I want to draw a 4pointed star using GLUT and openGL in C++. Here is my code
glBegin(GL_TRIANGLE_FAN);
glVertex3f(0.0f,6.0f,0.0f);
glVertex3f(1.0f,4.0f,0.0f);
glVertex3f(3.0f,3.0f,0.0f);
glVertex3f(1.0f,2.0f,0.0f);
glVertex3f(0.0f,0.0f,0.0f);
glVertex3f(-1.0f,2.0f,0.0f);
glVertex3f(-3.0f,3.0f,0.0f);
glVertex3f(-1.0f,4.0f,0.0f);
glEnd();
The problem is the shape directly goes to 3,3 from 0,6
can anyone help me how to fix this,
screenshot
I want something like this
desired output
The 1st point of the the GL_TRIANGLE_FAN primitiv is always held fixed (See Triangle primitives). Just start the GL_TRIANGLE_FAN primitiv at one of the "inner" points:
glBegin(GL_TRIANGLE_FAN);
glVertex3f(1.0f,4.0f,0.0f);
glVertex3f(3.0f,3.0f,0.0f);
glVertex3f(1.0f,2.0f,0.0f);
glVertex3f(0.0f,0.0f,0.0f);
glVertex3f(-1.0f,2.0f,0.0f);
glVertex3f(-3.0f,3.0f,0.0f);
glVertex3f(-1.0f,4.0f,0.0f);
glVertex3f(0.0f,6.0f,0.0f);
glEnd();
You are creating a triangle fan but setting its central vertex (the initial one) at 0,6,0.
You probably want to change your geometry so that your central vertex is at the origin (for symmetry). It also works to move the first vertex down to the bottom as #Rabbid76 shows.
Related
I am using gluCylinder() to create a cylinder in openGL and then plotting points inside the cylinder with Depth Test On .
When i see the front view of the cylinder, the points inside the cylinder are obstructed by front face.
To make front face of the cylinder translucent i am using Blending.
I am using below functions.
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
But whatever coloring or alpha value i assign to the cylinder the front face is not looking transparent due to its back face.
Tell whether it is possible to do with blending only or else i need to introduce lighting for both the faces of Cylinder.Here it clearly visible the change in the color of front face and back face of cylinder. And the points inside the cylinder are not visible due to being obstructed by front face of cylinder.
You should be able to accomplish this by drawing the cylinder twice, while culling the front faces the first time, and culling the back faces the second time. This way, you can draw the front and back parts differently, e.g. by making the front part transparent.
The code sequence could look like this:
// Draw back part of cylinder, opaque.
glEnable(GL_CULL_FACE);
glCullFace(GL_FRONT);
gluCylinder(...);
// Draw points.
// Draw front part of cylinder, transparent.
glCullFace(GL_BACK);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
gluCylinder(...);
glDisable(GL_BLEND);
If I understand you right then no, you can't do it with blending alone.
If the cylinder's normals all point outward then you also won't be able to see the cylinder's internal parts no matter what you do.
I do something similar to show characters behind walls and it goes like this - render your scene normally and save it all to a framebuffer. Then render what you want shown behind with the buffer contents on top, using a custom shader to make a bubble of transparency around the thing you want shown behind.
Not sure if I am explaining it well or not but it unfortunately requires multiple steps to get the results you want.
Your problem is still a bit unclear to me despite the image but I will attempt to answer based on my perception of your issue.
You are drawing a cylinder and have geometry (lines or other models) inside the cylinder. You want the cylinder to look translucent so the inner objects are visible. Here is one way to do it. Assuming your render functions are drawCylinder() and drawPoints().
init()
{
...
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
...
}
drawScene()
{
glDepthMask(GL_FALSE);
glEnable(GL_BLEND);
drawCylinder();
glDisable(GL_BLEND);
glDepthMask(GL_TRUE);
drawPoints();
}
doing so will make sure that the points are drawn regardless of the cylinder. Try using lower values of alpha for your cylinder color.
Please note this is one way to do it. I suggest using shaders to have more control over blending as well as exploring fragment/pixel discard options.
When drawing a quad, it vanishes when rotation brings in a position perpendicular to the screen. Ideally what I'd like to see is (b) but I get nothing
Is there something wrong with my code ? (warning old openGL code following)
void draw_rect(double vector[4][3], int rgb[3], double transp)
{
GLint is_depth, is_blend, blend_src, blend_dst;
glGetIntegerv(GL_DEPTH_WRITEMASK, &is_depth);
glGetIntegerv(GL_BLEND, &is_blend);
glGetIntegerv(GL_BLEND_SRC, &blend_src);
glGetIntegerv(GL_BLEND_DST, &blend_dst);
glEnable(GL_BLEND);
glDepthMask(0);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
// code to set the color ...
glBegin(GL_POLYGON);
glVertex3v(&vector[0][0]);
glVertex3v(&vector[1][0]);
glVertex3v(&vector[2][0]);
glVertex3v(&vector[3][0]);
glEnd();
if (!is_blend){ glDisable(GL_BLEND); }
glDepthMask(is_depth);
glBlendFunc(blend_src, blend_dst);
}
A quad (assuming it is defined by coplanar faces, as in this case) is by definition infinitely thin. It is correct behavior for it to be invisible when perpendicular to the camera.
The "correct" solution is to make a box rather than a single quad.
See Drawing cube 3D using Opengl for an example using a cube. You'll need to tweak the vertex positions to make the cube smaller along one dimension (probably Z), but it'll give you the effect that you're looking for.
Also, stop using the fixed function stuff (glVertex, etc.). It's been deprecated for years. Shaders aren't that difficult, and examples are easy to find via your favorite search engine.
try making it a line of some definite width when the quad is perpendicular to the screen
I drawed a triangle {(0,0), (1,0), (0,1)}. Now i want to draw a second one. But for some reason not any triangle draws. For example triangle: {(1.5654, 1.2), (1.1, 1.4564), (1.5, 1.15)} is drawn normal, but triangle {(1,1), (1,0), (0, 1)} doesn't appear. Hear the code i use to draw:
glBegin(GL_TRIANGLES);
invers_sh.setAttributeValue(b_colorLoc, colors[0]);
glVertex2d(1.5654, 1.2);
invers_sh.setAttributeValue(b_colorLoc, colors[1]);
glVertex2d(1.1, 1.4564);
invers_sh.setAttributeValue(b_colorLoc, colors[2]);
glVertex2d(1.5, 1.15);
glEnd();
For first triangle it's the same code (but coordinates are different). I tryed to unite both drawings (in one glBegin/glEnd) - same result. What i do wrong?
you need to draw all vertices in a clockwise or counter clockwise order depending on frontface setting, you can google it for more details.
As pointed out in the other answer, you need to draw the vertices for all triangles in the same order, clockwise or counter-clockwise (OpenGL default).
To correct the ordering, just swap out the first and last vertex.
You can control this behaviour (called FaceCulling) with OpenGL-commands like glCullFace, glFrontFace and glEnable/glDisable with GL_CULL_FACE.
I've drawn a simple quad with glBegin and glEnd. With a for-loop I create copies of the quad and rotate it around my y-Axis in 3D space.
Now the problem is that I only see the quads in the front. These in the back are not displayed. I assume that the problem lies within the normal vector, which direction is towards me. Is there a possibility to define two normal vectors for one quad.
Sounds like you need to disable backface culling:
glDisable(GL_CULL_FACE);
These in the back are not displayed. I assume that th problem lies within the normal-vector,
The problem is not the normal vector, but what OpenGL considers front side and backside. What's what is determined by the winding of the vertices on the screen. If the vertices are on screen in counterclockwise order, then by default OpenGL assumes the front face. If back face culling is enables, back faces will not be drawn. You can disable culling, but then you'll get odd lighting results.
The best way is to draw the back side explicitly with it's own set of quads; windings and normals adjusted.
My cube isn't rendering as expected when I use GL_BLEND.
glEnable(GL_CULL_FACE);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
I'm also having a similar problem with drawing some semi-opaque vertices in front, which could well be related.
Related: Why do my semi-opaque vertices make background objects brighter in OpenGL?
Here's what it's supposed to look like:
Normal cube http://img408.imageshack.us/img408/2853/normalcube.png
And here's what it actually looks like:
Dark cube http://img7.imageshack.us/img7/7133/darkcube.png
Please see the code used to create the colored cube, and the code used to actually draw the cube.
The cube is being drawn like so:
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glPushMatrix();
glLoadIdentity();
// ... do some translation, rotation, etc ...
drawCube();
glPopMatrix();
// ... swap the buffers ...
You could try disabling all lighting before drawing the cube:
glDisable(GL_LIGHTING);
It looks like you have lighting enabled on the second one,
try with a glShadeModel( GL_FLAT ) before drawing,
This has me stomped. What it looks like is that some vertices have some alpha values that are non-opaque. However the code you posted has all 1. for alpha. So... in order to debug more, did you try to change your clear color to something non-black ? Say green ?
From the code, I doubt lighting is turned on, since no normals were specified.
Last comment, offtopic... You should really not use glBegin/glEnd (2 function calls per vertex + 2 per primitive is really not a good usage of the recent developments in OpenGL). Try glDrawElements with QUAD_LIST, or even better, TRIANGLE_LIST. You already have the data nicely laid out for that.