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As per subject I have the following pseudo-code to setup window capture in X (Linux):

xdisplay = XOpenDisplay(NULL);
win_capture = ...find the window to capture...
XCompositeRedirectWindow(xdisplay, win_capture, CompositeRedirectAutomatic);
XGetWindowAttributes(xdisplay, win_capture, &win_attr); // attributes used later
GLXFBConfig *configs = glXChooseFBConfig(xdisplay, win_attr.root, config_attrs, &nelem);
// cycle through the configs to
// find a valid one
...
win_pixmap = XCompositeNameWindowPixmap(xdisplay, win_capture);
const int pixmap_attrs[] = {GLX_TEXTURE_TARGET_EXT, GLX_TEXTURE_2D_EXT,
                    GLX_TEXTURE_FORMAT_EXT,
                    GLX_TEXTURE_FORMAT_RGBA_EXT, None};
gl_pixmap = glXCreatePixmap(xdisplay, config, win_pixmap, pixmap_attrs);
gl_ctx = glXCreateNewContext(xdisplay, config, GLX_RGBA_TYPE, 0, 1);
glXMakeCurrent(xdisplay, gl_pixmap, gl_ctx);
glEnable(GL_TEXTURE_2D);
glGenTextures(1, &gl_texmap);
glBindTexture(GL_TEXTURE_2D, gl_texmap);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, win_attr.width, win_attr.height, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

Then, much later on, this would be the loop to capture the frames:

glXMakeCurrent(xdisplay, gl_pixmap, gl_ctx);
glBindTexture(GL_TEXTURE_2D, gl_texmap);
glXBindTexImageEXT(xdisplay, gl_pixmap, GLX_FRONT_LEFT_EXT, NULL);
glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_BYTE, data); // data is output RGBA buffer
glXReleaseTexImageEXT(xdisplay, gl_pixmap, GLX_FRONT_LEFT_EXT);

I basically do glXBindTexImageEXT -> glGetTexImage -> glXReleaseTexImageEXT so that I get an updated picture. It does work, but not sure I'm doing the right/optimal thing.

Is there a better/more optimized way to get such picture/context?

Emanuele
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  • It seems quite a bit of overhead to use the whole GLX pipeline for this, only to read back the texture data into main memory again. Is there a particular reason why you need OpenGL? – Florian Echtler May 08 '20 at 16:07
  • I have also developed a version where I use PBO to transfer data and manage memory buffers. You won't believe, but on some hardware many operations should be DMA-like, which would bring 2 benefits: stability in terms of not flickering and true capture. If you do the same with some other API (like x11grab in libavcodec), you get choppy frames: with this capture, the quality is much better - because GL pipeline is involved in the capture. – Emanuele May 09 '20 at 06:40

1 Answers1

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As of now I've found a slightly better way to implement fetching the composite window through OpenGL, via PBO; the advantages of this way is that you could initiate the command asynchronously and then retrieve the RGBA buffer from system memory, whilst the OpenGL driver does data transfer.

Sample pseudocode:

// setup a PBO
GLuint cur_pbo;
glGenBuffers(1, &cur_pbo);
glBindBuffer(GL_PIXEL_PACK_BUFFER, cur_pbo);
glBufferData(GL_PIXEL_PACK_BUFFER, size, NULL, GL_STREAM_READ);

Then much later on

glXMakeCurrent(xdisplay, gl_pixmap, gl_ctx);
glBindTexture(GL_TEXTURE_2D, gl_texmap);
glXBindTexImageEXT(xdisplay, gl_pixmap, GLX_FRONT_LEFT_EXT, NULL);
glBindBuffer(GL_PIXEL_PACK_BUFFER, cur_pbo);
// This will initiate the data transfer, the previous
// buffer pointer is now an offset in the index bound by previous
// glBufferData call
glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
// do something else
...
...
...
// then later on when we _really_ need to get the data
// perform this call which will make wait if the RGBA 
// data is not avilable yet
void* rgba_ptr = glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_ONLY);
// Then when finished to use rgba_ptr, release it
glUnmapBuffer(GL_PIXEL_PACK_BUFFER);
glXReleaseTexImageEXT(xdisplay, gl_pixmap, GLX_FRONT_LEFT_EXT);

This approach is definitely better than original approach (in the question) if you can use the CPU/same thread to do something between the calls to glGetTexImage and glMapBuffer.
It's worth thinking it may be still better even if you perform these calls sequentially (instead of glGetTexImage without PBO) because the driver may still optimize the transfer and would manage the system memory buffer itself.

Emanuele
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