ShaderCam | Demo app to discuss rendering camera frame using OpenGL ES

I am currently programming an application for image processing. To achieve the needed performance, I have to use the GPU to compute the camera input, more specifically use OpenGL ES 2.0. With the help of this project (https://github.com/yulu/ShaderCam) I achieved to pass the image to the pipeline and do simple operations with the fragment shader (like inverting colors etc).

My knowledge of GLSL, fragment shaders and vertex shaders is fairly limited but I am aware of pipeline constraints and what the two shaders do in the pipeline. So - formulating the problem - I would like to calculate the average color of a row in my received image and return it (per row) to my application. I read here https://stackoverflow.com/a/13866636/8038866 that this is generally possible, however I can't seem to find out the following things:

1 (edit: SOLVED by simply passing the w and h of my texture to the vertex and fragment shader): Knowing where the row ends (and having that information in the fragment shader). For this I assume that I would have to pass the width of the picture to the vertex shader and from there to the fragment shader, right?

2.: How to calculate the average the color values of each row in the fragment shader and then pass them to the application. If I understand it correctly - the fragment shader only excutes the code per pixel, so I am not sure how to achieve this.

Here are the two very basic shaders vertex shader: