flip_y_requires_shift

[EriKWDev]

I have just spent a some time debugging why my set_viewport calls seem to cause inconsistent behaviour between metal on macos and my vulkan linux. After some investigation into wgpu-hal and blade, I found a difference in how viewports are created.

In blade, the viewport in vulkan always has the y set to the height upon starting a render

/// blade:s begin_render
let viewport = vk::Viewport {
    x: 0.0,
    y: target_size[1] as f32, // AHA! Missed this.. We shift it here already
    width: target_size[0] as f32,
    height: -(target_size[1] as f32),
    min_depth: 0.0,
    max_depth: 1.0,
};

but wgpu-hal sometimes has it set to 0

/// wgpu-hal:s, begin_render
let vk_viewports = [vk::Viewport {
    x: 0.0,
    y: if self.device.private_caps.flip_y_requires_shift {
        desc.extent.height as f32
    } else {
        0.0
    },
    width: desc.extent.width as f32,
    height: -(desc.extent.height as f32),
    min_depth: 0.0,
    max_depth: 1.0,
}];

let vk_viewports = [vk::Viewport {
    x: rect.x,
    y: if self.device.private_caps.flip_y_requires_shift {
        rect.y + rect.h // shift is applied..
    } else {
        rect.y
    },
    width: rect.w,
    height: -rect.h, // flip Y
    min_depth: depth_range.start,
    max_depth: depth_range.end,
}];

In wgpu-hal vulkan/mod.rs they state:

struct PrivateCapabilities {
    /// Y-flipping is implemented with either `VK_AMD_negative_viewport_height` or `VK_KHR_maintenance1`/1.1+. The AMD extension for negative viewport height does not require a Y shift.
    ///
    /// This flag is `true` if the device has `VK_KHR_maintenance1`/1.1+ and `false` otherwise (i.e. in the case of `VK_AMD_negative_viewport_height`).
    flip_y_requires_shift: bool,
    // ..
}

[EriKWDev]

perhaps a throwback, but here was a PR that introduced the flip_y_requires_shift flag in gfx instead of the maintenance_level: u8

gfx-rs/gfx#3624

[EriKWDev]

Do I understand it correctly that since blade requires vulkan be >= API_VERSION_1_1, and always has a flipped y, blade should always set the viewport:s y to rect.y + rect.h?