GL_LINES drawing back to first point at the end of verticies - iphone

Question

Using GLKit (OpenGLES 2.0) on iOS.
I'm modifying some code that renders cubes in a 3d environment so that it will render waveforms in 2 dimensions in a 3d scene. I have it working great except for one problem which is:
Suppose I render n x/y coordinates using GL_LINES. It will draw a line from 0 -> 1, then 1 -> 2, and so on. But then point N will always have a line drawn back to point 0. I would expect GL_LINE_LOOP to this, but not GL_LINES.
I have a work around in mind if I cannot solve this and that is to use 2 GL_TRIANGLE_STRIP to draw each line segment. I'd really rather just understand why this isn't working though.
Here are the important parts of the code:
typedef struct {
float Position[3];
} Vertex;
const NSUInteger kVerticiesSize = 320;
Vertex Vertices[kVersiciesSize] = {};
const GLubyte Indices[] = {
0,1,
1,2,
2,3,
3,4,
4,5,
5,6,
6,7,
7,8,
8,9,
9,10,
// keeps going to kVerticiesSize
The GL setup
- (void)setupGL {
[EAGLContext setCurrentContext:self.context];
self.effect = [[GLKBaseEffect alloc] init];
glGenVertexArraysOES(1, &_vertexArray);
glBindVertexArrayOES(_vertexArray);
glGenBuffers(1, &_vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, _vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
glGenBuffers(1, &_indexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Indices), Indices, GL_STATIC_DRAW);
glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(GLKVertexAttribPosition,
3,
GL_FLOAT,
GL_FALSE,
sizeof(Vertex),
offsetof(Vertex, Position));
float aspect = fabsf(self.bounds.size.width / self.bounds.size.height);
GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(65.0f), aspect, 0.0f, 50.0f);
self.effect.transform.projectionMatrix = projectionMatrix;
}
And the Update and Render methods:
// Always called once before "render"
-(void)update {
static bool hasRun = NO;
for(int x = 0; x < kVersiciesSize; x++){
// normalize x variable to x coord -1 .. 1 and set Y to math function
Vertex v = Vertices[x];
v.Position[0] = [self normalizedX:x];
v.Position[1] = cos(x);
v.Position[2] = 0;
Vertices[x] = v;
if(!hasRun){
NSLog(#"x=%f y=%f z=%f", v.Position[0], v.Position[1], v.Position[2]);
}
}
hasRun = YES;
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
[self updateTransforms];
}
-(void)render {
[self.effect prepareToDraw];
glBindVertexArrayOES(_vertexArray);
glDrawElements(GL_LINE_STRIP,
sizeof(Indices)/sizeof(Indices[0]),
GL_UNSIGNED_BYTE,
0);
}

Answer 1

You have:
glVertexAttribPointer(GLKVertexAttribPosition,
3,
GL_FLOAT,
GL_FALSE,
sizeof(Vertex),
offsetof(Vertex, Position));
But the signature of glVertexAttribPointer is:
glVertexAttribPointer (GLuint indx, // GLKVertexAttribPosition
GLint size, // 3 floats per vertex
GLenum type, // Vertices are GL_FLOAT-s
GLboolean normalized, // GL_FALSE
GLsizei stride, // *** Should be 0, no?
const GLvoid* ptr); // *** Should be &Vertices[0], no?
I'm pretty sure that, if you make the commented suggestions above, it would work. Then you just render the whole thing with:
glDrawArrays(GL_LINES, 0, kVerticiesSize);
or
glDrawArrays(GL_LINES, startIndex, lineCount);
...right?
DISCLAIMER: I'm not fluent with the whole bound-buffers pipeline, but it strikes me as unnecessary complication, in this case.
ALSO NOTE: Your text mentions GL_LINES, but your sample code uses GL_LINE_STRIP. A line-strip works like a triangle-strip, where the first 2 vertices define the first line segment but, after that, each single vertex defines the next line-segment, which picks-up where the previous one left off. Hence, your vertex indices are correct for GL_LINES but, for GL_LINE_STRIP, you'd just want { 0, 1, 2, 3, 4, 5... }