问题
I would like to do offscreen rendering (in console environment without any WS) with FBOs. I know that it is necessary to create an OpenGL context, and at least a dummy window for any operation, therefore I did the following initialization:
// Step 1 - Get the default display.
eglDisplay = eglGetDisplay((EGLNativeDisplayType)0);
// Step 2 - Initialize EGL.
EGLint iMajorVersion, iMinorVersion;
if (!eglInitialize(eglDisplay, &iMajorVersion, &iMinorVersion))
{
printf("Error: eglInitialize() failed.\n");
goto cleanup;
}
// Step 3 - Make OpenGL ES the current API.
eglBindAPI(EGL_OPENGL_ES_API);
if (!TestEGLError("eglBindAPI"))
{
goto cleanup;
}
// Step 4 - Specify the required configuration attributes.
EGLint pi32ConfigAttribs[5];
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_WINDOW_BIT;
pi32ConfigAttribs[2] = EGL_RENDERABLE_TYPE;
pi32ConfigAttribs[3] = EGL_OPENGL_ES2_BIT;
pi32ConfigAttribs[4] = EGL_NONE;
// Step 5 - Find a config that matches all requirements.
int iConfigs;
if (!eglChooseConfig(eglDisplay, pi32ConfigAttribs, &eglConfig, 1, &iConfigs) || (iConfigs != 1))
{
printf("Error: eglChooseConfig() failed.\n");
goto cleanup;
}
// Step 6 - Create a surface to draw to.
EGLSurface eglSurface;
eglSurface = eglCreateWindowSurface(eglDisplay, eglConfig, (EGLNativeWindowType)NULL, NULL);
if (!TestEGLError("eglCreateWindowSurface")) goto cleanup;
// Step 7 - Create a context.
eglContext = eglCreateContext(eglDisplay, eglConfig, NULL, ai32ContextAttribs);
if (!TestEGLError("eglCreateContext")) goto cleanup;
// Step 8 - Bind the context to the current thread
eglMakeCurrent(eglDisplay, eglSurface, eglSurface, eglContext);
if (!TestEGLError("eglMakeCurrent")) goto cleanup;
This type of initialization works on the target system, but if I draw something into the FBO, and after read it back (getReadPixels) always only get a black image. I have tried the same code above X11, where it (already) fails on eglCreateWindowSurface call with error: EGL_BAD_NATIVE_WINDOW WORKS, BUT it WORKS if I pass to this call a real X11window native window handler (This case I can read back the valid rendered image also)
Please clarify me about what type of surface should I need to use for FBO rendering?? or what do I do wrong??
The rest of the drawing code: //init and start opengl es shanders Shaders_Init();
{
// create a framebuffer object
glGenFramebuffers(1, &fboId);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);
// create a texture object
GLuint textureId;
glGenTextures(1, &textureId);
glBindTexture(GL_TEXTURE_2D, textureId);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); //GL_LINEAR_MIPMAP_LINEAR
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
//glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP_HINT, GL_TRUE); // automatic mipmap
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, renderBufferWidth, renderBufferHeight, 0,
GL_RGB, GL_UNSIGNED_BYTE, 0);
//glBindTexture(GL_TEXTURE_2D, 0);
// attach the texture to FBO color attachment point
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
GL_TEXTURE_2D, textureId, 0);
// check FBO status
GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
if(status != GL_FRAMEBUFFER_COMPLETE) {
printf("Problem with OpenGL framebuffer after specifying color render buffer: \n%x\n", status);
} else {
printf("FBO creation succedded\n");
}
}
// Sets the clear color.
// The colours are passed per channel (red,green,blue,alpha) as float values from 0.0 to 1.0
glClearColor(0.6f, 0.8f, 1.0f, 1.0f); // clear blue
// We're going to draw a triangle to the screen so create a vertex buffer object for our triangle
{
// Interleaved vertex data
GLfloat afVertices[] = { -0.4f,-0.4f,0.0f, // Position
0.4f ,-0.4f,0.0f,
0.0f ,0.4f ,0.0f};
// Generate the vertex buffer object (VBO)
glGenBuffers(1, &ui32Vbo);
// Bind the VBO so we can fill it with data
glBindBuffer(GL_ARRAY_BUFFER, ui32Vbo);
// Set the buffer's data
unsigned int uiSize = 3 * (sizeof(GLfloat) * 3); // Calc afVertices size (3 vertices * stride (3 GLfloats per vertex))
glBufferData(GL_ARRAY_BUFFER, uiSize, afVertices, GL_STATIC_DRAW);
}
// Draw a triangle
{
glClear(GL_COLOR_BUFFER_BIT);
if (!TestEGLError("glClear")) goto cleanup;
// First gets the location of that variable in the shader using its name
int i32Location = glGetUniformLocation(uiProgramObject, "myPMVMatrix");
// Then passes the matrix to that variable
glUniformMatrix4fv( i32Location, 1, GL_FALSE, pfIdentity);
/*
Enable the custom vertex attribute at index VERTEX_ARRAY.
We previously binded that index to the variable in our shader "vec4 MyVertex;"
*/
glEnableVertexAttribArray(VERTEX_ARRAY);
// Sets the vertex data to this attribute index
glVertexAttribPointer(VERTEX_ARRAY, 3, GL_FLOAT, GL_FALSE, 0, 0);
/*
Draws a non-indexed triangle array from the pointers previously given.
This function allows the use of other primitive types : triangle strips, lines, ...
For indexed geometry, use the function glDrawElements() with an index list.
*/
glDrawArrays(GL_TRIANGLES, 0, 3);
if (!TestEGLError("glDrawArrays")) goto cleanup;
// get the image data
long imageSize = x * y * 3;
unsigned char *data = new unsigned char[imageSize];
glReadPixels(0,0,x,y,GL_RGB,GL_UNSIGNED_BYTE,data);
THANKS in advance!!!!! Regards, Geza
回答1:
The solution was the following (without error handling):
#ifdef CONTEXT_ES20
EGLint ai32ContextAttribs[] = { EGL_CONTEXT_CLIENT_VERSION, 2,
EGL_NONE };
#endif
// Step 1 - Get the default display.
eglDisplay = eglGetDisplay((EGLNativeDisplayType)0);
// Step 2 - Initialize EGL.
eglInitialize(eglDisplay, 0, 0);
#ifdef CONTEXT_ES20
// Step 3 - Make OpenGL ES the current API.
eglBindAPI(EGL_OPENGL_ES_API);
// Step 4 - Specify the required configuration attributes.
EGLint pi32ConfigAttribs[5];
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_WINDOW_BIT;
pi32ConfigAttribs[2] = EGL_RENDERABLE_TYPE;
pi32ConfigAttribs[3] = EGL_OPENGL_ES2_BIT;
pi32ConfigAttribs[4] = EGL_NONE;
#else
EGLint pi32ConfigAttribs[3];
pi32ConfigAttribs[0] = EGL_SURFACE_TYPE;
pi32ConfigAttribs[1] = EGL_WINDOW_BIT;
pi32ConfigAttribs[2] = EGL_NONE;
#endif
// Step 5 - Find a config that matches all requirements.
int iConfigs;
eglChooseConfig(eglDisplay, pi32ConfigAttribs, &eglConfig, 1,
&iConfigs);
if (iConfigs != 1) {
printf("Error: eglChooseConfig(): config not found.\n");
exit(-1);
}
// Step 6 - Create a surface to draw to.
EGLSurface eglSurface;
eglSurface = eglCreateWindowSurface(eglDisplay, eglConfig,
(EGLNativeWindowType)NULL, NULL);
// Step 7 - Create a context.
#ifdef CONTEXT_ES20
eglContext = eglCreateContext(eglDisplay, eglConfig, NULL,
ai32ContextAttribs);
#else
eglContext = eglCreateContext(eglDisplay, eglConfig, NULL, NULL);
#endif
// Step 8 - Bind the context to the current thread
eglMakeCurrent(eglDisplay, eglSurface, eglSurface, eglContext);
来源：https://stackoverflow.com/questions/12662227/opengl-es2-0-offscreen-context-for-fbo-rendering