THREE.FBOHelper | FrameBuffer Object inspector for three.js | Graphics library

My application is coded in Javascript + Three.js / WebGL + GLSL. I have 200 curves, each one made of 85 points. To animate the curves I add a new point and remove the last. So I made a positions shader that stores the new positions onto a texture (1) and the lines shader that writes the positions for all curves on another texture (2). The goal is to use textures as arrays: I know the first and last index of a line, so I need to convert those indices to uv coordinates.

I use FBOHelper to debug FBOs.

1) This 1D texture contains the new points for each curve (200 in total): positionTexture

2) And these are the 200 curves, with all their points, one after the other: linesTexture

The black parts are the BUG here. Those texels shouldn't be black. How does it work: at each frame the shader looks up the new point for each line in the positionTexture and updates the linesTextures accordingly, with a for loop like this: