Subviewport with transparent background darkening sprites

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1

Godot Version

v4.5.1.stable.mono.official [f62fdbde1]

Question

When I display an image using a SubviewportTexture in a Sprite2D to see a SubViewport with transparent_bg = true, it darkens the image. This seems to be a known issue but none of the proposed solutions I’ve found work for me. An example looks like this. The two sprites here are the same. Apparently, the issue is that the background color is black and so the black transparency is being blended in, darkening the colors.

Screenshot 2026-08-11 214121
Screenshot 2026-08-11 214121729×289 14.1 KB

[gd_scene load_steps=3 format=3 uid="uid://b7f5flni7mks3"]

[ext_resource type="Texture2D" uid="uid://bpoefbef82uie" path="res://Art/blurred_laser.png" id="1_58y1e"]

[sub_resource type="ViewportTexture" id="ViewportTexture_oyi3a"]
viewport_path = NodePath("SubViewport")

[node name="Node2D" type="Node2D"]

[node name="SubViewport" type="SubViewport" parent="."]
transparent_bg = true
size = Vector2i(100, 100)

[node name="Sprite" type="Sprite2D" parent="SubViewport"]
position = Vector2(50, 50)
texture = ExtResource("1_58y1e")

[node name="DarkerSprite" type="Sprite2D" parent="."]
texture = SubResource("ViewportTexture_oyi3a")

[node name="ViewportTextureSprite" type="Sprite2D" parent="."]
position = Vector2(-6, 0)
texture = ExtResource("1_58y1e")

Here are some of the relevant discussions I’ve found.

viewport texture using transparency is darker colour (or brighter with premult on) · Issue #88603 · godotengine/godot · GitHub suggests changing the renderer to compatibility mode. I can’t do this, I neeed some of the features in Forward+ like particle trails.
Transparent color become different when inside sub viewport · Issue #32615 · godotengine/godot · GitHub suggests using a material with blend mode = Premult Alpha on the ViewportContainer. But I’m not using a ViewportContainer and Subviewports do not have a material parameter. I tried putting doing this on the “Sprite” and actually this does work for this case. However, in my use case I am actually using a GPUParticles2D node instead of just the Sprite2D, and in that case using the material with blend mode = Premult Alpha on the GPUParticles2D lightens all the colors to white for some reason.

Another suggestion I saw somewhere would be to change the background color for the SubViewport, but there no longer is an option in Project Settings for this.

Any help? Thanks so much.

2

You’ve basically diagnosed it correctly, the transparent SubViewport outputs premultiplied alpha and the default canvas blending expects straight alpha, so where alpha is below 1 the colors read darker. The Premult Alpha blend mode fixes it on the Sprite2D because it tells the renderer the data is already premultiplied.

For the GPUParticles2D, come at it from the other side. Instead of changing the blend mode, convert the texture back to straight alpha with a small shader and leave blending at the default Mix. Try putting a ShaderMaterial on the GPUParticles2D with this:

shader_type canvas_item;

void fragment() {
	COLOR.rgb /= max(COLOR.a, 0.0001);
}

That un premultiplies the viewport texture (dividing the color by alpha undoes the premultiply), and since the blend mode stays Mix the particles render normally instead of washing out. The max() just guards against dividing by zero on fully transparent pixels.