Partial transparency blended with black in screenshot images saved using get_viewport().get_texture().get_image().save_png()

[mellowminx]

Tested versions

• Tested on Godot v4.5.stable.official [876b290]
• Appears to have been an issue since 4 years ago, described in this Reddit thread

System information

Godot v4.5.stable - macOS Sequoia (15.7.1) - Multi-window, 1 monitor - OpenGL 3 (Compatibility) - Apple M1 Pro - Apple M1 Pro (10 threads) - 16.00 GiB memory

Issue description

When using Godot's get_viewport().get_texture().get_image().save_png() function to save screenshot images of a game with partially transparent visuals (such as gradients which fade to transparent), using a transparent background window, the partially transparent pixels are blended with black, resulting in a darker (and uglier)

Steps to reproduce

1. Display a Sprite2D that has partial transparency (such as a gradient which fades to transparent)
2. Make sure background transparency settings are on (as in MRP)
3. Take a screenshot using get_viewport().get_texture().get_image().save_png()

Minimal reproduction project (MRP)

MRP uploaded here: https://github.com/mellowminx/godot-4-5-transparent-bg-screenshot-test/

[mellowminx]

Just a note that in the MRP link readme I also described the hacky workaround I did based on the linked reddit thread -- applying CanvasItemMaterial with Premultiplied alpha blend mode. It works but it totally changes how the visuals look in-game. In my own game I implemented it so that the material is only applied for a split-second while taking the screenshot, but it's perceptible as a white flash, very unpleasant.

[AThousandShips]

Sounds related to:

• SubViewport with transparent background results in premultiplied alpha colors in viewport texture #99715
• viewport texture using transparency is darker colour (or brighter with premult on) #88603
• Viewport.get_texture() not correct when alpha!=1.0 #91828

[mellowminx]

Godot forum user normalized has shared a really good workaround for this! Copied below:

You’re plucking the image from the main screen which afaik means that you can’t get linear/hdr data so the image is RGBA8 with premultiplied alpha. As a quick solution, you can simply un-premultiply the pixels by dividing RGB values by their alpha before saving. Skip the zero divisions.

# just before saving
for j in image_data.get_height():
		for i in image_data.get_width():
			var c = image_data.get_pixel(i, j)
			if c.a > 0:
				c.r /= c.a
				c.g /= c.a
				c.b /= c.a
			image_data.set_pixel(i, j, c)

This unpremultiplied workaround actually results in slightly more accurate and vibrate colors compared to the CanvasItemMaterial premultiplied alpha blend mode workaround. I hope this thread hopes someone else struggling with the same issue.

normalized also said:

It’s a bit strange that Godot’s Image class has premultiply_alpha() function but doesn’t have its reverse.

Maybe there should be a built-in unpremultiply_alpha() function? It's unintuitive that translucent pixels are rendered darker in screenshots by default.

Another user on the same Godot forum thread commented about a similar issue of darkened renders from Godot's "movie maker" mode.

[AThousandShips]

So is this different from the above linked issues?

[romgerman]

Is this related to #78207?