Method onDraw

snap_drawing/src/snap_drawing/cpp/Layers/ExternalLayer.cpp:65–86

    return Image::makeFromBitmap(bitmap.value(), false);
}

void ExternalLayer::onDraw(DrawingContext& drawingContext) {
    if (_externalSurface != nullptr) {
        auto frameSize = getFrame().size();
        _externalSurface->setRelativeSize(frameSize);

        if (shouldRasterizeExternalSurface()) {
            // Rasterize at min(frameSize, outputSize): a layer pinched/zoomed larger than the
            // final output has no need for a raster buffer bigger than what will ever be shown.
            // drawImage below stretches the (possibly smaller) result back to frameSize, which is
            // information-equivalent for image content -- unlike vector content, there's no
            // absolute-unit geometry (border width, radius, text) that a uniform scale would distort.
            auto* root = getRoot();
            auto outputSize = root != nullptr ? root->getOutputSize() : OutputSize{};
            auto rasterWidth = (outputSize.width > 0) ? std::min(frameSize.width, outputSize.width) : frameSize.width;
            auto rasterHeight =
                (outputSize.height > 0) ? std::min(frameSize.height, outputSize.height) : frameSize.height;

            // A degenerate (empty or sub-pixel) layer truncates to a zero-sized raster target,
            // which createBitmap rejects. Nothing to draw here, so skip it rather than take the
            // error path on every frame.
            if (static_cast<int>(rasterWidth) <= 0 || static_cast<int>(rasterHeight) <= 0) {
                return;
            }