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.

Understanding the issue

Shader bugs manifest visually but trace to invisible state. Triage requires understanding the runtime context as much as the source.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

The triage path for this kind of bug is long. The symptom appears in gameplay, but the cause is in a different system. The reporter describes the gameplay effect; the engineer has to translate that into a hypothesis about the underlying cause. Misdirection is common.

At the engine level, the behavior comes from a deliberate design decision in Godot. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

Diagnosing this class of bug benefits from a structured approach: confirm the symptom, isolate the variables, hypothesize the cause, and verify the hypothesis before writing fix code. Skipping the isolation step is the most common mistake; without it, fixes often address symptoms while the underlying cause continues to produce other variations.

For Godot-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

Within Godot, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“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.