function makeDemoImage() {
                    var canvas = document.createElement('canvas');
                    canvas.width = 1000;
                    canvas.height = 720;
                    var ctx = canvas.getContext('2d');
                    var sky = ctx.createLinearGradient(0, 0, 0, canvas.height);
                    sky.addColorStop(0, '#f7f3e7');
                    sky.addColorStop(.62, '#cdded8');
                    sky.addColorStop(1, '#7fa5a3');
                    ctx.fillStyle = sky;
                    ctx.fillRect(0, 0, canvas.width, canvas.height);

                    ctx.fillStyle = 'rgba(33,70,74,.15)';
                    for (let i = 0; i < 6; i++) {
                        ctx.beginPath();
                        ctx.ellipse(160 + i * 175, 164 + (i % 2) * 42, 126, 28, -.08, 0, Math.PI * 2);
                        ctx.fill();
                    }

                    var ridges = [{
                        y: 420,
                        color: '#7b9995',
                        amp: 130,
                        step: 145
                    }, {
                        y: 510,
                        color: '#496f72',
                        amp: 170,
                        step: 125
                    }, {
                        y: 610,
                        color: '#193f48',
                        amp: 190,
                        step: 112
                    }];
                    ridges.forEach( (ridge, layer) => {
                        ctx.beginPath();
                        ctx.moveTo(0, canvas.height);
                        ctx.lineTo(0, ridge.y);
                        for (let x = 0; x <= canvas.width + ridge.step; x += ridge.step) {
                            var peak = ridge.y - ridge.amp * (.54 + .46 * Math.sin(x * .014 + layer));
                            ctx.quadraticCurveTo(x + ridge.step * .45, peak, x + ridge.step, ridge.y + Math.sin(x * .021) * 22);
                        }
                        ctx.lineTo(canvas.width, canvas.height);
                        ctx.closePath();
                        ctx.fillStyle = ridge.color;
                        ctx.globalAlpha = .44 + layer * .16;
                        ctx.fill();
                    }
                    );
                    ctx.globalAlpha = 1;

                    ctx.strokeStyle = '#163d45';
                    ctx.lineWidth = 7;
                    ctx.lineCap = 'round';
                    ctx.beginPath();
                    ctx.moveTo(742, 620);
                    ctx.bezierCurveTo(716, 476, 760, 340, 700, 192);
                    ctx.stroke();
                    ctx.lineWidth = 3;
                    for (let i = 0; i < 13; i++) {
                        var y = 230 + i * 25;
                        var side = i % 2 ? -1 : 1;
                        ctx.beginPath();
                        ctx.moveTo(716 + Math.sin(i) * 16, y);
                        ctx.quadraticCurveTo(790 + side * 18, y - 32, 842 + side * 44, y - 45);
                        ctx.stroke();
                    }
                    ctx.fillStyle = '#244f55';
                    for (let i = 0; i < 42; i++) {
                        var x = 690 + Math.sin(i * 2.1) * (55 + i % 7 * 9);
                        var y = 205 + (i * 17) % 310;
                        ctx.beginPath();
                        ctx.ellipse(x, y, 13, 5, (i % 5) * .42, 0, Math.PI * 2);
                        ctx.fill();
                    }

                    var img = new Image();
                    img.src = canvas.toDataURL('image/png');
                    return img;
                }

                function imageLuminance(image, u, v) {
                    if (!image)
                        return 1;
                    var px = clamp(u * (image.width - 1), 0, image.width - 1);
                    var py = clamp(v * (image.height - 1), 0, image.height - 1);
                    var x0 = Math.floor(px)
                      , y0 = Math.floor(py);
                    var x1 = Math.min(x0 + 1, image.width - 1)
                      , y1 = Math.min(y0 + 1, image.height - 1);
                    var tx = px - x0
                      , ty = py - y0;
                    var sample = (x, y) => {
                        var i = (y * image.width + x) * 4;
                        return (image.data[i] * .2126 + image.data[i + 1] * .7152 + image.data[i + 2] * .0722) / 255;
                    }
                    ;
                    var top = sample(x0, y0) * (1 - tx) + sample(x1, y0) * tx;
                    var bottom = sample(x0, y1) * (1 - tx) + sample(x1, y1) * tx;
                    var lum = top * (1 - ty) + bottom * ty;
                    var contrast = Number(ui.contrast.value) / 100;
                    lum = clamp((lum - .5) * contrast + .5, 0, 1);
                    if (ui.invert.checked)
                        lum = 1 - lum;
                    return lum;
                }

                function sampleLuminance(x, y) {
                    var bounds = modelBounds();
                    var u = clamp((y - bounds.yMin) / (bounds.yMax - bounds.yMin), 0, 1);
                    var v = clamp((x - bounds.xMin) / (bounds.xMax - bounds.xMin), 0, 1);
                    return imageLuminance(state.sourcePixels, u, v);
                }

                function sampleBladeLuminance(bladeIndex, radius, cross) {
                    var pixels = state.bladeConfigs[bladeIndex].pixels;
                    if (!pixels)
                        return 1;
                    var u = clamp((radius - FAN.rMin) / (FAN.rMax - FAN.rMin), 0, 1);
                    var v = clamp((1 - cross) / 2, 0, 1);
                    return imageLuminance(pixels, u, v);
                }

                function readImageFile(file) {
                    return new Promise( (resolve, reject) => {
                        var reader = new FileReader();
                        reader.onload = () => {
                            var img = new Image();
                            img.onload = () => resolve({
                                file,
                                img
                            });
                            img.onerror = () => reject(new Error(`${file.name} 无法读取`));
                            img.src = reader.result;
                        }
                        ;
                        reader.onerror = () => reject(new Error(`${file.name} 无法读取`));
                        reader.readAsDataURL(file);
                    }
                    );
                }

                async function loadFiles(fileList) {
                    var files = [...(fileList || [])].filter( (file) => file.type.startsWith('image/'));
                    if (!files.length) {
                        ui.status.textContent = '请选择 JPG、PNG 或 WEBP 图片。';
                        return;
                    }
                    try {
                        showBusy(true);
                        if (state.imageMode === 'whole') {
                            var loaded = await readImageFile(files[0]);
                            state.globalConfig.image = loaded.img;
                            state.globalConfig.name = loaded.file.name;
                            state.globalConfig.isDemo = false;
                            ui.status.textContent = '整扇图片已载入，正在生成浮雕。';
                        } else {
                            var available = FAN.angles.length - state.selectedBlade;
                            var loaded = await Promise.all(files.slice(0, available).map(readImageFile));
                            loaded.forEach( ({file, img}, offset) => {
                                var config = state.bladeConfigs[state.selectedBlade + offset];
                                config.image = img;
                                config.name = file.name;
                                config.isDemo = false;
                            }
                            );
                            ui.status.textContent = `已从第 ${state.selectedBlade + 1} 片开始载入 ${loaded.length} 张图片。`;
                        }
                        ui.fileInput.value = '';
                        applyConfigToControls();
                        scheduleRebuild({
                            redrawImage: true
                        });
                    } catch (error) {
                        console.error(error);
                        ui.status.textContent = `图片载入失败：${error.message}`;
                        showBusy(false);
                    }
                }