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Member Since 07 Oct 2004
Online Last Active Today, 10:43 PM

Topics I've Started

c++, is a std::map's value ever copied?

01 November 2013 - 12:05 AM

 Can I guarantee that once inserted, the value part will not be copied?

// i.e. can i do this:
std::map< int, int > my_map;
int *int_ptr = &my_map::find(5)->second; // assuming 5 exists.
// will int_ptr always be at the same place if I add / remove map entries (not including the int_ptr entry).

 And if this is true, is it valid to say an unordered_map will do this as well?


quad rendering to ortho view set to my window leaves lines between quads

18 September 2013 - 02:29 AM

 Hi, when I render quads to my ortho view (set up as (0, window_x_size), (0, window_y_size), (-1, 1)) opengl leaves a blank line between neighbouring quads.


heres my ortho matrix setup:

matrix4x4 &matrix4x4::make_ortho(float left_, float right_,
 float top_, float bottom_, float near_, float far_)
  mat[0] = 2.0f / (right_ - left_);
  mat[5] = 2.0f / (top_ - bottom_);
  mat[10] = -2.0f / (far_ - near_);
  mat[12] = -((right_ + left_) / (right_ - left_));
  mat[13] = -((top_ + bottom_) / (top_ - bottom_));
  mat[14] = -((far_ + near_) / (far_ - near_));
  mat[15] = 1.0f;
  return *this;
I call the above with make_ortho(0, 600, 0, 400, -1, 1) where the window size is 600 x 400.
This is the contents of my vertex array right before I enable the vertex attrib array and glVertexAttribPointer it.
(2 quads, one under the other)

verts : (0, 300, 0)
verts : (19, 300, 0)
verts : (19, 319, 0)
verts : (0, 319, 0)
verts : (0, 320, 0)
verts : (19, 320, 0)
verts : (19, 339, 0)
verts : (0, 339, 0)

Unfortunately, this is leaving a blank line between the top quad and the bottom one and I can't see why.
My vertex shader: (trans is set to my ortho matrix)

#version 330
uniform mat4 trans;
in vec3 pos;
in vec3 color;
out vec3 frag_color;
void main()
  gl_Position = trans * vec4(pos, 1.0);
  frag_color = color;

Fragment shader:

#version 330
in vec3 frag_color;
out vec4 out_color;
void main()
  out_color = vec4(frag_color, 1.0); 

 Everything works fine (quad placement is correct, color is correct), except there is a blank (black) horizontal line between the 2 quads.
 Any help would be appreciated.

problem with glDrawElements and vbo's

14 February 2013 - 12:37 AM

 Hi. I'm having a problem getting glDrawElements working right with a vbo and index buffer.


 Heres the render code: (error checking omitted for brevity).


// ... load shaders and program
GLfloat verts[] = ( 0.0, 0.0, 0.0, 0.5, 0.0, 0.0, 0.5, 0.5, 0.0 };
GLushort idx[] = ( 0, 1, 2 };
GLuint v1, v2;
glGenBuffers(1, &v1);
glBindBuffer(GL_ARRAY_BUFFER, v1);
glBufferData(GL_ARRAY_BUFFER, sizeof(GLfloat) * 9, verts, GL_STATIC_DRAW);
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(GLfloat) * 3, 0);
glGenBuffers(1, &v2);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(GLushort) * 3, idx, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glDeleteBuffers(1, &v1);
glDeleteBuffers(1, &v2);


Vertex shader:

#version 330
layout (location = 1) in vec3 position;
uniform mat4 from_identity = mat4(1.0); // identity for now, this will change
void main()
  gl_Position = from_identity * vec4(position, 0);


Frag shader:

#version 330
void main()
  gl_FragColor = vec4(0.5, 1, 0, 0.5);


 This renders nothing.


 If I change the vertex shader to "gl_Position = gl_Vertex;" and draw the triangle with the old stuff (glBegin(GL_TRIANGLES); ...; glEnd();) it draws fine.


 Any suggestions?







help me understand rendering a quad down the z-axis

26 November 2012 - 12:10 AM

I've used the code from nehe's lesson2 to build a simple app that demonstrates the problem I'm having.
The code is original, except I've removed the gluPerspective line, made it non-fullscreen and changed the drawGLScene() routine.

Here is the new drawGLScene() :
int drawGLScene(GLvoid)
	glTranslatef(0, 0.0f, -0.5f);
glColor3f(1, 1, 1);
		glVertex3f(-0.5f, 0.5f, 0.2f);
		glVertex3f(0.5f, 0.5f, 0.2f);
		glVertex3f(0.5f, -0.5f, 0.2f);
		glVertex3f(-0.5f, -0.5f, 0.2f);
glColor3f(1, 0, 0);
		glVertex3f(-0.5f, 0.5f, -0.2f);
		glVertex3f(0.5f, 0.5f, -0.2f);
		glVertex3f(0.5f, -0.5f, -0.2f);
		glVertex3f(-0.5f, -0.5f, -0.2f);
	if (GLWin.doubleBuffered)
		glXSwapBuffers(GLWin.dpy, GLWin.win);
	return True;  

As you can see, I'm drawing 2 rectangles at the same position, except along the z-axis.
The first is colored white, and has its vectices for z at 0.2f. The second is red, and z is at -0.2f.
For some reason, the red quad appears in front of the white one. Seeing how opengl treats the negative z axis as going into the screen, I'm trying to understand why the white quad (z at 0.2f) isn't drawn in front of the red one.

texture mapping really slow

28 May 2012 - 02:33 AM

Hi, I'm working on an opengl engine and for some reason, on my system, it seems that texture mapping is horribly slow.

I'm using ubuntu 64-bit, with ati's own drivers across two monitors.

I've modified a nehe example tutorial to make it as basic as I could to illustrate my problem.

(when I disable double buffering in my main app, I can see the texture rendering line-by-line (it draws maybe 5 lines per second)).

The modified tutorial prints out
Got Doublebuffer Visual!
glX-Version 1.4
Depth 24
Congrats, you have Direct Rendering!

Possibly the problem is that I'm driving 2 different monitors with one video card, causing texture mapping to happen on the cpu side of things?

Anyhow, here is the modified tutorial (stripped down as much as I could). It takes about 30 seconds to texture map a 512x512 texture to a simple QUAD.

* This code was created by Jeff Molofee '99
* (ported to Linux/GLX by Mihael Vrbanec '00)
* [edited for brevity]
* (modified by me to illustrate a test case)

#include <memory.h>

#include <stdio.h>
#include <GL/glx.h>
#include <GL/gl.h>

/* stuff about our window grouped together */
typedef struct {
	Display *dpy;
	int screen;
	Window win;
	GLXContext ctx;
	XSetWindowAttributes attr;
	int x, y;
	unsigned int width, height;
	unsigned int depth;	
} GLWindow;

static int attrListDbl[] = { GLX_RGBA, GLX_DOUBLEBUFFER,
	None };

GLWindow GLWin;

/* function called when our window is resized (should only happen in window mode) */
void resizeGLScene(unsigned int width, unsigned int height)
	glViewport(0, 0, width, height);

// <added by me, this is the generated texture I am rendering>
int tex;

/* general OpenGL initialization function */
int initGL(GLvoid)
	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
	/* we use resizeGLScene once to set up our initial perspective */
	resizeGLScene(GLWin.width, GLWin.height);

// <my additions for texture mapping setup>

// initial buffer to copy to texture
int size = 512;
unsigned zeros[size * size];
memset(zeros, 0, size * size * 4);

// make a red square
int xct, yct;
for (yct = 50; yct < 100; ++yct)
  for (xct = 50; xct < 100; ++xct)
	zeros[yct * size + xct] = 0x000000ff;

glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_2D, tex);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, size, size, 1,


	return True;

/* Here goes our drawing code */
int drawGLScene(GLvoid)

glFrustum(-1.0f, 1.0f, -1.0f, 1.0f, 1.0f, 5.0f);


glBindTexture(GL_TEXTURE_2D, tex);
	glColor3f(0.5f, 0.5f, 1.0f);
glTexCoord2f(0.0f, 0.0f);
		glVertex3f(-1.0f, 1.0f, -2.0f);
glTexCoord2f(1.0f, 0.0f);
		glVertex3f(1.0f, 1.0f, -2.0f);
glTexCoord2f(1.0f, 1.0f);
		glVertex3f(1.0f, -1.0f, -2.0f);
glTexCoord2f(0.0f, 1.0f);
		glVertex3f(-1.0f, -1.0f, -2.0f);
	glXSwapBuffers(GLWin.dpy, GLWin.win);
	return True;	

/* this function creates our window and sets it up properly */
/* FIXME: bits is currently unused */
Bool createGLWindow(char* title, int width, int height, int bits)
	XVisualInfo *vi;
	Colormap cmap;
	int dpyWidth, dpyHeight;
	int i;
	int glxMajorVersion, glxMinorVersion;
	Window winDummy;
	unsigned int borderDummy;

	/* set best mode to current */
	/* get a connection */
	GLWin.dpy = XOpenDisplay(0);
	GLWin.screen = DefaultScreen(GLWin.dpy);

	vi = glXChooseVisual(GLWin.dpy, GLWin.screen, attrListDbl);
	if (vi == NULL)
return 0;
		printf("Got Doublebuffered Visual!\n");
	glXQueryVersion(GLWin.dpy, &glxMajorVersion, &glxMinorVersion);
	printf("glX-Version %d.%d\n", glxMajorVersion, glxMinorVersion);
	/* create a GLX context */
	GLWin.ctx = glXCreateContext(GLWin.dpy, vi, 0, GL_TRUE);
	/* create a color map */
	cmap = XCreateColormap(GLWin.dpy, RootWindow(GLWin.dpy, vi->screen),
		vi->visual, AllocNone);
	GLWin.attr.colormap = cmap;
	GLWin.attr.border_pixel = 0;

		/* create a window in window mode*/
		GLWin.attr.event_mask = ExposureMask | KeyPressMask | ButtonPressMask |
		GLWin.win = XCreateWindow(GLWin.dpy, RootWindow(GLWin.dpy, vi->screen),
			0, 0, width, height, 0, vi->depth, InputOutput, vi->visual,
			CWBorderPixel | CWColormap | CWEventMask, &GLWin.attr);
		XMapRaised(GLWin.dpy, GLWin.win);
	/* connect the glx-context to the window */
	glXMakeCurrent(GLWin.dpy, GLWin.win, GLWin.ctx);
	XGetGeometry(GLWin.dpy, GLWin.win, &winDummy, &GLWin.x, &GLWin.y,
		&GLWin.width, &GLWin.height, &borderDummy, &GLWin.depth);
	printf("Depth %d\n", GLWin.depth);
	if (glXIsDirect(GLWin.dpy, GLWin.ctx))
		printf("Congrats, you have Direct Rendering!\n");
		printf("Sorry, no Direct Rendering possible!\n");
	return True;	

int main(int argc, char **argv)
	XEvent event;

	createGLWindow("NeHe's Color Tutorial", 640, 480, 24);

for (;;)

return 0;

Any help will be appreciated.