// Copyright (C) 2020 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \title QSB Manual
    \page qtshadertools-qsb.html

    \c qsb is a command-line tool provided by the \l{Qt Shader Tools}
    module. It integrates third-party libraries such as
    \l{https://github.com/KhronosGroup/glslang}{glslang} and
    \l{https://github.com/KhronosGroup/SPIRV-Cross}{SPIRV-Cross},
    optionally invokes external tools, such as, \c fxc or \c
    spirv-opt, and generates \c{.qsb} files. Additionally, it can be
    used to inspect the contents of a \c{.qsb} package.

    \badcode
Usage: qsb [options] file
Qt Shader Baker (using QShader from Qt 6.10.0)

Options:
  -?, -h, --help               Displays help on commandline options.
  --help-all                   Displays help, including generic Qt options.
  -v, --version                Displays version information.
  -b, --batchable              Also generates rewritten vertex shader for Qt
                               Quick scene graph batching.
  --zorder-loc <location>      The extra vertex input location when rewriting
                               for batching. Defaults to 7.
  --glsl <versions>            Comma separated list of GLSL versions to
                               generate. (for example, "100 es,120,330")
  --hlsl <versions>            Comma separated list of HLSL (Shader Model)
                               versions to generate. F.ex. 50 is 5.0, 51 is 5.1.
  --msl <versions>             Comma separated list of Metal Shading Language
                               versions to generate. F.ex. 12 is 1.2, 20 is 2.0.
  --qt6                        Equivalent to --glsl "100 es,120,150" --hlsl 50
                               --msl 12. This set is commonly used with shaders
                               for Qt Quick materials and effects.
  --msltess                    Indicates that a vertex shader is going to be
                               used in a pipeline with tessellation. Mandatory
                               for vertex shaders planned to be used with
                               tessellation when targeting Metal (--msl).
  --tess-vertex-count <count>  The output vertex count from the tessellation
                               control stage. Mandatory for tessellation
                               evaluation shaders planned to be used with Metal.
                               The default value is 3. If it does not match the
                               tess.control stage, the generated MSL code will
                               not function as expected.
  --tess-mode <mode>           The tessellation mode: triangles or quads.
                               Mandatory for tessellation control shaders
                               planned to be used with Metal. The default value
                               is triangles. Isolines are not supported with
                               Metal. If it does not match the tess.evaluation
                               stage, the generated MSL code will not function
                               as expected.
  --view-count <num_views>     The number of views the shader is used with.
                               num_views must be >= 2. Mandatory when multiview
                               rendering is used (gl_ViewIndex). Set only for
                               vertex shaders that really do rely on multiview
                               (as the resulting asset is tied to num_views).
                               Can be set for fragment shaders too, to get
                               QSHADER_VIEW_COUNT auto-defined. (useful for
                               ensuring uniform buffer layouts)
  -g                           Generate full debug info for SPIR-V and DXBC
  -O                           Invoke spirv-opt (external tool) to optimize
                               SPIR-V for performance.
  -o, --output <filename>      Output file for the shader pack.
  --orig-file <filename>       Filename to be be used for dependency tracking
                               instead of <file>.
  --qsbversion <version>       QSB version to use for the output file. By
                               default the latest version is automatically used,
                               use only to bake compatibility versions. F.ex. 64
                               is Qt 6.4.
  -c, --fxc                    In combination with --hlsl invokes fxc (SM
                               5.0/5.1) or dxc (SM 6.0+) to store DXBC or DXIL
                               instead of HLSL.
  -t, --metallib               In combination with --msl builds a Metal library
                               with xcrun metal(lib) and stores that instead of
                               the source. Suitable only when targeting macOS,
                               not iOS.
  -T, --metallib-ios           In combination with --msl builds a Metal library
                               with xcrun metal(lib) and stores that instead of
                               the source. Suitable only when targeting iOS, not
                               macOS.
  -D, --define <name[=value]>  Define macro. This argument can be specified
                               multiple times.
  -p, --per-target             Enable per-target compilation. (instead of
                               source->SPIRV->targets, do source->SPIRV->target
                               separately for each target)
  -d, --dump                   Switches to dump mode. Input file is expected to
                               be a shader pack.
  -x, --extract <what>         Switches to extract mode. Input file is expected
                               to be a shader pack. Result is written to the
                               output specified by -o. Pass -b to choose the
                               batchable variant.
                               <what>=reflect|spirv,<version>|glsl,<version>|...
  -r, --replace <what>         Switches to replace mode. Replaces the specified
                               shader in the shader pack with the contents of a
                               file. This argument can be specified multiple
                               times. Pass -b to choose the batchable variant.
                               Also supports adding a shader for a
                               target/variant that was not present before.
                               <what>=<target>,<filename> where
                               <target>=spirv,<version>|glsl,<version>|...
  -e, --erase <what>           Switches to erase mode. Removes the specified
                               shader from the shader pack. Pass -b to choose
                               the batchable variant.
                               <what>=spirv,<version>|glsl,<version>|...
  -s, --silent                 Enables silent mode. Only fatal errors will be
                               printed.
  --mediump                    Default to medium precision for floats in
                               fragment shaders instead of high. GLSL ES only;
                               ignored for everything else, including GLSL.
  --depfile <depfile>          Enables generating the depfile for the input
                               shaders, using the #include statements.

Arguments:
  file                         Vulkan GLSL source file to compile. The file
                               extension determines the shader stage, and can be
                               one of .vert, .tesc, .tese, .geom, .frag, .comp.
                               Note: Tessellation control/evaluation is not
                               supported with HLSL, instead use -r to inject
                               handcrafted hull/domain shaders. Some targets may
                               need special arguments to be set, e.g. MSL
                               tessellation will likely need --msltess,
                               --tess-vertex-count, --tess-mode, depending on
                               the stage. Geometry shaders are not supported
                               with Metal.
    \endcode

    \section1 Modes of Operation

    There are five major modes of operation:

    \list

    \li \c{.qsb} file generation.

    \li \c{.qsb} file inspection. For example, \c{qsb -d
    myshader.frag.qsb} will print the reflection metadata (in JSON
    form) and the included shaders.

    \li Extraction mode. This allows writing a given shader from an
    existing \c{.qsb} file into a separate file. For example, \c{qsb
    -x spirv,100 -o myshader.spv myshader.frag.qsb} writes the SPIR-V
    binary into \c myshader.spv.

    \li Replace mode. This allows replacing the contents of one or more shaders
    within the \c{.qsb} file with content read from the specified files. This
    way handcrafted shader code can be injected into the \c{.qsb} package.

    \li Erase mode. This removes the specified shader variant from the \c{.qsb}
    file.

    \endlist

    \section1 Example

    Take the following fragment shader:
    \badcode
    #version 440

    layout(location = 0) in vec2 v_texcoord;
    layout(location = 0) out vec4 fragColor;
    layout(binding = 1) uniform sampler2D tex;

    layout(std140, binding = 0) uniform buf {
        float uAlpha;
    };

    void main()
    {
        vec4 c = texture(tex, v_texcoord);
        fragColor = vec4(c.rgb, uAlpha);
    }
    \endcode

    Executing \c{qsb -o shader.frag.qsb shader.frag} results in generating \c
    shader.frag.qsb. Inspecting this file with \c{qsb -d shader.frag.qsb} gives
    us:

    \badcode
Stage: Fragment
QSB_VERSION: 5
Has 1 shaders:
  Shader 0: SPIR-V 100 [Standard]

Reflection info: {
    "combinedImageSamplers": [
        {
            "binding": 1,
            "name": "tex",
            "set": 0,
            "type": "sampler2D"
        }
    ],
    "inputs": [
        {
            "location": 0,
            "name": "v_texcoord",
            "type": "vec2"
        }
    ],
    "localSize": [
        0,
        0,
        0
    ],
    "outputs": [
        {
            "location": 0,
            "name": "fragColor",
            "type": "vec4"
        }
    ],
    "uniformBlocks": [
        {
            "binding": 0,
            "blockName": "buf",
            "members": [
                {
                    "name": "uAlpha",
                    "offset": 0,
                    "size": 4,
                    "type": "float"
                }
            ],
            "set": 0,
            "size": 4,
            "structName": "_27"
        }
    ]
}


Shader 0: SPIR-V 100 [Standard]
Entry point: main
Contents:
Binary of 864 bytes
    \endcode

    By default only SPIR-V is generated, so an application using this shader
    package would only be functional with Vulkan. Let's make it more useful:

    \badcode
    qsb --glsl "100 es,120,150" --hlsl 50 --msl 12 -o shader.frag.qsb shader.frag
    \endcode

    This leads to generating a shader package that makes it suitable for OpenGL,
    Direct 3D, and Metal as well. The features used in this shader are basic,
    and even GLSL ES 100 (the shading language of OpenGL ES 2.0) is suitable.

    Inspecting the result shows:

    \badcode
Stage: Fragment
QSB_VERSION: 5
Has 6 shaders:
  Shader 0: GLSL 120 [Standard]
  Shader 1: HLSL 50 [Standard]
  Shader 2: GLSL 100 es [Standard]
  Shader 3: MSL 12 [Standard]
  Shader 4: SPIR-V 100 [Standard]
  Shader 5: GLSL 150 [Standard]

Reflection info: {
    ... <same as above>
}


Shader 0: GLSL 120 [Standard]
Entry point: main
Contents:
#version 120

struct buf
{
    float uAlpha;
};

uniform buf _27;

uniform sampler2D tex;

varying vec2 v_texcoord;

void main()
{
    vec4 c = texture2D(tex, v_texcoord);
    gl_FragData[0] = vec4(c.xyz, _27.uAlpha);
}

************************************

Shader 1: HLSL 50 [Standard]
Entry point: main
Native resource binding map:
0 -> [0, -1]
1 -> [0, 0]
Contents:
cbuffer buf : register(b0)
{
    float _27_uAlpha : packoffset(c0);
};

Texture2D<float4> tex : register(t0);
SamplerState _tex_sampler : register(s0);

static float2 v_texcoord;
static float4 fragColor;

struct SPIRV_Cross_Input
{
    float2 v_texcoord : TEXCOORD0;
};

struct SPIRV_Cross_Output
{
    float4 fragColor : SV_Target0;
};

void frag_main()
{
    float4 c = tex.Sample(_tex_sampler, v_texcoord);
    fragColor = float4(c.xyz, _27_uAlpha);
}

SPIRV_Cross_Output main(SPIRV_Cross_Input stage_input)
{
    v_texcoord = stage_input.v_texcoord;
    frag_main();
    SPIRV_Cross_Output stage_output;
    stage_output.fragColor = fragColor;
    return stage_output;
}

************************************

...

Shader 3: MSL 12 [Standard]
Entry point: main0
Native resource binding map:
0 -> [0, -1]
1 -> [0, 0]
Contents:
#include <metal_stdlib>
#include <simd/simd.h>

using namespace metal;

struct buf
{
    float uAlpha;
};

struct main0_out
{
    float4 fragColor [[color(0)]];
};

struct main0_in
{
    float2 v_texcoord [[user(locn0)]];
};

fragment main0_out main0(main0_in in [[stage_in]], constant buf& _27 [[buffer(0)]], texture2d<float> tex [[texture(0)]], sampler texSmplr [[sampler(0)]])
{
    main0_out out = {};
    float4 c = tex.sample(texSmplr, in.v_texcoord);
    out.fragColor = float4(c.xyz, _27.uAlpha);
    return out;
}

************************************

...
    \endcode

    This package can now be used by Qt Quick with all supported graphics APIs:
    Vulkan, Direct 3D, Metal, OpenGL, and OpenGL ES. At run time the appropriate
    shader is picked up automatically by the Qt Rendering Hardware Interface
    that sits underneath Qt Quick and Qt Quick 3D.

    Besides translating the SPIR-V bytecode back to higher level source code,
    the system takes care of additional problems, such as ensuring correct
    mapping of SPIR-V binding numbers onto native resources. For example, with
    HLSL we saw a section like this above:

\badcode
Native resource binding map:
 0 -> [0, -1]
 1 -> [0, 0]
\endcode

    Internally, this allows mapping a SPIR-V style binding point \c 0 to the
    HLSL register \c b0 and binding \c 1 to \c t0 and \c s0. This helps making
    the differences in resource bindings between the various shading languages
    transparent to the users of the Rendering Hardware Interface, and allows
    everything in Qt to operate with Vulkan/SPIR-V style binding points as they
    are specified in the original Vulkan-style GLSL source code.

    \section1 Shader Types

    The type of shader is deduced from the input file extension. Thus, the
    extension must be one of the following:

    \list
    \li \c .vert - for vertex shaders
    \li \c .tesc - for tessellation control shaders
    \li \c .tese - for tessellation evaluation shaders
    \li \c .geom - for geometry shaders
    \li \c .frag - for fragment (pixel) shaders
    \li \c .comp - for compute shaders
    \endlist

    \note Tessellation control and evaluation shaders are not currently
    supported with Direct 3D (HLSL).

    \section1 Shading Languages and Versions

    SPIR-V 1.0 is always generated. What gets generated in addition depends on
    the command-line arguments \c{--glsl}, \c{--hlsl}, and \c{--msl}.

    These parameters are all followed by a comma-separated list. The list must
    include GLSL-style version numbers, with an optional suffix (\c{es},
    indicating GLSL ES). The space between the suffix and the version is
    optional (not having the space can help avoiding the need for quoting).

    For example, Qt Quick's built-in materials (the shaders backing items, such
    as \l Image, \l Text, \l Rectangle) all prepare their shaders with \c{--glsl
    "100 es,120,150" --hlsl 50 --msl 12}. This makes them compatible with OpenGL
    ES 2.0 and higher, OpenGL 2.1 and higher, and OpenGL core profile contexts
    of version 3.2 and higher.

    If the shader uses functions or constructs that do not have an equivalent in
    the specified targets, \c qsb will fail. If that is the case, the targets
    will need to be adjusted, and this also means that the application's minimum
    system requirements get adjusted implicitly. As an example, take the \c
    textureLod GLSL function that is only available with OpenGL ES 3.0 and up
    (meaning GLSL ES 300 or higher). When requesting GLSL \c{300 es} instead of
    \c{100 es}, \c qsb will succeed, but the application using that \c{.qsb}
    file will now require OpenGL ES 3.0 or higher and will not be compatible
    with OpenGL ES 2.0 based systems.

    Another obvious example of this is compute shaders: \c{.comp} shaders will
    need to specify \c{--glsl 310es,430} as compute shaders are only available
    with OpenGL ES 3.1 or newer and OpenGL 4.3 or newer.

    Adjusting the shader model version for HLSL, or the Metal Shading Language
    version is expected to be needed rarely. Shader Model 5.0 (\c{--hlsl 50}) and
    MSL 1.2 (\c{--msl 12}) will typically be sufficient.

    \section1 Qt Quick Scene Graph Batching

    The Qt Quick Scene Graph's renderer supports batching of geometry to reduce
    the number of draw calls. See \l{Qt Quick Scene Graph}{the Scene Graph
    pages} for details. This relies on injecting code to the vertex shader's
    main() function. In Qt 5.x this happened at run time, by modifying the
    supplied GLSL vertex shader code. In Qt 6 that is not an option. Instead,
    batchable variants of vertex shaders can be built by the \c qsb tool. This
    is requested by the \c{-b} argument. When the input is not a vertex shader
    with \c{.vert} extension, it has no effect. For vertex shaders, however, it
    will lead to generating two versions for each target. Qt Quick will then
    automatically pick the right variant (standard or batchable) at run time.

    \note Applications do not have to concern themselves with the details of
    batching. They must simply ensure that \c{-b} (or the equivalent \c
    BATCHABLE keyword if using the \l{Qt Shader Tools Build System
    Integration}{CMake integration}) is specified when processing the vertex
    shaders. This is relevant only for Qt Quick shaders used with \l
    ShaderEffect or \l QSGMaterialShader.

    Take the following example vertex shader:
    \badcode
#version 440
layout(location = 0) in vec4 position;
layout(location = 1) in vec2 texcoord;
layout(location = 0) out vec2 v_texcoord;
layout(std140, binding = 0) uniform buf {
    mat4 mvp;
} ubuf;

void main()
{
    v_texcoord = texcoord;
    gl_Position = ubuf.mvp * position;
}
    \endcode

    Running \c {qsb -b --glsl 330 -o example.vert.qsb example.vert} leads to:

\badcode
Stage: Vertex
QSB_VERSION: 5
Has 4 shaders:
  Shader 0: GLSL 330 [Standard]
  Shader 1: GLSL 330 [Batchable]
  Shader 2: SPIR-V 100 [Standard]
  Shader 3: SPIR-V 100 [Batchable]

Reflection info: {
  ...
\endcode

    Note how all target languages and versions now exist in two variants:
    Standard and a slightly modified Batchable.

    \section1 Invoking External Tools

    \c qsb can invoke certain external tools. These fall into two categories:
    tools for performing optimizations on the shader bytecode (SPIR-V), and
    platform-specific tools to perform the first phase of shader compilation,
    from source to some intermediate bytecode format.

    These are enabled by the following command-line options:

    \list

    \li \c{-O} - invokes \c spirv-opt as a post-processing step on the SPIR-V
    binary.  The \c{.qsb} file will include the optimized version. This assumes
    that \c spirv-opt is available on the system (for example, from the Vulkan
    SDK) and ready to be invoked.

    \li \c{-c} or \c{--fxc} - invokes \c{fxc.exe}, the Direct 3D shader
    compiler.  The resulting \c DXBC (DirectX Byte Code) data is what gets
    stored in the \c{.qsb} file instead of HLSL.  Qt will automatically pick it
    up at run time, so it is up to the \c{.qsb} file's creator to decide what to
    include, HLSL source or the intermediate format. Whenever possible, prefer
    the latter since it eliminates the need for parsing an HLSL source at run
    time, leading to potentially significant performance gains upon graphics
    pipeline creation. The downside is that this argument can only be used when
    \c qsb runs on Windows.

    \li \c{-t} or \c{--metallib} - invokves the appropriate XCode Metal tools to
    generate a .metallib file and includes that in the \c{.qsb} package instead
    of the MSL source code. This option is only available when \c qsb is running
    on macOS.

    \endlist

    \section1 Other Options

    \list

    \li \c{-D} - defines a macro. This allows using #ifdef and similar in the
    GLSL source code.

    \li \c{-g} - enables generating full debug information for SPIR-V, thus
    enabling tools like \l{https://renderdoc.org/}{RenderDoc} to display the
    full source when inspecting a pipeline or when performing vertex or fragment
    debugging. Also has an effect for Direct 3D when \c{-c} is specified,
    because \c fxc is then instructed to include debug information in the
    generated intermediate bytecode.

    \li \c{--mediump} - requests defaulting to medium precision for floats in
    GLSL ES fragment shaders. Ignored for any other targets, including (non-ES)
    GLSL, and for non-fragment shaders.

    \li \c{--orig-file} - specifies the filename that is used instead of the
    input source file when generating dependency files for CMake (--depfiles).
    This is useful when the file passed to qsb is an intermediate, generated
    shader source file. For dependency tracking purposes however, it is often
    the original file that needs to be used.

    \endlist

    \section1 Tessellation

    \list

    \li \c{--msltess} - Indicates that the vertex shader is used in a pipeline
    that includes tessellation stages. Has no effect for other types of
    shaders, and when MSL shader generation is not enabled. If not specified,
    the vertex shader will not be functional on Metal in combination with
    tessellation.

    \li \c{--tess-vertex-count <count>} - Specifies the output vertex count
    from the tessellation control stage. Specifying this is mandatory for
    tessellation evaluation shaders used with Metal. The default value is 3. If
    it does not match the tessellation control stage, the generated MSL code
    will not function as expected.

    \li \c{--tess-mode <mode>} - This option specifies the tessellation mode.
    It can take one of two values: \c triangles or \c quads. The default value
    is \c triangles. Specifying this is mandatory for tessellation control
    shaders used with Metal. The value must match the tessellation evaluation
    stage, otherwise the generated MSL code will not function as expected.

    \endlist

    \section1 Multiview

    Take the following vertex shader. It is written as Vulkan-compatible GLSL,
    enabling the \c{GL_EXT_multiview} extension to make using \c{gl_ViewIndex}
    legal.

\badcode
#version 440
#extension GL_EXT_multiview : require

layout(location = 0) in vec4 pos;
layout(std140, binding = 0) uniform buf
{
    mat4 mvp[2];
};

void main()
{
    gl_Position = mvp[gl_ViewIndex] * pos;
}
\endcode

    \note In practice the #extension GL_EXT_multiview line will not be needed
    in the source code passed to \c qsb, because passing the
    \c{--view-count} argument described below automatically injects that line
    into the shader source code before compiling to SPIR-V.

    For Vulkan this works as-is, as long as Vulkan 1.1 is supported at runtime.
    See
    \l{https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_KHR_multiview.html}{VK_KHR_multiview}
    for details.

    To generate a HLSL vertex shader from the above for Direct 3D 12 (see
    \l{https://microsoft.github.io/DirectX-Specs/d3d/ViewInstancing.html}{view
    instancing} for details), the minimum shader model version is 6.1, meaning
    \c qsb will fail when specifying e.g. \c{--hlsl 50}. Use at least \c{--hlsl
    61} when processing a multiview vertex shader. Multiview is not supported
    with Direct 3D 11.

    For OpenGL, additional metadata is needed:

    \list

    \li \c{--view-count} - When transpiling above shader (after compiling to
    SPIR-V) to OpenGL-compatible GLSL source code, it is not enough to map
    \c{gl_ViewIndex} to \c{gl_ViewID_OVR}, but the number of views needs to be
    declared in the shader as well. Passing a value of 2 to the \c{--view-count}
    argument results in injecting a \c{layout(num_views = 2) in;} statement into
    the generated GLSL source code, thus making it a valid (OpenGL) GLSL shader.
    See
    \l{https://registry.khronos.org/OpenGL/extensions/OVR/OVR_multiview.txt}{GL_OVR_multiview}
    for details and note that the generated GLSL shaders also require
    \l{https://registry.khronos.org/OpenGL/extensions/OVR/OVR_multiview2.txt}{GL_OVR_multiview2}
    to be supported at runtime since that is what is going to be required by the
    \c{#extension} directive in the generated shader source code.

    \endlist

    When targeting OpenGL (ES) with such a vertex shader, the generated GLSL
    version (\c{--glsl}) must be at least \c 330 or \c{300 es}. The former is
    not something that is enforced by \c qsb or QShaderBaker, but in practice
    OpenGL implementations are known to reject such shaders if the GLSL version
    is 150 or smaller. Hence it is recommended to pass \c{--glsl 330,300es}
    when conditioning vertex shaders enabling GL_EXT_multiview.

    Specifying \c{--view-count} automatically generates and injects a
    preprocessor define:
    \c{#define QSHADER_VIEW_COUNT n} where \c n is the number of views.
    When the view count is not provided, the define is not set at all.
    This allows writing code such as the following, and allows using the same
    source file for all view count-specific variants of the shader.

\badcode
layout(std140, binding = 0) uniform buf {
#if QSHADER_VIEW_COUNT >= 2
    mat4 matrix[QSHADER_VIEW_COUNT];
#else
    mat4 matrix;
#endif
    float opacity;
};
\endcode

    In addition, the \c{#extension GL_EXT_multiview : require} line is generated
    automatically in vertex shaders whenever a view count of 2 or greater is
    set. This reduces the number of extra lines needed to be added for multiview
    support in a vertex shader.

    Setting the view count can be relevant with other types of shaders as well.
    For example, when sharing a uniform buffer between the vertex and fragment
    shader, and both shaders have to ensure the same buffer layout, it can be
    useful to be able to write \c{#if QSHADER_VIEW_COUNT >= 2} in both source
    files. That can be ensured by specifying \c{--view-count} for both when
    invoking \c qsb.

    \note Directly relying on the \c{gl_ViewIndex} keyword in a non-vertex
    stage, for example in a fragment shader, is not portable at the moment and
    should be avoided.

    \section1 Working with GLSL Features Specific to OpenGL

    It can sometimes be necessary to use shading language constructs that are
    specific to OpenGL and GLSL, and are not applicable to other shading
    languages, intermediate formats, and graphics APIs.

    The prime examples of this are the external textures and samplers of OpenGL
    ES. Implementing video playback or showing a camera viewfinder may involve,
    depending on the platform, working with OpenGL texture objects that are not
    meant to be used as regular 2D textures, but are rather usable, with a
    limited feature set, via the
    \l{https://www.khronos.org/registry/OpenGL/extensions/OES/OES_EGL_image_external.txt}{GL_TEXTURE_EXTERNAL_OES}
    binding point in the OpenGL API and the \c{samplerExternalOES} sampler type
    in shaders. The latter presents a potential showstopper when using the
    SPIR-V based shader pipeline of Qt: running such a shader through qsb will
    result in a failure due to \c{samplerExternalOES} not being accepted as a
    valid type due to not being mappable to SPIR-V and other target shading
    languages.

    To overcome this, qsb offers the option to replace the contents of any given
    shader variant in a .qsb file with user-provided data that is read from a
    file, fully replacing the original qsb-generated shader source or byte code.

    Take the following fragment shader. Note the type of \c tex. What if the
    type needs to be \c samplerExternalOES when running with OpenGL ES?

\badcode
#version 440

layout(location = 0) in vec2 texCoord;
layout(location = 0) out vec4 fragColor;

layout(std140, binding = 0) uniform buf {
    float opacity;
} ubuf;

layout(binding = 1) uniform sampler2D tex;

void main()
{
    fragColor = texture(tex, texCoord).rgba * ubuf.opacity;
}
\endcode

   Just changing the type of samplerExternalOES is not feasible. That leads to
   compilation errors right away.

   There is a simple solution, however: writing a separate, pure OpenGL ES
   targeted version of the shader, and injecting it into the .qsb file. The
   following shader is only compatible with GLSL ES and cannot be run through
   qsb. However, we know that it can be processed by OpenGL ES at run time.

\badcode
precision highp float;
#extension GL_OES_EGL_image_external : require
varying vec2 texCoord;

struct buf
{
    float opacity;
};

uniform buf ubuf;
uniform samplerExternalOES tex;

void main()
{
    gl_FragColor = texture2D(tex, texCoord).rgba * ubuf.opacity;
}
\endcode

    Let's call this \c{shader_gles.frag}. Once \c{qsb --glsl 100es -o
    shader.frag.qsb shader.frag} completes, giving us a (half-ready) .qsb file,
    we can do \c{qsb -r glsl,100es,shader_gles.frag shader.frag.qsb} to update
    \c{shader.frag.qsb} by substituting the shader for GLSL 100 es with
    the contents of the specified file (\c{shader_gles.frag}). Now
    \c{shader.frag.qsb} is ready to be used at run time with OpenGL ES.

    \note Pay attention to keeping the interface between the shader and the
    application unchanged. Always inspect the qsb-generated GLSL code first,
    either by printing the .qsb file contents via \c{-d} option, or by
    extracting the GLSL ES 100 shader by running \c{qsb -x glsl,100es -o
    gles_shader.frag shader.frag.qsb}. The struct, struct member, and uniform
    names must not differ in the manually injected version either.

    \note The ability to place data from arbitrary files into the .qsb package
    can also be used to inject handcrafted hull and domain HLSL shaders, in
    order to make tessellation-based graphics pipelines functional with Direct
    3D as well.

*/
