Hi, I have a question about mglCanvasGL, would you please to tell me the answer?

[Anonymous]

Here is the question:

From the source code, I know that mglCanvas::B is responsible for converting mathematic coordinates to screen coordinates.

At the beginning,

B.x = w/2, 'w' is picture's width.
B.y = h/2, 'h' is picture's height.

   w  0  0

B.b = 0 h 0
0 0 sqrt(w*h)

Now let's transform the origin (0 0).

According source code in mglCanvas::PostScale, with mglCanvas object, the result is (w/2 h/2). It is easy to understand and the fact is so.

With mglCanvasGL, its constructor sets w =1 and h = 1, the transformed result should be (0.5 0.5), but the fact is that the transformed result is still (0 0) in OpenGL's coordinate system.

I have been confused for a few days, but I can not find the reason. would you please to tell me why?

BTW: Following code causes my program crashing:

gr = mgl_create_graph_gl();
...
mgl_add_legend( gr, "test-1", "r1" );
mgl_legend( gr, 3, "#", "" ); // crashing here

If I use mgl_create_graph, it's OK.

I am not sure whether it is a bug, or my code is not correct.

My compiler is vc2008 sp1, OS is winxp sp3.

[Alexey Balakin]

This depend on how coordinates are assumed to be in OpenGL and in bitmap. In bitmap coordinates naturally measured in range [0...w][0...h], and center should be at (w/2, h/2). In OpenGL coordiantes are supposed to be in range [-1,1][-1,1] by default, and center should be at (0,0).

I'll check yours code. Which MathGL version produce the bug? Do you test it with latest one (v.2.2)?

[Anonymous]

Thanks for your direction.
I traced the program agian and found the reason.
In OpenGL version, origin(0 0) is transformed to (0.5 0.5) which is stored in an array.
Before drawing, mglCanvas::pxl_transform is called, this function convers origin from (0.5 0.5) to (0 0).

As to the bug, I have tested the code with v2.2, the program is OK with mgl_create_graph_gl().
the library was created from source code with vs2008. I only used two option - enable-double, enable-opengl.

BTW: I found the line width does not work with OpenGL version (mglCanvasGL). The following code set the line width 4, but the result is 1.

mgl_line(gr, 0, 0, 0, 1, 1, 0, "b4", 2 );
mgl_add_legend( gr, "test-1", "r4" );

In bitmap version (mglCanvas), it's all right.

[Anonymous]

Sorry if this thread is a little bit older,
but I would also like to know, how to fix or if there will be a fix in the future for Plot (line width) problem using GLUT/Opengl.

I am currently trying the newest MathGL v2.3.1 (vs2012).