Why does glReadPixels seem to return garbage? - linux

Question

I'm trying to draw offscreen with OpenGL. For this I use EGL to initialize a pbuffer surface, and then draw to it, reading the results back with glReadPixels. But the following program gives me garbage on different (Mesa-based Intel on Linux) GPUs. Namely, on Atom N550 I get zeros, while on Xeon E3-1200 v3 I have 70 00 07 44 instead of the expected 40 80 bf ff.
With LIBGL_ALWAYS_SOFTWARE=1 environment variable set, I get the expected results. Also, if I comment out the line with eglBindAPI, I get good result on Xeon, but still zeros on Atom.
Here's my program:
#include <EGL/egl.h>
#include <GL/gl.h>
#include <iostream>
#include <iomanip>
#include <cstring>
int eglPrintError(std::string const& context)
{
const GLint error=eglGetError();
std::cerr << context << ": error 0x" << std::hex << int(error) << "\n";
return 1;
}
bool checkError(std::string const& funcName)
{
const GLenum error=glGetError();
if(error!=GL_NO_ERROR)
{
std::cerr << funcName << ": error 0x" << std::hex << int(error) << "\n";
return true;
}
return false;
}
constexpr int fbW=1, fbH=1;
bool initGL()
{
if(!eglBindAPI(EGL_OPENGL_API)) return !eglPrintError("eglBindAPI");
const EGLDisplay dpy=eglGetDisplay(EGL_DEFAULT_DISPLAY);
if(!dpy) return !eglPrintError("eglGetDisplay");
if(!eglInitialize(dpy,nullptr,nullptr)) return !eglPrintError("eglInitialize");
static const EGLint cfgAttribs[]={EGL_RENDERABLE_TYPE, EGL_OPENGL_BIT,
EGL_SURFACE_TYPE, EGL_PBUFFER_BIT,
EGL_RED_SIZE, 8,
EGL_GREEN_SIZE, 8,
EGL_BLUE_SIZE, 8,
EGL_ALPHA_SIZE, 8,
EGL_NONE};
EGLConfig cfg;
EGLint cfgCount;
if(!eglChooseConfig(dpy,cfgAttribs,&cfg,1,&cfgCount))
return !eglPrintError("eglChooseConfig");
if(cfgCount==0)
{
std::cerr << "Failed to get any usable EGL configs\n";
return false;
}
const EGLContext context=eglCreateContext(dpy,cfg,EGL_NO_CONTEXT,NULL);
if(!context) return !eglPrintError("eglCreateContext");
const EGLint surfaceAttribs[]={EGL_WIDTH, fbW, EGL_HEIGHT, fbH, EGL_NONE};
const EGLSurface surface=eglCreatePbufferSurface(dpy,cfg,surfaceAttribs);
if(!surface) return eglPrintError("eglCreatePbufferSurface");
if(!eglMakeCurrent(dpy,surface,surface,context))
return !eglPrintError("eglMakeCurrent");
return true;
}
int main(int argc, char** argv)
{
if(!initGL()) return 1;
glViewport(0,0,fbW,fbH);
glClearColor(0.25,0.5,0.75,1.);
glClear(GL_COLOR_BUFFER_BIT);
glFinish();
unsigned char data[4*fbW*fbH];
std::memset(data,0xb7,sizeof data); // to see unchanged values
glReadPixels(0,0,fbW,fbH,GL_RGBA,GL_UNSIGNED_BYTE,data);
if(checkError("glReadPixels")) return 1;
std::cout << "Data read: " << std::hex << std::setfill('0');
for(auto datum : data)
std::cout << std::setw(2) << +datum << " ";
std::cout << "\n";
return 0;
}
My question is, is there anything wrong in the above code which could lead to such behavior, or are my drivers just simply buggy?