How exactly can I do a Z buffer prepass with openGL.
I'v tried this:
glcolormask(0,0,0,0); //disable color buffer
//draw scene
glcolormask(1,1,1,1); //reenable color buffer
//draw scene
//flip buffers
But it doesn't work. after doing this I do not see anything. What is the better way to do this?
Thanks
// clear everything
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// z-prepass
glEnable(GL_DEPTH_TEST); // We want depth test !
glDepthFunc(GL_LESS); // We want to get the nearest pixels
glcolormask(0,0,0,0); // Disable color, it's useless, we only want depth.
glDepthMask(GL_TRUE); // Ask z writing
draw()
// real render
glEnable(GL_DEPTH_TEST); // We still want depth test
glDepthFunc(GL_LEQUAL); // EQUAL should work, too. (Only draw pixels if they are the closest ones)
glcolormask(1,1,1,1); // We want color this time
glDepthMask(GL_FALSE); // Writing the z component is useless now, we already have it
draw();
You're doing the right thing with glColorMask.
However, if you're not seeing anything, it's likely because you're using the wrong depth test function.
You need GL_LEQUAL, not GL_LESS (which happens to be the default).
glDepthFunc(GL_LEQUAL);
If i get you right, you are trying to disable the depth-test performed by OpenGL to determine culling. You are using color functions here, which does not make sense to me. I think you are trying to do the following:
glDisable(GL_DEPTH_TEST); // disable z-buffer
// draw scene
glEnable(GL_DEPTH_TEST); // enable z-buffer
// draw scene
// flip buffers
Do not forget to clear the depth buffer at the beginning of each pass.
Related
Well, I’m trying to draw tiles into a stencil buffer, but while drawing obviously happens something that I don’t understand and during the drawing disappearing (not drawn) part of the tiles.
So, how I draw:
// Enable blending.
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
// Enable testing.
glEnable(GL_STENCIL_TEST);
// Disable depth test.
glDisable(GL_DEPTH_TEST);
// Set stencil buff to 0.
glClearStencil(0);
glClear(GL_STENCIL_BUFFER_BIT);
// Here I got all visible tiles by a camera.
auto const visible_tiles = camera.visible_tiles();
// Draw tiles into a stencil.
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
glStencilMask(0xFF);
// Don't output the color.
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
// ...
// Here I had a loop where I go through each tile from all which was visible for the camera.
auto tile_id = 1;
visible_tiles.for_each_tile([this, &tile_id](auto const& tile_position) {
// Output tile ID into stencil buffer. I assume there will never be more than
// 255 tiles.
glStencilFunc(GL_ALWAYS, tile_id, 0xFF);
mesh_->draw(); // << Here I draw.
++tile_id;
});
// Disable testing.
glDisable(GL_STENCIL_TEST);
However, how my problem looks like?
]1
If I disable stencil testing at all, everything is OK.
And we draw it correctly.
UPD: With the help of the debug, I managed to narrow the circle of suspects )))
And I realized that the problem was somewhere in glEnable(GL_STENCIL_TEST), namely when I repeatedly call my method “draw(…)”.
So, I just did the following:
static bool flag = false;
if (!flag) {
glEnable(GL_STENCIL_TEST);
flag = true;
}
So, now I call glEnable(GL_STENCIL_TEST) only once, at first call of draw, and didn’t call glDisable(GL_STENCIL_TEST);
And it looks as if everything works correctly, at least now I didn’t see any defects.
But why this works?
I'm writing code that display hidden part of the object.
Here is example : the plate is larger polygonal object, and cylinder is smaller polygonal object. cylinder is hidden by plate. (See the lower half window : cylinder penetrates the plate. some part of the cylinder is hidden by plate. )
The image is made by below code.
draw plate (not draw it to RGB buffer. only catch the depth values)
draw cylinder (if depth test 'less' passes : that means visible part of the cylinder (smaller depth) is drawn)
The model is rotated along y axis for each frame. It gives me a correct result for every frame.
Now, I'd like to display hidden part of the cylinder as transparent.
Before using blending, I want to display only the hidden part of the cylinder. That is, I have to display cylinder's region that have more greater depth values than plate's depth. Then, I just change
glDepthFunc(GL_LESS);
to
glDepthFunc(GL_GREATER);
However, If I changed it to GL_GREATER, it does not give me a correct result.
I got correct result at first frame, but after then, the model is gone. (That means, the model is not displayed on window. Both of upper, and lower viewport)
I cannot catch the reason. Help me!
void MyDisplay()
{
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glRotatef(rot, 0, 1, 0);
glViewport(0,0, width, height/2);
glEnable(GL_DEPTH_TEST);
DrawPlate();
glColor4f(0,0,0,1);
DrawCylinder();
glDisable(GL_DEPTH_TEST);
glViewport(0,height/2, width, height/2);
glEnable(GL_DEPTH_TEST);
glClearDepth(1.0);
//glDrawBuffer(GL_NONE); // No color buffers are written
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
DrawPlate();
glDepthFunc(GL_LESS);
// glDepthFunc(GL_GREATER); // doesn't work !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glColor4f(0,0,0,0.5f);
DrawCylinder();
delay(1);
glFlush();
glutPostRedisplay();
}
Hey guys i found a solution.
That is : depth test (greater) does not initialized after one frame.
the depth goes to 1 after frame, then no pixel passes the test.
Thus I have to input glDepthFunc(GL_LESS) at first line.
I need to render a sphere to a texture (done using a Framebuffer Object (FBO)), and then alpha blend that texture with the back buffer. So far I'm not doing any processing with the texture except clearing it at the beginning of every frame.
I should say that my scene consists of nothing but a planet in empty space, the sphere should appear next to or around the planet (kind of like a moon for now). When I render the sphere directly to the back buffer, it displays correctly; but when I do the intermediary step of rendering it to a texture and then blending that texture with the back buffer, the sphere only shows up when it is in front of the planet, the part that isn't in front is just "cut off":
I render the sphere using glutSolidSphere to a RGBA8 fullscreen texture that's bound to an FBO, making sure that every sphere pixel receives an alpha value of 1.0. I then pass the texture to a fragment shader program, and use this code to render a fullscreen quad - with the texture mapped onto it - to the backbuffer while alpha blending:
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
glDisable(GL_DEPTH_TEST);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glBegin(GL_QUADS);
glTexCoord2i(0, 1);
glVertex3i(-1, 1, -1); // TOP LEFT
glTexCoord2i(0, 0);
glVertex3i(-1, -1, -1); // BOTTOM LEFT
glTexCoord2i(1, 0);
glVertex3i( 1, -1, -1); // BOTTOM RIGHT
glTexCoord2i(1, 1);
glVertex3i( 1, 1, -1); // TOP RIGHT
glEnd();
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
glEnable(GL_DEPTH_TEST);
glDisable(GL_BLEND);
This is the shader code (taken from an FX file written in Cg):
sampler2D BlitSamp = sampler_state
{
MinFilter = LINEAR;
MagFilter = LINEAR;
MipFilter = LINEAR;
AddressU = Clamp;
AddressV = Clamp;
};
float4 blendPS(float2 texcoords : TEXCOORD0) : COLOR
{
float4 outColor = tex2D(BlitSamp, texcoords);
return outColor;
}
I don't even know whether this is a problem with the depth buffer or with alpha blending, I've tried a lot of combinations of enabling and disabling depth testing (with a depth buffer attached to the FBO) and alpha blending.
EDIT: I tried just rendering a blank fullscreen quad straight to the back buffer and even that was cropped around the planet's edges. For some reason, enabling depth testing for rendering the quad (that is, removing the lines glDisable(GL_DEPTH_TEST) and glEnable(GL_DEPTH_TEST) in the code above) got rid of the problem, but now everything but the planet and the sphere appears white:
I made sure (and could confirm) that the alpha channel of the texture is 0 at every pixel but the sphere's, so I don't understand where the whiteness could be introduced. (Would also still be interested in an explanation why enabling depth testing has this effect.)
I see two possible sources of error here:
1. Rendering to the FBO
If the missing pixels are not even present in the FBO after rendering, there must be some mechanism which discarded the corresponding fragments. The OpenGL pipeline includes four different types of fragment tests which can lead to fragments being discarded:
Scissor Test: Unlikely to be the cause, as the scissor test only affects a rectangular portion of the screen.
Alpha Test: Equally unlikely, as your fragments should all have the same alpha value.
Stencil Test: Also unlikely, unless you use stencil operations when drawing the background planet and copy over the stencil buffer from the back buffer to the FBO.
Depth Test: Same as for stencil test.
So there's a good chance that rendering into FBO is not the issue here. But just to be absolutely sure, you should read back your color attachment texture and dump it into a file for inspection. You can use the following function for that:
void TextureToFile(GLuint texture, const char* filename) {
glBindTexture(GL_TEXTURE_2D, texture);
GLint width, height;
glGetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_WIDTH, &width);
glGetTexLevelParameteriv(GL_TEXTURE_2D, 0, GL_TEXTURE_HEIGHT, &height);
std::vector<GLubyte> pixels(3 * width * height);
glGetTexImage(GL_TEXTURE_2D, 0, GL_RGB, GL_UNSIGNED_BYTE, &pixels[0]);
std::ofstream out(filename, std::ios::out | std::ios::binary);
out << "P6\n"
<< width << '\n'
<< height << '\n'
<< 255 << '\n';
out.write(reinterpret_cast<const char*>(&pixels[0]), pixels.size());
}
The resulting file is a portable pixmap (.ppm). Be sure to unbind the FBO before reading back the texture.
2. Texture mapping
Assuming rendering into the FBO works as expected, the only other source of error is blending the texture over the previously rendered scene. There are two scenarios:
a) Fragments get discarded
The possible reasons for fragments to get discarded are the same as in 1.:
Scissor Test: Nope, affects rectangular areas only.
Alpha Test: Probably not, the texels covered sphere should all have the same alpha value.
Stencil Test: Might be the cause if you use stencil operations/stencil testing when drawing the background planet and the old stencil state is still active.
Depth Test: Might be the cause, but as you already disable it, it really shouldn't have any effect.
So you should make sure that all of these tests are disabled, especially the stencil test.
b) Wrong results from blending
Assuming all fragments reach the back buffer, blending is the only thing which could still cause the wrong result. With your blending function (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) the values in the back buffer are irrelevant for blending, and we assume that the alpha values in the texture are correct. So I see no reason for why blending should be the root cause here.
Conclusion
In conclusion, the only sensible cause for the observed result seems to be stencil testing. If it's not, I'm out of options :)
I solved it or at least came up with a work around.
First off, the whiteness stems from the fact that glClearColor had been set to glClearColor(1.0f, 1.0f, 1.0f, 1000.0f), so everything but the planet wasn't even written to in the end. I now copy the contents of the back buffer (which is the planet, the atmosphere, and the space around it) to the texture before rendering the sphere, and I render the atmosphere and space before that copy/blit operation, so they are included in it. Previously, everything but the planet itself was rendered after my quad, which - when using depth testing - apparently placed everything behind the quad, making it invisible.
The reference implementation of the effect I'm trying to achieve has always used this kind of blit operation in its code but I didn't think it was necessary for the effect. Now I feel like there might be no other way...
I just started working with OpenGL, but I ran into a problem after implementing a Font system.
My plan is to simply visualize several Pathfinding Algorithms.
Currently OpenGL gets set up like this (OnSize gets called once on window creation manually):
void GLWindow::OnSize(GLsizei width, GLsizei height)
{
// set size
glViewport(0,0,width,height);
// orthographic projection
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0,width,height,0.0,-1.0,1.0);
glMatrixMode(GL_MODELVIEW);
m_uiWidth = width;
m_uiHeight = height;
}
void GLWindow::InitGL()
{
// enable 2D texturing
glEnable(GL_TEXTURE_2D);
// choose a smooth shading model
glShadeModel(GL_SMOOTH);
// set the clear color to black
glClearColor(0.0, 0.0, 0.0, 0.0);
glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.0f);
}
In theory I don't need blending, because I will only use untextured Quads to visualize obstacles and line etc to draw paths... So everything will be untextured, except the fonts...
The Font Class has a push and pop function, that look like this (if I remember right my Font system is based on a NeHe Tutorial that I was following quite a while ago):
inline void GLFont::pushScreenMatrix()
{
glPushAttrib(GL_TRANSFORM_BIT);
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(viewport[0],viewport[2],viewport[1],viewport[3], -1.0, 1.0);
glPopAttrib();
}
inline void GLFont::popProjectionMatrix()
{
glPushAttrib(GL_TRANSFORM_BIT);
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glPopAttrib();
}
So the Problem:
If I don't draw a Text I can see the Quads I want to draw, but they are quite dark, so there must be something wrong with my general OpenGL Matrix Properties.
If I draw Text (so the font related push and pop functions get called) I can't see any Quads.
The question:
How do I solve this problem and some background information why this happened would also be nice, because I am still a beginner/student, who just started.
If your quads are untextured, you will run into undefined behaviour. What will probably happen is that any previous texture will be used, and the colour at point (0,0) will be used, which could be what is causing them to be invisible.
Really, you need to disable texturing before trying to draw untextured quads using glDisable(GL_TEXTURE_2D). Again, if you don't, it'll just use the previous texture and texture co-ordinates, which without seeing your draw() loop, I'm assuming to be undefined.
I have a simple particle effect in OpenGL using GL_POINTS. The following is called, being passed particles in order from the particle furthest from the camera first to the one nearest the camera last:
void draw_particle(particle* part) {
/* The following is how the distance is calculated when ordering.
* GLfloat distance = sqrt(pow(get_camera_pos_x() - part->pos_x, 2) + pow(get_camera_pos_y() - part->pos_y, 2) + pow(get_camera_pos_z() - part->pos_z, 2));
*/
static GLfloat quadratic[] = {0.005, 0.01, 1/600.0};
glPointParameterfvARB(GL_POINT_DISTANCE_ATTENUATION_ARB, quadratic);
glPointSize(part->size);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_POINT_SPRITE_ARB);
glEnable(GL_TEXTURE_2D);
glBegin(GL_POINTS);
glColor4f(part->r, part->g, part->b, part->a);
glVertex3f(part->pos_x, part->pos_y, part->pos_z);
glEnd();
glDisable(GL_BLEND);
glDisable(GL_POINT_SPRITE_ARB);
}
However, there is some artifacting when rendering as can be seen in the following effect:artifacted image http://img199.imageshack.us/img199/9574/particleeffect.png
The problems go away if I disable depth testing, but I need the effects to be able to interact with other elements of the scene, appearing in front of and behind elements of the same GL_TRIANGLE_STRIP depending on depth.
If your particules are already sorted you can render like this :
Render particule with GL WRITE DEPTH but no depth testing (I don't remember exactly the constants)
Render the rest of the scene with depth test.
This way you are sure to get scene interaction with nice-looking particules.
Note: Please specify which OpenGL version you use when you post questions. That goes for any API.
When you want to render primitives with alpha blending, you can't have depth writes enabled. You need to draw your blended primitives sorted back-to-front. If you have opaque objects in the scene, render them first, and then draw your transparent primitives in a back-to-front sorted fashion with depth test enabled and depth writes disabled.
glEnable(GL_DEPTH_TEST);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_POINT_SPRITE);
glEnable(GL_CULL_FACE);
while(1)
{
...
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glDisable(GL_BLEND);
glDepthMask(1);
RenderOpaque();
SortSprites();
glEnable(GL_BLEND);
glDepthMask(0);
DrawSprites();
...
}