I have read that the first parameter of the glDrawElements is mode:
http://www.opengl.org/sdk/docs/man3/xhtml/glDrawElements.xml
Symbolic constants GL_POINTS, GL_LINE_STRIP, GL_LINE_LOOP, GL_LINES, GL_LINE_STRIP_ADJACENCY, GL_LINES_ADJACENCY, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_TRIANGLES, GL_TRIANGLE_STRIP_ADJACENCY and GL_TRIANGLES_ADJACENCY are accepted.
I do not see there GL_POLYGON. Is that means that I can not use GL_POLYGON? and if I got 10 indices? Am I need to transform it to a few polygons which contains 3 indices each one? If it is true, How do I do it?
The GL3 and GL4 level man pages on www.opengl.org only document the Core Profile of OpenGL. GL_POLYGON is deprecated, and was not part of the Core Profile when the spec was split into Core and Compatibility profiles in OpenGL 3.2.
You can still use GL_POLYGON if you create a context that supports the Compatibility Profile. But if you start out, I would suggest that you stick to Core Profile features. If you do need documentation for the deprecated features, you'll have to go back to the GL2 man pages.
To draw a polygon, GL_TRIANGLE_FAN is the easiest replacement. You can use the same set of vertices for a triangle fan as you would use for GL_POLYGON, and it will produce the same result.
You are linking to the GL3 manual pages, by the way.
Since GL_POLYGON was deprecated in 3.0 and removed in 3.1, you are not going to find it listed there. In fact, you will find some tokens there that are only supported in GL 3.2 (adjacency primitives, which were introduced when Geometry Shaders were); fortunately that is actually documented in the manual page itself unlike the fact that GL_POLYGON was deprecated.
For compatibility profiles (which you are using), you should view the GL2 manual page. The GL2 man page can be found here.
Related
The glDrawArrays document does not mention GL_QUADS in mode.
https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glDrawArrays.xhtml
But my PC is rendered with glDrawArrays (GL_QUADS, 0, 4); the rectangle is drawn. My PC is running on OpenGL 4.3.
Why is this?
Probably because your PC implements the compatibility profile.
However the OpenGL spec is just a spec. Vendors are free to implement other stuff as well to make users happy, for example by supporting legacy software.
I'm trying to write code to draw shapes on my JOGL canvas. I have the canvas on screen, but I can't figure out how to draw shapes. In GL2 examples, I see examples like:
gl.glBegin( GL2.GL_LINES );
gl.glVertex3f( 0.0f,0.75f,0 );
gl.glVertex3f( -0.75f,0f,0 );
gl.glEnd();
However, this doesn't work for me when gl is an instance of GL4 (gl is an instance of GL2 in this example).
OpenGL 3.x Core Profile has deprecated immediate mode statements.
Use vertex arrays or vertex buffers.
I cannot offer an example in JOGL, but in other languages, one of related calls is glDrawArrays(). You'll need to enable and set up the data source arrays before calling glDrawArrays(); if memory serves well, you'll be interested in glEnableClientState(), glVertexPointer() et al.
I'd like to lay out some things I think I've learned, but am unsure about:
VBOs are the way to go. They're created with glGenBuffers and glBufferData.
For maximum flexibility, it's best to pass generic vertex attributes to shaders with glVertexAttribPointer, rather than glVertex, glNormal, etc..
glDrawElements can be used with vertex buffers and an index buffer to efficiently render geometry with lots of shared vertices, such as a landscape mesh.
Assuming all of that is correct so far, here's my question. All of the tutorials I've read about modern OpenGL completely omit glEnableClientState. But the OpenGL man pages say that without glEnableClientState, glDrawElements will do nothing:
http://www.opengl.org/sdk/docs/man/xhtml/glDrawElements.xml
The key passage is: "If GL_VERTEX_ARRAY is not enabled, no geometric primitives are constructed."
This leads me to the following questions:
None of the tutorials use glEnableClientState before calling glDrawElements. Does this mean the man page is wrong or outdated?
GL_VERTEX_ARRAY would seem to be the thing you enable if you're going to use glVertexPointer, and likewise you'd use GL_NORMAL_ARRAY with glNormalPointer, and so on. But if I'm not using those functions, and am instead using generic vertex attributes with glVertexAttribPointer, then why would it be necessary to enable GL_VERTEX_ARRAY?
If GL_VERTEX_ARRAY is not enabled, no geometric primitives are constructed.
That's because the man page is wrong. The man page covers GL 2.1 (and it's still wrong for that), and for whatever reason, the people updating the man page refuse to update the older GL versions for bug fixes.
In GL 2.1, you must use either generic attribute index 0 or GL_VERTEX_ARRAY. In GL 3.1+, you don't need to use any specific attribute indices.
This is because, in GL versions before 3.1, all array rendering functions were defined in terms of calls to glArrayElement, which used immediate mode-based rendering. That means that you need something to provoke the vertex. Recall that, in immediate mode, calling glVertex*() not only sets the vertex position, it also causes the vertex to be sent with the other attributes. Calling glVertexAttrib*(0, ...) does the same thing. That's why older versions require you to use either attribute 0 or GL_VERTEX_ARRAY.
In GL 3.1+, once they took out immediate mode, they had to specify array rendering differently. And because of that, they didn't have to limit themselves to using attribute 0.
If you want API docs for core GL 3.3 works, I suggest you look at the actual API docs for core GL 3.3. Though to be honest, I'd just look at the spec if you want accurate information. Those docs have a lot of misinformation in them. And since they're not actually wiki-pages, that information never gets corrected.
Your first 3 points are correct. And to answer your last half of your question, do not use glEnableClientState for modern OpenGL. Start coding!
VBOS are the way to go. They're created with glGenBuffers and
glBufferData.
VBOs are often used in high-performance applications. You might want to do something simpler first. Vertex arrays can be a good way to go to get started quickly. Because VBOs sit on top of vertex arrays anyway, you'll be using much of the same code when you switch to VBOs, and it might be good to run a test using vertex arrays or indexed vertex arrays first.
For maximum flexibility, it's best to pass generic vertex attributes
to shaders with glVertexAttribPointer, rather than glVertex, glNormal,
etc..
That's a good approach.
glDrawElements can be used with vertex buffers and an index buffer to
efficiently render geometry with lots of shared vertices, such as a
landscape mesh.
Perhaps. You have to be sure the vertices truly are shared, though. A vertex that is in the same location but has a different normal (e.g. for flat shading) isn't truly shared.
Does version OpenGL 3+ only uses "GL_TRIANGLES" ?
That's what I read, but in the documentation for OpenGL 3.3, http://www.opengl.org/sdk/docs/man3/, "glDrawArrays()" takes the following parameters:
GL_POINTS,
GL_LINE_STRIP,
GL_LINE_LOOP,
GL_LINES,
GL_LINE_STRIP_ADJACENCY,
GL_LINES_ADJACENCY,
GL_TRIANGLE_STRIP,
GL_TRIANGLE_FAN,
GL_TRIANGLES,
GL_TRIANGLE_STRIP_ADJACENCY,
GL_TRIANGLES_ADJACENCY
Does version OpenGL 3+ only uses "GL_TRIANGLES"
You mean "instead of also offering GL_QUADS and GL_POLYGON"?
Yes indeed. Quads and Polygons have been removed altogether. Most polygons needed to be tesselated into triangles anyway, since OpenGL can deal with convex polygons only (convex also imples planar!). Similar holds for quads.
Lines and points remain to be supported of course.
Does version OpenGL 3+ only uses "GL_TRIANGLES" ?? That's what I read
Where? Please provide a link.
There is a difference between "GL_TRIANGLES" and "triangles".
GL_TRIANGLES is a specific primitive type. It has a specific interpretation. It's base primitive type is "triangles" (as in, it generates triangles), but there's more to it than that.
"triangles" are exactly that: assemblages of 3 vertices that represent a planar area. GL_TRIANGLES, GL_TRIANGLE_STRIP, and GL_TRIANGLE_FAN produce triangles.
OpenGL 3.1+ core does not allow the use of the specific primitive types GL_QUADS, GL_QUAD_STRIP (ie: all "quad" types), and GL_POLYGON. Everything else is fair game.
According to section 2.6.1 of the specification commands like glDrawArrays() accept the primitives you posted. So, no, OpenGL 3.3 doesn't accept just GL_TRIANGLES.
What you read was probably meant to explain that OpenGL doesn't support primitives like GL_QUADS and GL_POLYGON anymore.
Quad and polygon primitives have been removed according to appendix E.2.2 of the specification (since version 3.1, prior versions still support them, although they're deprecated from version 3.0).
You can find the specification here.
I've just finished my basic OpenGL model loader, and now I want to move over to VBOs instead of glBegin() and glEnd(). I read in an article that even VBOs are deprecated.
My question is: Are VBOs really deprecated and is there a better way to draw objects in OpenGL? Should I be using display lists perhaps?
Vertex Buffer Objects are not deprecated. In fact, I believe they are the only (non-deprecated) way to render in OpenGL 3.0 and above.
See the OpenGL page on Vertex Buffer Object.
Legacy Note: Versions of OpenGL prior to 3.0 allowed the use of
client-side data for vertex rendering, but GL 3.0 deprecated this.
Core GL 3.1 and greater forbid client-side vertex data, so VBOs are
the only means for rendering.