Answer
One of the situations where you might get this warning in Xcode is when using an app that uses shaders such as the Maps app with an MKMapView. You'll find that the map view works as expected without that warning on a real device with real hardware/native OS.
In the sim the SolidRibbonShader fragment shader is not able to read the output of the v_gradient vertex shader probably because it's in beta or there might be an incompatibility between Xcode version and SIM version. However the shaders are recognized on a real device.
Explanation
Those shaders belong to the OpenGL Rendering Pipeline. The Rendering Pipeline is the sequence of steps that OpenGL takes when rendering objects.
The rendering pipeline is responsible for things like applying texture, converting the vertices to the right coordinate system and displaying the character on the screen etc.
There are six stages in this pipeline.
- Per-Vertex Operation
- Primitive Assembly
- Primitive Processing
- Rasterization
- Fragment Processing
- Per-Fragment Operation
Finally, an image appears on the screen of your device. These six stages are called the OpenGL Rendering Pipeline and all data used for rendering must go through it.
What is a shader?
A shader is a small program developed by you that lives in the GPU. A shader is written in a special graphics language called OpenGL Shading Language(GLSL).
A shader takes the place of two important stages in the OpenGL Rendering Pipeline: Per-Vertex Processing and Per-Fragment Processing stage. There is one shader for each of these two stages.
The ultimate goal of the Vertex Shader is to provide the final transformation of the mesh vertices to the rendering pipeline. The goal of the Fragment shader is to provide Coloring and Texture data to each pixel heading to the framebuffer.
Vertex shaders transform the vertices of a triangle from a local model coordinate system to the screen position. Fragment shaders compute the color of a pixel within a triangle rasterized on screen.
Separate Shader Objects Speed Compilation and Linking
Many OpenGL ES apps use several vertex and fragment shaders, and it is often useful to reuse the same fragment shader with different vertex shaders or vice versa. Because the core OpenGL ES specification requires a vertex and fragment shader to be linked together in a single shader program, mixing and matching shaders results in a large number of programs, increasing the total shader compile and link time when you initialize your app.
