Content Analysis: XorShaders has some of the better

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Shader Tutorial for GameMaker 19c - Luma Mask: coloured rim
Shader Tutorial for GameMaker 19b - Luma Mask: remap UVs & vertex buffers
1 Minute Shader Tutorial for GameMaker
Shader Tutorial for GameMaker 24 - Flame Distortion
Shader Tutorial for GameMaker 15d - Blur by Scaling
Shader Tutorial for GameMaker 21 - Magnifying Glass & Chromatic Aberration
Shader Tutorial for GameMaker 12 - Invisibility Silhouette (in_Position)
How to Create Shaders? | GameMaker Coaching
Shader Tutorial for GameMaker 15b - 2-Pass Blur & Multiple Render Passes
Shader Tutorial for GameMaker 05 - Gamma/Brightness/Contrast/Saturation
GameMaker - Introduction to Shaders
Shader Tutorial for GameMaker 19a - Luma Mask: basic dissolve shader