Writing OpenEXR 16bit image file in C++ - c++

Question

I am trying to write a 16bit texture rendered with OpenGL using OpenEXR, following the example in page 4 from the documentation, but for some reason my code crashes when executing file_exr.writePixels(512). Is there anything I am missing here?
Update: I did check that fboId and pboId are well initialized and no OpenGL errors exist until this point.
const Imf::Rgba * dest;
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboId);
glReadPixels(0, 0, 512, 512, GL_BGRA, GL_HALF_FLOAT_NV, 0);
dest = (const Imf::Rgba *)glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB);
Imf::RgbaOutputFile file_exr("/tmp/file.exr", 512, 512, Imf::WRITE_RGBA);
file_exr.setFrameBuffer(dest, 1, 512);
file_exr.writePixels(512);
glUnmapBufferARB(GL_PIXEL_PACK_BUFFER_ARB);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, 0);

Answer 1

Did you just copy and paste that code (and just that code)? Then the reason for it failing is that:
The buffer object you want to read the pixels from OpenGL into does not exist; hence mapping it will fail, which meant you point OpenEXR to a null pointer
There's no single error condition check at all in above code.
Do this instead:
First a helper, to clean up the OpenGL error stack (which may accumulate multiple error conditions):
int check_gl_errors()
{
int errors = 0;
while( GL_NO_ERROR != glGetError() ) { errors++; }
return errors;
}
Then this
int const width = 512;
int const height = 512;
size_t const sizeof_half_float = 2;
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
glBindBufferARB(GL_PIXEL_PACK_BUFFER_ARB, pboId);
glBufferDataARB(GL_PIXEL_PACK_BUFFER_ARB,
width * height * sizeof_half_float,
NULL,
GL_STATIC_READ_ARB);
if( !check_gl_errors() ) {
glPixelStorei(GL_PACK_ALIGNMENT, 1);
glPixelStorei(GL_PACK_ROW_LENGTH, 0);
glPixelStorei(GL_PACK_SKIP_PIXELS, 0);
glPixelStorei(GL_PACK_SKIP_ROWS, 0);
/* BTW: You have to check that your system actually supports the
GL_HALF_FLOAT_NV format extension at all. */
glReadPixels(0, 0, width, width, GL_BGRA, GL_HALF_FLOAT_NV, 0);
if( !check_gl_errors() ) {
Imf::Rgba const * const dest = (Imf::Rgba const*)
glMapBufferARB(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY_ARB);
if( !check_gl_errors() && nullptr != dest ) {
Imf::RgbaOutputFile file_exr(
"/tmp/file.exr",
width, height,
Imf::WRITE_RGBA);
file_exr.setFrameBuffer(dest, 1, width);
file_exr.writePixels(height);
glUnmapBufferARB(GL_PIXEL_PACK_BUFFER_ARB);
}
else {
/* glMapBuffer failed */
}
}
else {
/* glReadPixels failed */
}
}
else {
/* glBufferDataARB failed => no valid buffer object
to work with in the first place */
}
All these error checks are important. They make your program not crash, but give diagnostics, what went wrong.
Anyway, the use of a PBO in the very order of operations doesn't help anyway, because it gets mapped immediately after the glReadPixels operation, which makes the whole thing synchronous.