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Continue to ChatQuery : If the brighter area are in one general area (like at the top of your image) then just add a layer mask and then with the mask icon selected on the layer palette, use your gradient tool https://vmapp.org/query/14169/If-The-Brighter-Area-Are-In-One-General-Area-Like-At-The-Top-Of-Your-Image-Then-Just-Add-A-Layer-Mask-And-Then-With-The-Mask-Icon-Selected-On-The-Layer-Palette-Use-Your-Gradient-Tool/