java - OpenGL not rendering as expected -

if have screen setup following

    gl.glviewport(0,0,width,height);  //reset current viewport     gl.glmatrixmode(gl10.gl_projection); //select projection matrix     gl.glloadidentity();            //reset projection matrix      gl.glorthof(0f,1f,0f,1f,-1f,1f); 

and vertices set follows

this._vertices=new float[]{                 0.0f,0.5f,0.0f,   //v1 bottom left                 0.0f,1.0f,0.0f,    //v2 top left                 0.5f,0.5f,0.0f,  //v3 bottom right                 0.5f,1.0f,0.0f   //v4 top right         }; 

then when drawing do

gl.gltranslatef(0.0f,0.0f,0.0f); 

it places square in top left (i thought 0,0 bottom left?)

and then

gl.gltranslatef(0.5f,-0.5f,0.0f); 

places square in middle of screen (suggesting bottom -1.0f rather 0.0f)

how make bottom left of screen start 0,0?

update------

i have found if change line

gl.glorthof(0f,1f,0f,1f,-1f,1f); 

to

gl.glorthof(0f,1f,1f,0f,-1f,1f); 

then nothing changes (ie top left still 0,0 , bottom left 0,-1)

however if leave line out origin in centre of screen (ie top left -1,-1)

the glorthof used create screen coordinate system, since input parameters left, right, top , bottom can not understand confusion lies. if wish top left corner (0,0) point set top , left parameters 0 , bottom , right positive values positive x right , positive y downwards.

some common cases of usage of glorthof are:

• view coordinate system insert (0,0) top-left , (screenheight , screenwidth) bottom-right. in quite few cases bottom-left @ (0,0) though.
• graph coordinate system screen centre @ (0,0) , top-left @ (1,-1) , bottom-right @ (-1,1)
• normalized view system has screen centre @ (0,0) height relative width depending on screen ratio. top-left @ (-1,-(screenheight/screenwidth)) , bottom-right @ (1, screenheight/screenwidth).

you should note in 1st , 3rd cases square drawn square , not rectangle (where a/b = screenwidth/screenheight). choose suits application best.

you should know glorthof useful panning , zooming. instead of doing strange translations , scales can change glorthof parameters display part of scene.