Misadventures in Shaderland: the jumping uniform array element
I work on OpenGL ES for an Android and iOS app, and from time to time a weird glitch shows up on a specific device (or a set of devices with the same GPU series). This was one of those times. So, I was writing a new GLSL shader and all was going well. This shader has a uniform array as input, that is used to move the vertices dynamically along a polyline, without the need to reload geometry: #define POLYLINE_SIZE 16 uniform highp vec2 u_polyline[POLYLINE_SIZE]; Then, the vertex shader is doing the following to interpolate a position between two points of the polyline: vec2 pt1 = u_polyline[int(idx)]; vec2 pt2 = u_polyline[int(idx)+1]; vec2 pos = pt1 + (pt2 - pt1) * idx_diff; That worked correctly, and the vertices were moving smoothly along the polyline, based on some data that is specific to each vertex. That was until I tested the app on a Nexus 7 (2nd Gen). On that device, which has an Adreno 320 GPU, vertex positions seemed to be a displaced. After checking