
 	}
 }
 
+void set_alpha(Image& img, Byte* al, const wxSize& alpha_size) {
+	if (img.GetWidth() != alpha_size.GetWidth() || img.GetHeight() != alpha_size.GetHeight()) {
+		throw Error(_("Image must have same size as mask"));
+	}
+	if (!img.HasAlpha()) {
+		// copy
+		img.InitAlpha();
+		memcpy(img.GetAlpha(), al, img.GetWidth() * img.GetHeight());
+	} else{
+		// merge
+		Byte *im = img.GetAlpha();
+		size_t size = img.GetWidth() * img.GetHeight();
+		for (size_t i = 0 ; i < size ; ++i) {
+			im[i] = (im[i] * al[i]) / 255;
+		}
+	}
+}
+
 void set_alpha(Image& img, double alpha) {
 	Byte b_alpha = alpha * 255;
 	if (!img.HasAlpha()) {

src/gfx/gfx.hpp

@@ -130,6 +130,8 @@ void draw_combine_image(DC& dc, UInt x, UInt y, const Image& img, ImageCombine c
 
 /// Use the red channel of img_alpha as alpha channel for img
 void set_alpha(Image& img, const Image& img_alpha);
+/// Use the given bytes as alpha channel for img
+void set_alpha(Image& img, Byte* alphas, const wxSize& alphas_size);
 /// Set the transparency of img
 void set_alpha(Image& img, double alpha);
 

src/gfx/mask_image.cpp

@@ -30,11 +30,7 @@ AlphaMask::~AlphaMask() {
 }
 
 void AlphaMask::setAlpha(Image& img) const {
-	if (img.GetWidth() != size.GetWidth() || img.GetHeight() != size.GetHeight()) {
-		throw InternalError(_("Image used with maks must have same size as mask"));
-	}
-	if (!img.HasAlpha()) img.InitAlpha();
-	memcpy(img.GetAlpha(), alpha, size.GetWidth() * size.GetHeight());
+	set_alpha(img, alpha, size);
 }
 
 void AlphaMask::setAlpha(Bitmap& bmp) const {

src/gfx/resample_image.cpp

@@ -126,6 +126,10 @@ void resample(const Image& img_in, Image& img_out) {
 }
 
 void resample_and_clip(const Image& img_in, Image& img_out, wxRect rect) {
+	// mask to alpha
+	if (img_in.HasMask() && !img_in.HasAlpha()) {
+		const_cast<Image&>(img_in).InitAlpha();
+	}
 	// starting position in data
 	int offset_in = (rect.x + img_in.GetWidth() * rect.y);
 	if (img_out.GetHeight() == rect.height) {