Qt Quick 3D - 体积渲染示例

演示如何在 Qt Quick 3D 中进行体积渲染。

简介

本示例演示如何使用自定义着色器和三维体积纹理以及名为“体积光投射”（Volume ray casting）的技术进行体积渲染。这个示例是一个应用程序，它可以读取原始体积文件并在渲染的同时能够交互式地修改各种渲染设置，如色图、alpha 值和切片平面等。它旨在与位于https://klacansky.com/open-scivis-datasets/ 的体积兼容，并能自动设置正确的尺寸和缩放。

实现

该应用程序使用 QML，是一个包含ApplicationWindow 的 View3D，其中包含体积和一个包含设置的 ScrollView。为了渲染我们的体积，我们在 View3D 对象中创建了一个仅包含中间立方体模型的场景。

Model {
    id: cubeModel
    source: "#Cube"
    visible: true
    materials: CustomMaterial {
        shadingMode: CustomMaterial.Unshaded
        vertexShader: "alpha_blending.vert"
        fragmentShader: "alpha_blending.frag"

        property TextureInput volume: TextureInput {
            texture: Texture {
                textureData: VolumeTextureData {
                    id: volumeTextureData
                    source: "file:///default_colormap"
                    dataType: dataTypeComboBox.currentText ? dataTypeComboBox.currentText : "uint8"
                    width: parseInt(dataWidth.text)
                    height: parseInt(dataHeight.text)
                    depth: parseInt(dataDepth.text)
                }
                minFilter: Texture.Nearest
                mipFilter: Texture.None
                magFilter: Texture.Nearest
                tilingModeHorizontal: Texture.ClampToEdge
                tilingModeVertical: Texture.ClampToEdge
                //tilingModeDepth: Texture.ClampToEdge // Qt 6.7
            }
        }

        property TextureInput colormap: TextureInput {
            enabled: true
            texture: Texture {
                id: colormapTexture
                tilingModeHorizontal: Texture.ClampToEdge
                source: getColormapSource(colormapCombo.currentIndex)
            }
        }
        property real stepLength: Math.max(0.0001, parseFloat(
                                               stepLengthText.text,
                                               1 / cubeModel.maxSide))
        property real minSide: 1 / cubeModel.minSide
        property real stepAlpha: stepAlphaSlider.value
        property bool multipliedAlpha: multipliedAlphaBox.checked

        property real tMin: tSlider.first.value
        property real tMax: tSlider.second.value
        property vector3d sliceMin: sliceSliderMin(
                                        xSliceSlider.value,
                                        xSliceWidthSlider.value,
                                        ySliceSlider.value,
                                        ySliceWidthSlider.value,
                                        zSliceSlider.value,
                                        zSliceWidthSlider.value)
        property vector3d sliceMax: sliceSliderMax(
                                        xSliceSlider.value,
                                        xSliceWidthSlider.value,
                                        ySliceSlider.value,
                                        ySliceWidthSlider.value,
                                        zSliceSlider.value,
                                        zSliceWidthSlider.value)

        sourceBlend: CustomMaterial.SrcAlpha
        destinationBlend: CustomMaterial.OneMinusSrcAlpha
    }
    property real maxSide: Math.max(parseInt(dataWidth.text),
                                    parseInt(dataHeight.text),
                                    parseInt(dataDepth.text))
    property real minSide: Math.min(parseInt(dataWidth.text),
                                    parseInt(dataHeight.text),
                                    parseInt(dataDepth.text))
    scale: Qt.vector3d(parseFloat(scaleWidth.text),
                       parseFloat(scaleHeight.text),
                       parseFloat(scaleDepth.text))

    Model {
        visible: drawBoundingBox.checked
        geometry: LineBoxGeometry {}
        materials: DefaultMaterial {
            diffuseColor: "#323232"
            lighting: DefaultMaterial.NoLighting
        }
        receivesShadows: false
        castsShadows: false
    }

    Model {
        visible: drawBoundingBox.checked
        geometry: LineBoxGeometry {}
        materials: DefaultMaterial {
            diffuseColor: "#323232"
            lighting: DefaultMaterial.NoLighting
        }
        receivesShadows: false
        castsShadows: false
        position: sliceBoxPosition(xSliceSlider.value,
                                   ySliceSlider.value,
                                   zSliceSlider.value,
                                   xSliceWidthSlider.value,
                                   ySliceWidthSlider.value,
                                   zSliceWidthSlider.value)
        scale: Qt.vector3d(xSliceWidthSlider.value,
                           ySliceWidthSlider.value,
                           zSliceWidthSlider.value)
    }
}

这个立方体使用一个自定义着色器和一个用于体积的三维纹理以及一个用于色图的图像纹理。还有许多用于转换函数、切片平面等的属性。体积纹理的纹理数据是一个名为 VolumeTextureData 的自定义 QML 类型，定义在 volumetexturedata.cpp 和 volumetexturedata.h 中。

property TextureInput volume: TextureInput {
    texture: Texture {
        textureData: VolumeTextureData {
            id: volumeTextureData
            source: "file:///default_colormap"
            dataType: dataTypeComboBox.currentText ? dataTypeComboBox.currentText : "uint8"
            width: parseInt(dataWidth.text)
            height: parseInt(dataHeight.text)
            depth: parseInt(dataDepth.text)
        }
        minFilter: Texture.Nearest
        mipFilter: Texture.None
        magFilter: Texture.Nearest
        tilingModeHorizontal: Texture.ClampToEdge
        tilingModeVertical: Texture.ClampToEdge
        //tilingModeDepth: Texture.ClampToEdge // Qt 6.7
    }
}

它包含 source、dataType、width、height 和 depth 选项，用于定义如何解释原始体积文件。 VolumeTextureData 还包含一个用于异步加载体积的 loadAsync 函数。它将发送一个 loadSucceeded 或 loadFailed 信号。

这个立方体模型还包含两个包含一个 LineBoxGeometry 的模型。这些方块显示了体积的边界框和切片平面。

Model {
    visible: drawBoundingBox.checked
    geometry: LineBoxGeometry {}
    materials: DefaultMaterial {
        diffuseColor: "#323232"
        lighting: DefaultMaterial.NoLighting
    }
    receivesShadows: false
    castsShadows: false
}

Model {
    visible: drawBoundingBox.checked
    geometry: LineBoxGeometry {}
    materials: DefaultMaterial {
        diffuseColor: "#323232"
        lighting: DefaultMaterial.NoLighting
    }
    receivesShadows: false
    castsShadows: false
    position: sliceBoxPosition(xSliceSlider.value,
                               ySliceSlider.value,
                               zSliceSlider.value,
                               xSliceWidthSlider.value,
                               ySliceWidthSlider.value,
                               zSliceWidthSlider.value)
    scale: Qt.vector3d(xSliceWidthSlider.value,
                       ySliceWidthSlider.value,
                       zSliceWidthSlider.value)
}

让我们看看着色器。顶点着色器非常简单，除了对位置进行 MVP 投影外，还将计算从相机到模型的射线方向。

void MAIN()
{
    POSITION = MODELVIEWPROJECTION_MATRIX * vec4(VERTEX, 1.0);
    ray_direction_model = VERTEX - (inverse(MODEL_MATRIX) * vec4(CAMERA_POSITION, 1.0)).xyz;
}

片元着色器将从计算在模型空间中射线投射射线将开始的位置开始，考虑切片平面。在 while 循环中，它将沿着射线以相等的距离采样体素，将色彩图中的体素的值添加颜色和透明度。

void MAIN()
{
    FRAGCOLOR = vec4(0);

    // The camera position (eye) in model space
    const vec3 ray_origin_model = (inverse(MODEL_MATRIX) * vec4(CAMERA_POSITION, 1)).xyz;

    // Get the ray intersection with the sliced box
    float t_0, t_1;
    const vec3 top_sliced = vec3(100)*sliceMax - vec3(50);
    const vec3 bottom_sliced = vec3(100)*sliceMin - vec3(50);
    if (!ray_box_intersection(ray_origin_model, ray_direction_model, bottom_sliced, top_sliced, t_0, t_1))
        return; // No ray intersection with sliced box, nothing to render

    // Get the start/end points of the ray in original box
    const vec3 top = vec3(50, 50, 50);
    const vec3 bottom = vec3(-50, -50, -50);
    const vec3 ray_start = (ray_origin_model + ray_direction_model * t_0 - bottom) / (top - bottom);
    const vec3 ray_stop =  (ray_origin_model + ray_direction_model * t_1 - bottom) / (top - bottom);

    vec3 ray = ray_stop - ray_start;
    float ray_length = length(ray);
    vec3 step_vector = stepLength * ray / ray_length;

    vec3 position = ray_start;

    // Ray march until reaching the end of the volume, or color saturation
    while (ray_length > 0) {
        ray_length -= stepLength;
        position += step_vector;

        float val = textureLod(volume, position, 0).r;
        if (val == 0 || val < tMin || val > tMax)
            continue;

        const float alpha = multipliedAlpha ? val * stepAlpha : stepAlpha;
        vec4 val_color = vec4(textureLod(colormap, vec2(val, 0.5), 0).rgb, alpha);
        // Opacity correction
        val_color.a = 1.0 - pow(max(0.0, 1.0 - val_color.a), 1.0);
        FRAGCOLOR.rgb += (1.0 - FRAGCOLOR.a) * val_color.a * val_color.rgb;
        FRAGCOLOR.a += (1.0 - FRAGCOLOR.a) * val_color.a;
        if (FRAGCOLOR.a >= 0.95)
            break;
    }
}

为了控制体积模型，我们添加了一个自定义的Item，称为ArcballController，它实现了一个弧球控制器，这样我们就可以自由地旋转模型。当我们在模型上点击并移动鼠标时，DragHandler将向ArcballController发送命令。The WheelHandler向相机添加缩放功能。

ArcballController {
    id: arcballController
    controlledObject: cubeModel

    function jumpToAxis(axis) {
        cameraRotation.from = arcballController.controlledObject.rotation
        cameraRotation.to = originGizmo.quaternionForAxis(
                    axis, arcballController.controlledObject.rotation)
        cameraRotation.duration = 200
        cameraRotation.start()
    }

    function jumpToRotation(qRotation) {
        cameraRotation.from = arcballController.controlledObject.rotation
        cameraRotation.to = qRotation
        cameraRotation.duration = 100
        cameraRotation.start()
    }

    QuaternionAnimation {
        id: cameraRotation
        target: arcballController.controlledObject
        property: "rotation"
        type: QuaternionAnimation.Slerp
        running: false
        loops: 1
    }
}

DragHandler {
    id: dragHandler
    target: null
    acceptedModifiers: Qt.NoModifier
    onCentroidChanged: {
        arcballController.mouseMoved(toNDC(centroid.position.x,
                                           centroid.position.y))
    }

    onActiveChanged: {
        if (active) {
            view.forceActiveFocus()
            arcballController.mousePressed(toNDC(centroid.position.x,
                                                 centroid.position.y))
        } else
            arcballController.mouseReleased(toNDC(centroid.position.x,
                                                  centroid.position.y))
    }

    function toNDC(x, y) {
        return Qt.vector2d((2.0 * x / width) - 1.0,
                           1.0 - (2.0 * y / height))
    }
}

WheelHandler {
    id: wheelHandler
    orientation: Qt.Vertical
    target: null
    acceptedDevices: PointerDevice.Mouse | PointerDevice.TouchPad
    onWheel: event => {
                 let delta = -event.angleDelta.y * 0.01
                 cameraNode.z += cameraNode.z * 0.1 * delta
             }
}

我们还有一个名为OriginGizmo的自定义Item，它是一个用来显示旋转后模型方向的简单Gizmo。

OriginGizmo {
    id: originGizmo
    anchors.top: parent.top
    anchors.right: parent.right
    anchors.margins: 10
    width: 120
    height: 120
    targetNode: cubeModel

    onAxisClicked: axis => {
                       arcballController.jumpToAxis(axis)
                   }
}

要控制所有设置，我们在左侧使用ScrollView和一系列UI元素。

ScrollView {
    id: settingsPane
    height: parent.height
    property bool hidden: false

    function toggleHide() {
        if (settingsPane.hidden) {
            settingsPaneAnimation.from = settingsPane.x
            settingsPaneAnimation.to = 0
        } else {
            settingsPaneAnimation.from = settingsPane.x
            settingsPaneAnimation.to = -settingsPane.width
        }
        settingsPane.hidden = !settingsPane.hidden
        settingsPaneAnimation.running = true
    }

    NumberAnimation on x {
        id: settingsPaneAnimation
        running: false
        from: width
        to: width
        duration: 100
    }

    Column {
        topPadding: 10
        bottomPadding: 10
        leftPadding: 20
        rightPadding: 20

        spacing: 10

        Label {
            text: qsTr("Visible value-range:")
        }

        RangeSlider {
            id: tSlider
            from: 0
            to: 1
            first.value: 0
            second.value: 1
        }

        Image {
            width: tSlider.width
            height: 20
            source: getColormapSource(colormapCombo.currentIndex)
        }

        Label {
            text: qsTr("Colormap:")
        }

        ComboBox {
            id: colormapCombo
            model: [qsTr("Cool Warm"), qsTr("Plasma"), qsTr("Viridis"), qsTr("Rainbow"), qsTr("Gnuplot")]
        }

        Label {
            text: qsTr("Step alpha:")
        }

        Slider {
            id: stepAlphaSlider
            from: 0
            value: 0.2
            to: 1
        }

        Grid {
            horizontalItemAlignment: Grid.AlignHCenter
            verticalItemAlignment: Grid.AlignVCenter
            spacing: 5
            Label {
                text: qsTr("Step length:")
            }

            TextField {
                id: stepLengthText
                text: "0.00391" // ~1/256
                width: 100
            }
        }

        CheckBox {
            id: multipliedAlphaBox
            text: qsTr("Multiplied alpha")
            checked: true
        }

        CheckBox {
            id: drawBoundingBox
            text: qsTr("Draw Bounding Box")
            checked: true
        }

        CheckBox {
            id: autoRotateCheckbox
            text: qsTr("Auto-rotate model")
            checked: false
        }

        // X plane
        Label {
            text: qsTr("X plane slice (position, width):")
        }

        Slider {
            id: xSliceSlider
            from: 0
            to: 1
            value: 0.5
        }

        Slider {
            id: xSliceWidthSlider
            from: 0
            value: 1
            to: 1
        }

        // Y plane
        Label {
            text: qsTr("Y plane slice (position, width):")
        }

        Slider {
            id: ySliceSlider
            from: 0
            to: 1
            value: 0.5
        }

        Slider {
            id: ySliceWidthSlider
            from: 0
            value: 1
            to: 1
        }

        // Z plane
        Label {
            text: qsTr("Z plane slice (position, width):")
        }

        Slider {
            id: zSliceSlider
            from: 0
            to: 1
            value: 0.5
        }

        Slider {
            id: zSliceWidthSlider
            from: 0
            value: 1
            to: 1
        }

        // Dimensions
        Label {
            text: qsTr("Dimensions (width, height, depth):")
        }

        Row {
            spacing: 5
            TextField {
                id: dataWidth
                text: "256"
                validator: IntValidator {
                    bottom: 1
                    top: 2048
                }
            }
            TextField {
                id: dataHeight
                text: "256"
                validator: IntValidator {
                    bottom: 1
                    top: 2048
                }
            }
            TextField {
                id: dataDepth
                text: "256"
                validator: IntValidator {
                    bottom: 1
                    top: 2048
                }
            }
        }

        Label {
            text: qsTr("Scale (x, y, z):")
        }

        Row {
            spacing: 5
            TextField {
                id: scaleWidth
                text: "1"
                validator: DoubleValidator {
                    bottom: 0.001
                    top: 1000
                    decimals: 4
                }
            }
            TextField {
                id: scaleHeight
                text: "1"
                validator: DoubleValidator {
                    bottom: 0.001
                    top: 1000
                    decimals: 4
                }
            }
            TextField {
                id: scaleDepth
                text: "1"
                validator: DoubleValidator {
                    bottom: 0.001
                    top: 1000
                    decimals: 4
                }
            }
        }

        Label {
            text: qsTr("Data type:")
        }

        ComboBox {
            id: dataTypeComboBox
            model: ["uint8", "uint16", "int16", "float32", "float64"]
        }

        Label {
            text: qsTr("Load Built-in Volume:")
        }

        Row {
            spacing: 5

            Button {
                text: qsTr("Helix")
                onClicked: {
                    volumeTextureData.loadAsync("file:///default_helix",
                                                256, 256, 256, "uint8")
                    spinner.running = true
                }
            }

            Button {
                text: qsTr("Box")
                onClicked: {
                    volumeTextureData.loadAsync("file:///default_box", 256,
                                                256, 256, "uint8")
                    spinner.running = true
                }
            }

            Button {
                text: qsTr("Colormap")
                onClicked: {
                    volumeTextureData.loadAsync("file:///default_colormap",
                                                256, 256, 256, "uint8")
                    spinner.running = true
                }
            }
        }

        Button {
            text: qsTr("Load Volume...")
            onClicked: fileDialog.open()
        }
    }
}

当所有这些部分协同工作时，应用程序能够渲染并对我们的体积进行交互式控制。请注意，此示例可以渲染的体积大小以及性能将受您的显卡限制。

文件

图片

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