TerrainData.DirtyTextureRegion

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public void DirtyTextureRegion (string textureName, RectInt region, bool allowDelayedCPUSync);

Parameters

textureNameThe name of the Terrain texture.
regionThe rectangular region to mark as dirty.
allowDelayedCPUSyncSpecifies whether to allow delayed CPU synchronization of the texture.

Description

Marks the specified part of the Terrain texture as dirty.

Use this function only after you manually change the GPU part of the Terrain texture, such as by using Graphics.CopyTexture. Set the allowDelayedCPUSync parameter to true if you want Unity to perform immediate synchronization of the CPU part. If you set it to false, Unity queues the reading back of the dirty region until the next call to SyncTexture.

If the current active RenderTexture contains your changes, and you want to copy a part of it into the Terrain texture, use CopyActiveRenderTextureToTexture instead.