# OpenGL glDrawElements is drawing all of my vertices with one vertex at the origin *solved*

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Ok, so i've written an OBJ model loader, all seems to be well, ive tested to make sure that the arrays holding the vertex and indice information is 100% on spot, and they are, yet for some reason when i call glDrawElements, all of my triangles share one point at the origin, so instead of rendering a sphere or a cube, it ends up looking like a bloomin onion instead:

(note: im calling gldrawelements twice, once with triangles, the other with lines, this is how i saw that they all shared one origin)
Here is what my render method looks like (note i am using opengl es):
gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);gl.glVertexPointer(3, GL10.GL_FLOAT, 0, fbVertices);gl.glDrawElements(GL10.GL_TRIANGLES, ((fbVertices.array().length - 1) * 3), GL10.GL_UNSIGNED_SHORT, ibIndices);gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);

You can see the specific model class here (it says its c, its actually java, codepad doesnt support java yet)

(im using eclipse, android sdk 2.1, and opengl es)

Thanks in advance for any help, im really stuck, its quite frustrating :)

Edit: I believe that my problem is where i am calling glDrawElements and defining the size, i assumed since my floatbuffer was holding an array of each vertex, i could just pass the size of that, but then it renders about 1/3 of the points, so i tryed passing fbVertices.array().length * 3, and that seems to render all but about 2 or 3 of the points, but no matter how much i play with it, trying to add 1 or subtract one, it still renders funky ???

[Edited by - Silv3rLogic on September 28, 2010 7:20:16 PM]

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Are you properly adjusting your indices to be from 0 instead of from 1? I recall OBJ the first element was indexed as '1' instead of '0', this screws up a lot of people if you just treat obj indexes as glDrawElements indices.

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Im pretty sure they indexed correctly, but i could be wrong, both the vertice and indice information are packed into buffers the same way, so they both look something like this internally:
float[] vertices = { 1.0f, 0.0f, 0.0f ... };int[] indices = { 1, 2, 3 ... };

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Well I don't really follow what you were trying to say with your last post, but let me elaborate.

In your obj file you have a bunch of vertices

v (first)
v (second)
v (third)
v (fourth)
v (fifth)

To draw a triangle with the first three vertices, you get a face argument something like this:

f 1// 2// 3//

If you grab those face indices and throw them into opengl, you're getting something totally different. You upload your array of five vertices, and then you say "hey draw a triangle with indices 1 2 and 3"

However vertex[1] in opengl represents the SECOND vertex in your list, where you really meant the first vertex (which is index 0). So if you send the indexes from OBJ to opengl all your vertices will be wrong by 1, and you'll get the kind of jumbled polygon mush that you see in your images.

Basically when you read indexes from OBJ, subtract 1 from them.

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I was just trying to describe how i parse the obj file and store the coordinates in arrays, i understand what you are saying, but I just can't get it to want to work right, i tried offsetting the indices by -1, and now my cube looks like a paper airplane. The strange thing is, it renders the first 2 or 3 triangles just fine, then the rest are all jumbled.

This is basically what my engine is doing:

> OBJ loader opens OBJ file, parses it line by line
> when a line beginning with "v" is found, it adds a float[] containing the 3 vertex values to an arraylist
> same thing happens with the indices, when a line beginning with "f" is found, it parses the first value (index), so if i had
f 1//3 2//5 3//7
it would add { 1, 2, 3 } to the arraylist
> when the file is done being parsed, a floatbuffer is built by iterating through the arraylist, adding each individual value (so essentially its taking the arraylist and building it into one array containing all of the values in order), same thing happens with the indices (except its an intbuffer)

these two buffers (essentially just arrays containing all of the values in order) are passed to opengl

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The count that you pass to glDrawElements should be ibIndices.array(). length surely? DrawElements takes the number of indices, not vertices.

Have you tried with a simpler OBJ file of a quad? Maybe try that and paste the index and vertex lists here for us to checkout. (assuming the quad also fails to draw correctly)

Also, your index array appears to be an int[] wheres your call to glDrawElements states that the indices are GL_UNSIGNED_SHORT. This will cause it to read 2-bytes per index rather than 4.

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Okay, i changed my code a little, I am now passing the size of the indice array rather than the vertice array (it seems to be rendering 1/3 of the model now). I also changed all of the int[] arrays to short[] arrays, and now am passing GL_UNSIGNED_BYTE rather than GL_UNSIGNED_SHORT, but the results are the same.

I also created a simple triangle.obj file
# Simple Wavefront filev 0.0 0.0 0.0v 0.0 1.0 0.0v 1.0 0.0 0.0f 1 2 3

This seems to render just fine, yet my cube will not:

cube.obj
# Blender3D v249 OBJ File: # www.blender3d.orgv 1.000000 -1.000000 -1.000000v 1.000000 -1.000000 1.000000v -1.000000 -1.000000 1.000000v -1.000000 -1.000000 -1.000000v 1.000000 1.000000 -1.000000v 1.000000 1.000000 1.000001v -1.000000 1.000000 1.000000v -1.000000 1.000000 -1.000000f 5 1 4f 5 4 8f 3 7 8f 3 8 4f 2 6 3f 6 7 3f 1 5 2f 5 6 2f 5 8 6f 8 7 6f 1 2 3f 1 3 4

[Edited by - Silv3rLogic on September 28, 2010 5:06:21 PM]

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Sweet, thanks rewolfer you were right, i changed all of my int[] arrays to short[] arrays, and continued to pass GL_UNSIGNED_SHORT to glDrawElements. I also had to offset the indices by -1, now it is rendering beautifully ;D

I even whipped up a pretty complex model with objects, normals, tex coords, and everything, and it is working like a charm, i knew i wasn't crazy!

[Edited by - Silv3rLogic on September 28, 2010 6:45:05 PM]

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haha awesome. good job!

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Yes, this is a very old topic but thank you guys so much. Not only for being specific in what you did to fix it but also being specific in the problem. I had all of my triangles drawing the third vertex at the origin and I was pulling my hair out all weekend.

Again, I am sorry to bump an old post but you both deserve medals.

Cheers,
Chanz

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