SFD-populate.js

    populate(features, options) {
        const layout = this.layers[0].layout;
        const iconImage = layout['sdficon-image'];

        const hasIcon = iconImage;

        this.features = [];

        if (!hasIcon) {
            return;
        }

        const icons = options.iconDependencies;

        for (let i = 0; i < features.length; i++) {
            const feature = features[i];
            if (!this.layers[0].filter(feature)) {
                continue;
            }

            let icon;
            if (hasIcon) {
                icon = resolveTokens(feature.properties, iconImage);
            }

            if (!icon) {
                continue;
            }

            this.features.push({
                icon,
                index: i,
                sourceLayerIndex: feature.sourceLayerIndex,
                geometry: loadGeometry(feature),
                properties: feature.properties,
                type: VectorTileFeature.types[feature.type]
            });

            if (icon) {
                icons[icon] = true;
            }

            options.iconDependencies[icon] = true;
        }
    }

SYMBOL-populate.js

    populate(features, options) {
        const layout = this.layers[0].layout;
        const textField = layout['text-field'];
        const textFont = layout['text-font'];
        const iconImage = layout['icon-image'];

        const hasText = textField && textFont;
        const hasIcon = iconImage;

        this.features = [];

        if (!hasText && !hasIcon) {
            return;
        }

        const icons = options.iconDependencies;
        const stacks = options.glyphDependencies;
        const stack = stacks[textFont] = stacks[textFont] || {};

        for (let i = 0; i < features.length; i++) {
            const feature = features[i];
            if (!this.layers[0].filter(feature)) {
                continue;
            }

            let text;
            if (hasText) {
                text = resolveText(feature, layout);
            }

            let icon;
            if (hasIcon) {
                icon = resolveTokens(feature.properties, iconImage);
            }

            if (!text && !icon) {
                continue;
            }

            this.features.push({
                text,
                icon,
                index: i,
                sourceLayerIndex: feature.sourceLayerIndex,
                geometry: loadGeometry(feature),
                properties: feature.properties,
                type: VectorTileFeature.types[feature.type]
            });

            if (icon) {
                icons[icon] = true;
            }

            if (text) {
                for (let i = 0; i < text.length; i++) {
                    stack[text.charCodeAt(i)] = true;
                }
            }
        }

        if (layout['symbol-placement'] === 'line') {
            // Merge adjacent lines with the same text to improve labelling.
            // It's better to place labels on one long line than on many short segments.
            this.features = mergeLines(this.features);
        }
    }