Track the region where text is painted.

Source/core/rendering/RenderImage.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011-2012. All rights reserved.
  *
@@ skipping 323 matching lines @@
                 context->drawImage(image.get(), style()->colorSpace(), pixelSnappedIntRect(LayoutRect(paintOffset + imageOffset, imageSize)), CompositeSourceOver, shouldRespectImageOrientation());
                 errorPictureDrawn = true;
             }
 
             if (!m_altText.isEmpty()) {
                 String text = document()->displayStringModifiedByEncoding(m_altText);
                 context->setFillColor(style()->visitedDependentColor(CSSPropertyColor), style()->colorSpace());
                 const Font& font = style()->font();
                 const FontMetrics& fontMetrics = font.fontMetrics();
                 LayoutUnit ascent = fontMetrics.ascent();
-                LayoutPoint altTextOffset = paintOffset;
-                altTextOffset.move(leftBorder + leftPad + (paddingWidth / 2) - borderWidth, topBorder + topPad + ascent + (paddingHeight / 2) - borderWidth);
+                LayoutPoint textRectOrigin = paintOffset;
+                textRectOrigin.move(leftBorder + leftPad + (paddingWidth / 2) - borderWidth, topBorder + topPad + (paddingHeight / 2) - borderWidth);
+                LayoutPoint textOrigin(textRectOrigin.x(), textRectOrigin.y() + ascent);
 
                 // Only draw the alt text if it'll fit within the content box,
                 // and only if it fits above the error image.
                 TextRun textRun = RenderBlock::constructTextRun(this, font, text, style());
                 LayoutUnit textWidth = font.width(textRun);
+                FloatRect textRect(textRectOrigin, FloatSize(textWidth, fontMetrics.height()));
                 if (errorPictureDrawn) {
                     if (usableWidth >= textWidth && fontMetrics.height() <= imageOffset.height())
-                        context->drawText(font, textRun, altTextOffset);
+                        context->drawText(font, textRun, textOrigin, textRect);
                 } else if (usableWidth >= textWidth && usableHeight >= fontMetrics.height())
-                    context->drawText(font, textRun, altTextOffset);
+                    context->drawText(font, textRun, textOrigin, textRect);
             }
         }
     } else if (m_imageResource->hasImage() && cWidth > 0 && cHeight > 0) {
         RefPtr<Image> img = m_imageResource->image(cWidth, cHeight);
         if (!img || img->isNull()) {
             if (page && paintInfo.phase == PaintPhaseForeground)
                 page->addRelevantUnpaintedObject(this, visualOverflowRect());
             return;
         }
 
@@ skipping 212 matching lines @@
 #if ENABLE(SVG)
     CachedImage* cachedImage = m_imageResource->cachedImage();
     if (cachedImage && cachedImage->image() && cachedImage->image()->isSVGImage())
         return static_cast<SVGImage*>(cachedImage->image())->embeddedContentBox();
 #endif
 
     return 0;
 }
 
 } // namespace WebCore