Re: [PATCH] ui/gtk: Properly apply x/y scale when rendering GL area

[liuweife <weifeng.liu.z@gmail.com>]

Sorry, just realized that I forgot to cc Marc-André :D.

Here is a description of the issue this patch is trying to solve:

--------------------------
How to reproduce the issue
--------------------------
Launch the guest with gtk backed virtual display using command like

IMAGE_FOLDER=$HOME/workspace/images/fedora
DISK_PATH=$IMAGE_FOLDER/rootfs.qcow2
UEFI_CODE_PATH=$IMAGE_FOLDER/OVMF_CODE.fd
UEFI_DATA_PATH=$IMAGE_FOLDER/OVMF_VARS.fd
ISO_PATH=$IMAGE_FOLDER/Fedora-Workstation-Live-42-1.1.x86_64.iso
MEM=4G
$QEMU \
    -enable-kvm                                                    \
    -M q35                                                         \
    -object memory-backend-memfd,id=mem1,size=$MEM                 \
    -machine memory-backend=mem1                                   \
    -smp 4                                                         \
    -cpu host                                                      \
    -boot order=c,menu=on,splash-time=3                            \
    -cdrom $ISO_PATH                                               \
    -drive if=pflash,format=raw,readonly=on,file=$UEFI_CODE_PATH   \
    -drive if=pflash,format=raw,file=$UEFI_DATA_PATH               \
    -hda $DISK_PATH                                                \
    -display gtk,gl=on,show-cursor=on                              \
    -device virtio-vga-gl,venus=false,blob=true,hostmem=$MEM       \
    -usb -device usb-tablet

When guest GUI is ready, click the guest display and we are likely to
find that the cursor coordinates are incorrectly reported to the guest
with an offset.

------------------
Cause of the issue
------------------
There is a mapping from the gtk glarea widget coordinates to the guest
display coordinates, and the scale factor plays an essential role in
the calculation. However, the scale factor defaults to 1 and is not
updated to reflect the fact on startup, rendering incorrect cursor
coordinates.

Best regards,
Weifeng

On Sat, 2025-04-19 at 17:48 +0800, Weifeng Liu wrote:
> On startup, scale_x and scale_y were set to 1 that didn't reflect the
> real situation of the scan-out, resulting in incorrect cursor
> coordinates to be sent when moving the mouse pointer. Simply updating
> the scales before rendering the image fixes this issue.
> 
> Cc: hikalium <hikalium@hikalium.com>
> Cc: Alexander Orzechowski <orzechowski.alexander@gmail.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Signed-off-by: Weifeng Liu <weifeng.liu.z@gmail.com>
> ---
>  ui/gtk-gl-area.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/ui/gtk-gl-area.c b/ui/gtk-gl-area.c
> index 2c9a0db425..01235f876a 100644
> --- a/ui/gtk-gl-area.c
> +++ b/ui/gtk-gl-area.c
> @@ -42,6 +42,7 @@ void gd_gl_area_draw(VirtualConsole *vc)
>  #ifdef CONFIG_GBM
>      QemuDmaBuf *dmabuf = vc->gfx.guest_fb.dmabuf;
>  #endif
> +    int fbw, fbh;
>      int ww, wh, ws, y1, y2;
>  
>      if (!vc->gfx.gls) {
> @@ -53,6 +54,11 @@ void gd_gl_area_draw(VirtualConsole *vc)
>      ww = gtk_widget_get_allocated_width(vc->gfx.drawing_area) * ws;
>      wh = gtk_widget_get_allocated_height(vc->gfx.drawing_area) * ws;
>  
> +    fbw = surface_width(vc->gfx.ds);
> +    fbh = surface_height(vc->gfx.ds);
> +    vc->gfx.scale_x = (double)ww / fbw / ws;
> +    vc->gfx.scale_y = (double)wh / fbh / ws;
> +
>      if (vc->gfx.scanout_mode) {
>          if (!vc->gfx.guest_fb.framebuffer) {
>              return;