Hey @arturo thx for the quick reply and explanation.
In order to explain my problem i tried to reduce the complexity but left out an important note i think.
Actually I want to pre process the image within an fbo and custom shaders (warp, scale, blur, erode ect)...before sending it to another thread calculating flow, hence they are already on a texture and not pixels anymore.
Wouldnt it be then the quickest version?