gistfile1.txt

// quad pos: 0.5, 0.5, 0
// quad scale: 2, 1, 0
// texture size: 1280x720

Vector3 wPos = new Vector3(x,y,0); // x,y = texture pixel pos

float localX = wPos.x*2 / tex.width - 0.5f * 2; // 2 = quad transform scale
float localY = wPos.y / tex.height - 0.5f * 1;

wPos = transform.TransformPoint(new Vector3(localX, localY, 0));