Why does my RenderTarget format reset on resize?

InitAutoFormat() chooses the default format based on the engine’s current settings, which is often RTF_RGBA8 sRGB. If you previously authored the RT as RTF_RGBA16f for HDR work, calling InitAutoFormat downgrades it. Use InitCustomFormat with explicit format flags to preserve the bit depth and gamma.

How do I resize a RenderTarget without changing format?

Call RenderTarget->ResizeTarget(NewWidth, NewHeight). This preserves format flags. If you instead recreate by calling Init* methods, you must pass the same format arguments each time to keep behavior consistent.

What's the difference between linear and sRGB on a RenderTarget?

sRGB format applies gamma encoding on write and decoding on read. Useful for storing pre-tone-mapped color that you sample as if from a regular texture. Linear (bForceLinearGamma=true) stores raw values without gamma adjustment — required for normal maps, data textures, or HDR scene captures.

Fix: Unreal RenderTarget Format Mismatch After Resize

Quick answer: Use InitCustomFormat(W, H, PF_FloatRGBA, /*bForceLinearGamma=*/true) instead of InitAutoFormat. Or call ResizeTarget(W, H) which preserves the existing format.

A scene capture renders to a UTextureRenderTarget2D for use as a mirror. It looks correct until the window resizes — after which colors are washed out and HDR values clipped. The render target was recreated at the new resolution with the wrong format.

Two Init Methods, Two Behaviors

UTextureRenderTarget2D has multiple init paths:

InitAutoFormat(W, H) — resizes and picks a default format (RTF_RGBA8 sRGB on most platforms). Overrides any custom format you previously set.
InitCustomFormat(W, H, Format, bForceLinearGamma) — resizes with the format you specify. Use for HDR, normal maps, depth, or any non-default needs.
ResizeTarget(W, H) — resizes without touching the format. Use when only the resolution should change.

Many resize code paths call InitAutoFormat by reflex, silently downgrading HDR RTs to 8-bit sRGB.

The Fix

// Before (wrong if RT was previously HDR):
RenderTarget->InitAutoFormat(NewWidth, NewHeight);

// After:
RenderTarget->InitCustomFormat(NewWidth, NewHeight, PF_FloatRGBA, true);
RenderTarget->UpdateResourceImmediate(true);

PF_FloatRGBA is 16-bit half-float per channel — 64 bits per pixel. bForceLinearGamma = true avoids gamma encoding so HDR values pass through. UpdateResourceImmediate commits the change on the rendering thread without waiting for the next frame.

Alternative: Preserve Existing Format

RenderTarget->ResizeTarget(NewWidth, NewHeight);

Simpler and safer. Use whenever the goal is purely a resolution change — ResizeTarget keeps whatever format you originally created the RT with.

Common Format Choices

PF_B8G8R8A8 + sRGB — standard low-bit color. 32 bpp.
PF_FloatRGBA (RTF_RGBA16f) — HDR, 64 bpp.
PF_R32_FLOAT — single channel float, e.g., depth or shadow.
PF_R16G16B16A16 — integer HDR, 64 bpp.

For real-time HDR with bloom/tonemapping, FloatRGBA is the typical choice; for normal map captures, use a linear 8/8 with R8G8 packing.

UpdateResourceImmediate vs UpdateResource

The standard UpdateResource defers the underlying resource creation to the next render frame. If you read from the RT in the same frame as the resize, you may sample undefined data. UpdateResourceImmediate blocks until the resource is ready — safer for runtime resize/use in the same tick.

Verifying

Capture the RT before and after resize. Sample a pixel that should hold an HDR value (e.g., 5.0 red). Before fix: clamped to 1.0 after resize. After fix: still 5.0. Use the Output Log r.SceneRenderTargetResizeMethod 2 to debug RT lifetimes during resize sequences.

“InitAutoFormat is convenient and wrong for anything non-default. Either be explicit with InitCustomFormat or use ResizeTarget to keep format intact.”

For HDR render targets, write a helper EnsureHDR(RT, W, H) that always calls InitCustomFormat with the right flags. Centralizes the choice.