webkit引擎源码
CanvasRenderingContext2D* HTMLCanvasElement::createContext2d(const String& type)
{
    ASSERT_UNUSED(HTMLCanvasElement::is2dType(type), type);
    ASSERT(!m_context);

    bool usesDashboardCompatibilityMode = false;
#if ENABLE(DASHBOARD_SUPPORT)
    usesDashboardCompatibilityMode = document().settings().usesDashboardBackwardCompatibilityMode();
#endif

    // Make sure we don't use more pixel memory than the system can support.
    size_t requestedPixelMemory = 4 * width() * height();//计算占用内存
    if (activePixelMemory + requestedPixelMemory > maxActivePixelMemory()) {
        StringBuilder stringBuilder;
        stringBuilder.appendLiteral("Total canvas memory use exceeds the maximum limit (");
        stringBuilder.appendNumber(maxActivePixelMemory() / 1024 / 1024);
        stringBuilder.appendLiteral(" MB).");
        document().addConsoleMessage(MessageSource::JS, MessageLevel::Warning, stringBuilder.toString());
        return nullptr;
    }

    m_context = CanvasRenderingContext2D::create(*this, document().inQuirksMode(), usesDashboardCompatibilityMode);

    downcast<CanvasRenderingContext2D>(*m_context).setUsesDisplayListDrawing(m_usesDisplayListDrawing);
    downcast<CanvasRenderingContext2D>(*m_context).setTracksDisplayListReplay(m_tracksDisplayListReplay);

#if USE(IOSURFACE_CANVAS_BACKING_STORE) || ENABLE(ACCELERATED_2D_CANVAS)
    // Need to make sure a RenderLayer and compositing layer get created for the Canvas.
    invalidateStyleAndLayerComposition();
#endif

    return static_cast<CanvasRenderingContext2D*>(m_context.get());
}
100 * 100 canvas 占的内存是 100 * 100 * 4 bytes = 40,000 bytes
https://github.com/WebKit/webkit/blob/master/Source/WebCore/html/HTMLCanvasElement.cpp#L365