Add hasalpha and addalpha methods

spec/convenience_spec.lua

@@ -117,5 +117,16 @@ describe("test convenience functions", function()
         assert.are.equal(result:bands(), 1)
         assert.are.equal(result:avg(), 17 / 4)
     end)
-end)
 
+    it("can call hasalpha", function()
+        local im1 = vips.Image.new_from_file("./spec/images/Gugg_coloured.jpg")
+        local im2 = vips.Image.new_from_file("./spec/images/watermark.png")
+
+        assert.is_false(im1:hasalpha())
+        assert.is_true(im2:hasalpha())
+    end)
+
+    it("can call addalpha", function ()
+        assert.are.equal(im:addalpha():avg(), 128.75)
+    end)
+end)

src/vips/Image_methods.lua

@@ -575,6 +575,26 @@ end
 
 -- convenience functions
 
+function Image_method:hasalpha()
+    local result = vips_lib.vips_image_hasalpha(self.vimage)
+    if result == nil then
+        error(verror.get())
+    end
+
+    return result ~= 0
+end
+
+function Image_method:addalpha()
+    local max_alpha
+    if self:interpretation() == "rgb16" or self:interpretation() == "grey16" then
+        max_alpha = 65535
+    else
+        max_alpha = 255
+    end
+
+    return self:bandjoin(max_alpha)
+end
+
 function Image_method:bandsplit()
     local result
 

src/vips/cdefs.lua

@@ -177,6 +177,8 @@ ffi.cdef [[
     void vips_image_set (VipsImage *image, const char *name, GValue *value);
     int vips_image_remove (VipsImage *image, const char *name);
 
+    int vips_image_hasalpha(VipsImage *image);
+
     char *vips_filename_get_filename (const char *vips_filename);
     char *vips_filename_get_options (const char *vips_filename);