Render OpenGL scene to texture using FBO in fixed function pipeline drawing
The Problem
I work on the open source game torcs (http://torcs.sourceforge.net/). The game's graphic pipeline is still using the fixed function pipeline (FFP) of OpenGL 1.3.
I try to render the game scenes to textures in a FBO (Framebuffer Object) in order to do some post-processing on the rendered textures. I use OpenGL 3.3. on my machine.
Currently I have set up the FBO with attached textures at GL_COLOR_ATTACHMENT0&1 (2 in order to have two consecutive frames readable in the shader) and an attached renderbuffer at GL_DEPTH_ATTACHMENT .
After binding the FBO the game rendering function is executed. When I afterwards unbind the FBO and for validation write back the texture via a shader program to the window buffer the scene is incomplete. To be more specific, only the outline of the car is rendered, so are skidmarks of the tires and some smoke. This shows that something is rendered to the texture of the FBO, but not everything . Among others no textures (for trees, houses, grass etc.) are rendered to the texture in the FBO. This suggests that my texture set up is incorrect, but unfortunately my knowledge of OpenGL is limited, which is why I hope for your help.
One other thing worth noting is, that if I leave out the line glActiveTexture(GL_TEXTURE0); before the drawing happens then one texture will be displayed (ie will be written to the FBO and written back to the window system framebuffer instead of the car outline.
The Code
The following code shows the initialization of the FBO (from https://en.wikibooks.org/wiki/OpenGL_Programming/Post-Processing):
int screen_width = 640;
int screen_height = 480;
/* Texture A*/
glGenTextures(1, &fbo_texture);
glBindTexture(GL_TEXTURE_2D, fbo_texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, screen_width, screen_height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glBindTexture(GL_TEXTURE_2D, 0);
/* Texture B*/
//glActiveTexture(GL_TEXTURE1);
glGenTextures(1, &fbo_texture_a);
glBindTexture(GL_TEXTURE_2D, fbo_texture_a);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, screen_width, screen_height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glBindTexture(GL_TEXTURE_2D, 0);
/* Depth buffer */
glGenRenderbuffers(1, &rbo_depth);
glBindRenderbuffer(GL_RENDERBUFFER, rbo_depth);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, screen_width, screen_height);
glBindRenderbuffer(GL_RENDERBUFFER, 0);
/* Framebuffer to link everything together */
glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, fbo_texture, 0);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, fbo_texture_a, 0);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rbo_depth);
GLenum status;
if ((status = glCheckFramebufferStatus(GL_FRAMEBUFFER)) != GL_FRAMEBUFFER_COMPLETE) {
fprintf(stderr, "glCheckFramebufferStatus: error 0x%x", status);
return 0;
}
glBindFramebuffer(GL_FRAMEBUFFER, 0);
/* Compile and link shaders */
...
The following code shows where the drawing happens: Edit : if use_fbo=false then everything will be rendered to screen directly as before. The only changes I made are within the brackets.
if (use_fbo)
{
glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glPushAttrib(GL_VIEWPORT_BIT);
glViewport(0,0,grWinw, grWinh);
if (fbo_a) // drawing to fbo_texture_a
{
glDrawBuffer(GL_COLOR_ATTACHMENT1);
glActiveTexture(GL_TEXTURE0+11);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, fbo_texture_a);
}
else
{
glDrawBuffer(GL_COLOR_ATTACHMENT0);
glActiveTexture(GL_TEXTURE0+12);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, fbo_texture);
}
glActiveTexture(GL_TEXTURE0);
glEnable(GL_TEXTURE_2D);
}
glClearColor(0.7f, 0.1f, 0.1f, 1.0f); //clear with red to see what is drawn to the fbo
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
grScreens[0]->update(s, grFps);//THIS IS WHERE THE DRAWING HAPPENS unchanged from original drawing in TORCS
if (use_fbo)
{
glPopAttrib();
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glEnable(GL_TEXTURE_2D);
glDrawBuffer(GL_BACK);
glClearColor(1.0, 1.0, 1.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glUseProgram(program_postproc);
if (fbo_a) // drawn to fbo_texture_a
{
glUniform1i(uniform_fbo_texture, 11);
glUniform1i(uniform_fbo_texture_a, 12);
fbo_a=!fbo_a;
}
else
{
glUniform1i(uniform_fbo_texture, 12);
glUniform1i(uniform_fbo_texture_a, 11);
fbo_a=!fbo_a;
}
glEnableVertexAttribArray(attribute_v_coord_postproc);
glBindBuffer(GL_ARRAY_BUFFER, vbo_fbo_vertices);
glVertexAttribPointer(
attribute_v_coord_postproc, 2, GL_FLOAT, GL_FALSE, 0, 0);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glDisableVertexAttribArray(attribute_v_coord_postproc);
glUseProgram(0);
}
I hope I provided enough information for you to help me with this. Any advice is appreciated.
EDIT: I checked my shader code and FBO implementation again (simplified it to just one color attachment etc. with a simplified drawing) and it all worked. I think the trouble is the mix of fixed function pipeline for drawing and my implementation of the FBO...
EDIT: here are two images of what happens with use_fbo=true vs. false: (Note: the red color is the clear color after the FBO is bound, to see what gets rendered to the fbo: nothing apart from the shadow and skid marks)
I also tried to visualize the depth buffer (changed the implementation to a texture attachment for depth) and even though I linearized, there was no information. I suppose the depth is not correctly written to the FBO either.
When I compared your code with mine working engine I see these differences so try them one by one:
texture format
you are using:
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, screen_width, screen_height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
so merging all your to:
GL_COLOR_ATTACHMENT0: GL_RGBA,GL_RGBA,GL_UNSIGNED_BYTE
GL_COLOR_ATTACHMENT1: GL_RGBA,GL_RGBA,GL_UNSIGNED_BYTE
I am using:
GL_COLOR_ATTACHMENT0 : GL_RGBA , GL_RGBA8 , GL_UNSIGNED_BYTE
GL_DEPTH_ATTACHMENT : GL_DEPTH_COMPONENT, GL_DEPTH_COMPONENT16, GL_UNSIGNED_BYTE
GL_STENCIL_ATTACHMENT: GL_STENCIL_INDEX , GL_STENCIL_INDEX8 , GL_UNSIGNED_BYTE
You need to change internal pixel format of textures to specify bit width. If my memory serves well when I code this (some years back) it did not work with just GL_RGBA,GL_RGBA .
depth target
I am using depth and stencil textures the same way as color attachment I do not use any RenderBuffer calls. That does not mean your code is wrong but mine is tested and works.
texture size
This is most likely not valid anymore as most gfx cards support rectangle texture extension but OpenGL textures should be power of 2 resolution. So for starters try 512x512 instead of your 640x480 And change back when your code is working (just to be sure ...).
In case it helps here is my C++ FBO class taken from mine engine so you got something to compare to (will not work alone as it uses textures and stuff from the engine):
//------------------------------------------------------------------------------
//--- Open GL FBO object ver 2.31 ----------------------------------------------
//------------------------------------------------------------------------------
#ifndef _OpenGL_FBO_h
#define _OpenGL_FBO_h
//------------------------------------------------------------------------------
class OpenGL_FBO
{
public:
GLuint fbo;
int xs,ys;
struct _dst
{
GLint txr; // texture id
GLenum dst; // GL_DEPTH_COMPONENT, GL_COLOR_ATTACHMENT0, ...
_dst() { txr=-1; dst=GL_COLOR_ATTACHMENT0; }
_dst(_dst& a) { *this=a; }
~_dst() {}
_dst* operator = (const _dst *a) { *this=*a; return this; }
//_dst* operator = (const _dst &a) { ...copy... return this; }
};
List<_dst> dst;
OpenGL_FBO() { fbo=0xFFFFFFFF; xs=1; ys=1; dst.reset(); }
OpenGL_FBO(OpenGL_FBO& a) { fbo=0xFFFFFFFF; dst.reset(); *this=a; }
~OpenGL_FBO() { if (fbo!=0xFFFFFFFF) glDeleteFramebuffers(1,&fbo); }
OpenGL_FBO* operator = (const OpenGL_FBO *a) { *this=*a; return this; }
//OpenGL_FBO* operator = (const OpenGL_FBO &a) { ...copy... return this; }
void resize(OpenGLscreen &scr,int _xs=-1,int _ys=-1)
{
int i;
_dst *d;
if (_xs<=0) _xs=scr.xs;
if (_ys<=0) _ys=scr.ys;
// for (xs=1;xs<_xs;xs<<=1);
// for (ys=1;ys<_ys;ys<<=1);
xs=_xs; ys=_ys; // ****
if (fbo==0xFFFFFFFF) glGenFramebuffers(1,&fbo);
glBindFramebuffer(GL_FRAMEBUFFER,fbo);
for (d=dst.dat,i=0;i<dst.num;i++,d++)
{
scr.txrs.bind(d->txr);
scr.txrs.resize(d->txr,xs,ys,1);
// glFramebufferTexture2D(GL_FRAMEBUFFER,t->dst,GL_TEXTURE_2D,scr.txrs.names[d->txr],0);
glFramebufferTexture(GL_FRAMEBUFFER,d->dst,scr.txrs.names[d->txr],0);
// glCheckFramebufferStatus(GL_FRAMEBUFFER);
}
scr.txrs.unbind();
glBindFramebuffer(GL_FRAMEBUFFER,0);
}
int add(OpenGLscreen &scr,int _dest=GL_COLOR_ATTACHMENT0) // add txr to fbo
{
_dst d;
OpenGL_TXR tmp;
// colro atachments
tmp.pixelformat =GL_RGBA;
tmp.pixeliformat=GL_RGBA8;
tmp.pixeltype=GL_UNSIGNED_BYTE;
tmp.mag=GL_NEAREST;
tmp.min=GL_NEAREST;
if (_dest==GL_DEPTH_ATTACHMENT)
{
tmp.pixelformat =GL_DEPTH_COMPONENT;
tmp.pixeliformat=GL_DEPTH_COMPONENT16;
// tmp.pixeltype=GL_FLOAT;
tmp.pixeltype=GL_UNSIGNED_BYTE;
}
if (_dest==GL_STENCIL_ATTACHMENT)
{
tmp.pixelformat =GL_STENCIL_INDEX;
tmp.pixeliformat=GL_STENCIL_INDEX8;
tmp.pixeltype=GL_UNSIGNED_BYTE;
}
tmp.xs=xs;
tmp.ys=ys;
tmp.zs=1;
tmp._mipmap=0;
tmp.txrtype=GL_TEXTURE_2D;
d.txr=scr.txrs.add(tmp);
d.dst=_dest;
dst.add(d);
return d.txr;
}
void bind(OpenGLscreen &scr) // init fbo >> txr
{
// init and resize
if (fbo==0xFFFFFFFF) glGenFramebuffers(1,&fbo);
glBindFramebuffer(GL_FRAMEBUFFER,fbo);
glViewport(0,0,xs,ys);
scr.cls();
}
void unbind(OpenGLscreen &scr)
{
glBindFramebuffer(GL_FRAMEBUFFER,0);
glViewport(scr.x0,scr.y0,scr.xs,scr.ys);
}
};
//------------------------------------------------------------------------------
#endif
//------------------------------------------------------------------------------
//--- end. ---------------------------------------------------------------------
//------------------------------------------------------------------------------
where:
OpenGLscreen scr is my rendering engine
scr.cls() is just glClear and stuff to init frame
scr.x0,y0,xs,ys is viewport of target window
scr.txrs is texture system class (handles all the textures) like add new texture loading/saving from/to file, conversion between CPU/GPU and much much more.
I also use mine dynamic list template so:
List<double> xxx; is the same as double xxx[];
xxx.add(5); adds 5 to end of the list
xxx[7] access array element (safe)
xxx.dat[7] access array element (unsafe but fast direct access)
xxx.num is the actual used size of the array
xxx.reset() clears the array and set xxx.num=0
xxx.allocate(100) preallocate space for 100 items
typical usage is:
// [globals and init]
OpenGLScreen scr; // can ignore this
OpenGL_FBO fbo;
scr.init(window_handle); // init OpenGL stuff can ignore this
fbo.add(scr,GL_COLOR_ATTACHMENT0);
fbo.add(scr,GL_DEPTH_ATTACHMENT);
fbo.resize(scr,512,512);
// [render loop]
fbo.bind(scr);
// here render
fbo.unbind(scr);
// here you can use the textures fbo.dst[].txr
Take a look here for specific example:
Of coarse for those of you that are stuck with older Intel HD graphics do not expect that this will work due to bug in drivers. See this slow workaround:
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