Why is my SubViewport texture upside-down on the target mesh?

Godot's framebuffer Y origin differs from imported texture Y origin. When sampling a ViewportTexture in a shader, V appears flipped. Sample with vec2(uv.x, 1.0 - uv.y) to flip back.

Should I use TextureRect or a custom shader?

For 2D UI, a TextureRect with the SubViewport's texture handles the flip automatically. For custom 3D mesh that samples the SubViewport, you must flip Y in the shader explicitly.

What about HDR vs sRGB mismatches?

If the Viewport renders HDR but you sample as sRGB (or vice versa), colors look wrong. Match the Viewport's Use HDR setting to what your shader expects, or apply the inverse gamma in the shader.

Fix: Godot 4 Viewport Texture Shader UV Flipped Vertically

Quick answer: Sample the ViewportTexture with vec2(uv.x, 1.0 - uv.y). For 2D UI rendering of a SubViewport, use a TextureRect (handles flip automatically) instead of a custom shader.

A SubViewport renders a mini-map. You assign its texture to a 3D quad shader. The mini-map appears upside-down. Y axis differs between framebuffer and texture.

The Symptom

SubViewport content displayed on a custom mesh appears flipped vertically. Same content in a TextureRect (UI) is right-side-up. Or: appears correct in editor preview, flipped in PIE.

The Fix

shader_type spatial;

uniform sampler2D viewport_tex : filter_linear;

void fragment() {
    vec2 flipped = vec2(UV.x, 1.0 - UV.y);
    ALBEDO = texture(viewport_tex, flipped).rgb;
}

Pass the SubViewport’s texture to the shader from script:

$Quad.material_override.set_shader_parameter(
    "viewport_tex",
    $SubViewport.get_texture()
)

HDR / sRGB Match

SubViewport → tick Use HDR if the target mesh expects linear values; untick if you want sRGB. Mismatch produces washed-out or over-saturated colors. The flip-only fix is for orientation; HDR is a separate dial.

Transparent Background

If the SubViewport should overlay other geometry, set transparent_bg = true on the SubViewport. Without it, the texture has a solid clear color around the rendered content.

Verifying

Place text in the SubViewport. Display on the quad. Text should read correctly. If still flipped, the shader sampling line wasn’t edited; if upside-down on a TextureRect (rare), the SubViewport’s render direction setting is wrong.

“Flip Y in the shader. Match HDR/sRGB. Mini-map reads right.”

Related Issues

For shader uniform arrays, see uniform arrays. For shader uniform runtime, see uniform update.

Flip Y. Match color space. Right way up.