Hi all, I was working on Weston when I noticed that the weston-simple-dmabuf-intel demo shows the window content in different orientations depending on whether it is composited with GL or directly scanned out. Obviously something is a miss. Then I went out to find what the orientation should be according to specs, rather than just flipping the y-invert flag in Weston without thinking.
The buffer is drawn with manual pixel-poking via mmap, and as a raw memory buffer, the image origin (orientation) is at top-left. When this is passed as a dmabuf to Weston and direct to scanout without any attempts in y-flipping (imported through GBM -> drmAddFB2), the image on the screen is on the same orientation as I would expect by reading the pixel-poking code. So that looks good to me. However, when the dmabuf is imported as an EGLImage and then composited from a GL texture, the orientation flips. The image on screen is y-inverted. Because Weston's GL-renderer works assuming the GL texture coordinate orientation, it y-inverts the texture coordinates when the image does *not* want y-flip. The dmabuf imported image is not flagged with y-invert, hence the problem. I could easily fix that by just toggling the flag, but I am not sure that's actually right. The above means, that the pixel 0,0 at top-left corner of the buffer ends up at GL texture coordinates 0,0. GL texture coordinates are specified to have origin at the bottom-left. This is the conflict. Importing the dmabuf semantically y-flips it. Is the Mesa implementation correct or not? I didn't find anything to answer that question. I skimmed through https://www.khronos.org/registry/egl/extensions/EXT/EGL_EXT_image_dma_buf_import.txt https://www.khronos.org/registry/gles/extensions/OES/OES_EGL_image_external.txt Did I miss something? Does it even matter if Mesa was incorrect, since it is now the de facto spec, and presumably everyone has adjusted their implementations and programs to match that already? For comparison, doing an import of a (wl_drm) wl_buffer as an EGLImage via EGL_WL_bind_wayland_display extension makes the image in GL texture coordinate orientation. That is, the EGL query for the y-invert flag returns the flag such, that the texture coordinate origin is at the bottom-left of the image. This matches the GL spec and the assumption in Weston's GL-renderer, so the image appears the right way up. I'm tracking the issue so far at: https://phabricator.freedesktop.org/T7475 Thanks, pq
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