Text Picking

OpenGL OpenGL: Advanced Coding
1

I am drawing text using the OpenGL bitmaps(glBitmap) on to the drawing area created using X-Motif. I want to pick that text using the mouse as like any other geometric objects. So, please give me the coding details using OpenGL.

2

OpenGL has the selection buffer and that can be used for picking. You need to determine the dimensions of the glBitmap to be drawn and render a invisible primitive in the exact location.

Or better yet, just use textured primitives for your text and use those for the selection buffer as well.

V-man

3

Hi,
Thanks alot,
But , how to find the dimensions of a bitmap character.So that , I can draw a rectangle around that bitmap , then I can pick.
Please , give me an idea.
Thank you
Sadhu

4

The glBitmap will be a raster operation, so you know for certain which pixels are gone get rendered to.

You need to know the current raster position. Its also best for this case to use a ortho projection, and to have ortho be the size of your screen.

glOrtho(0, width, 0, height, -1.0, 1.0);

Now call
glRaster2i(rasterX, rasterY);

The primitive can be
glBegin(GL_TRIANGLE_STRIP);
glVertex2f(rasterX, rasterY);
glVertex2f(rasterX, rasterY+height_of_glBitmap);
glVertex2f(rasterX+width_of_glBitmap, rasterY);
glVertex2f(rasterX+width_of_glBitmap, rasterY+height_of_glBitmap);
glEnd();

I hope this is correct. You can do a test and see. Also, there are the rasterization rules of opengl that may effect the exact pixels that get rendered too (something about a tiny offset needed in glVertex calls - 0.375 I think)

PS: cross your fingers!
V-man

5

That may need to be

glBegin(GL_TRIANGLE_STRIP);
glVertex2f(rasterX, rasterY);
glVertex2f(rasterX, rasterY-height_of_glBitmap);
glVertex2f(rasterX+width_of_glBitmap, rasterY);
glVertex2f(rasterX+width_of_glBitmap, rasterY-height_of_glBitmap);
glEnd();

V-man

6

Hi V-man,
thank you very much.

Regards.
sadhu