Overlay colored text (with an alpha layer) on the picture

[AndrePScope]

Hi Net-Vips!

On our project, we decided to use your product instead of ImageMagick.
Net-vips is not documented well, and I'm having difficulties with implementation simple things.

I need to overlay colored text (with an alpha layer) on the picture.
I tried to do this as shown in one of the php-vips examples.

libvips/php-vips#94

		private void Test()
		{
			using var image = Image.NewFromFile("sample.jpg", access: Enums.Access.Sequential);
			using var text = Image.Text("Hello", "Arial 36px", width: image.Width -100);

			using var overlay = text
				.NewFromImage(Image.NewFromArray(new[,] { { 255, 0, 0 } }))
				.Copy(interpretation: Enums.Interpretation.Srgb)
				.Bandjoin(text);

			image.Composite(overlay, Enums.BlendMode.Over, 400, 400).
				WriteToFile("_result.jpg");
		}

But I get an exception on image.Composite line .

	Message: unable to call composite2
	         composite2: images do not have same numbers of bands

Please, correct me, what I am doing something wrong.

[kleisauke]

Hi @AndrePScope,

libvips and this binding is rather low-level and requires some experience in image processing. I admit that the documentation could be a bit more extensive (see #99).

NewFromImage has a params overload where you can pass a single-dimensional array, for example:

@@ -4,7 +4,7 @@
 			using var text = Image.Text("Hello", "Arial 36px", width: image.Width -100);
 
 			using var overlay = text
-				.NewFromImage(Image.NewFromArray(new[,] { { 255, 0, 0 } }))
+				.NewFromImage(255, 0, 0)
 				.Copy(interpretation: Enums.Interpretation.Srgb)
 				.Bandjoin(text);
 

If you want a bit of opacity in the text, you could use:

@@ -3,10 +3,13 @@
 			using var image = Image.NewFromFile("sample.jpg", access: Enums.Access.Sequential);
 			using var text = Image.Text("Hello", "Arial 36px", width: image.Width -100);
 
+			// we'll use that as the alpha ... scale down to make it transparent
+			using var alpha = (text * 0.3).Cast(Enums.BandFormat.Uchar);
+
 			using var overlay = text
 				.NewFromImage(255, 0, 0)
 				.Copy(interpretation: Enums.Interpretation.Srgb)
-				.Bandjoin(text);
+				.Bandjoin(alpha);
 
 			image.Composite(overlay, Enums.BlendMode.Over, 400, 400).
 				WriteToFile("_result.jpg");

Also see libvips/pyvips#204 for other effects you could apply to text (such as shadows or neon effects).

[AndrePScope]

Thank you @kleisauke!

Thank you for correcting the code and for the useful link!