Codejock Developer Community : Markup rendering with alpha channel http://forum.codejock.com/ Copyright (c) 2006-2013 Web Wiz Forums - All Rights Reserved. Sun, 19 Apr 2026 12:38:16 +0000 Tue, 17 Jan 2017 09:13:24 +0000 http://blogs.law.harvard.edu/tech/rss Web Wiz Forums 12.04 360 forum.codejock.com/RSS_post_feed.asp?TID=23247 <![CDATA[Codejock Developer Community]]> http://forum.codejock.com/forum_images/codejock-logo.gif http://forum.codejock.com/ <![CDATA[Markup rendering with alpha channel : Hi,I&#039;m trying to rendering...]]> http://forum.codejock.com/forum_posts.asp?TID=23247&PID=75499&title=markup-rendering-with-alpha-channel#75499 Author: Marc.Gregoire
Subject: 23247
Posted: 17 January 2017 at 9:13am

Hi,

I'm trying to rendering some markup text to a bitmap but I would like to have a correct alpha channel in that bitmap.
For example, take the following code:
CXTPMarkupContext* ctx = XTPMarkupCreateContext();
CXTPMarkupUIElement* pUIElement = XTPMarkupParseText(ctx, 
 _T("<Page VerticalAlignment='Top' HorizontalAlignment='Left'>")
 _T("<Border BorderThickness='5' BorderBrush='#500000FF' Background='white' CornerRadius='16' Padding='8'>")
 _T("<TextBlock>Test</TextBlock>")
 _T("</Border>")
 _T("</Page>"));
 
CSize desiredSize = XTPMarkupMeasureElement(pUIElement);
CRect rc (CPoint(0, 0), desiredSize);
pUIElement->Arrange(rc);
CDC memoryDC;
memoryDC.CreateCompatibleDC(nullptr);
memoryDC.SetBkMode(TRANSPARENT);
BITMAPINFO pbiDIB;
pbiDIB.bmiHeader.biBitCount = 32;
pbiDIB.bmiHeader.biClrImportant = 0;
pbiDIB.bmiHeader.biClrUsed = 0;
pbiDIB.bmiHeader.biCompression = BI_RGB;
pbiDIB.bmiHeader.biPlanes = 1;
pbiDIB.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
pbiDIB.bmiHeader.biSizeImage = 0;
pbiDIB.bmiHeader.biXPelsPerMeter = 0;
pbiDIB.bmiHeader.biYPelsPerMeter = 0;
pbiDIB.bmiHeader.biHeight = rc.Height();
pbiDIB.bmiHeader.biWidth = rc.Width();
BYTE* pDIBitmap = nullptr;
HBITMAP bmpRender = ::CreateDIBSection(memoryDC, &pbiDIB, DIB_RGB_COLORS, (void **)&pDIBitmap, 0, 0);
HBITMAP oldBmp = (HBITMAP)SelectObject(memoryDC, bmpRender);
XTPMarkupRenderElement(pUIElement, memoryDC.GetSafeHdc(), rc);
XTPMarkupReleaseElement(pUIElement);
XTPMarkupReleaseContext(ctx);
ctx = nullptr;
SelectObject(memoryDC, oldBmp);

The R, G, and B channels are perfect.
But, the resulting alpha channel looks as follows:


The border is nicely anti-aliased, but the alpha values for the text are quite wrong.
These are completely black meaning the text would be completely transparent.

What am I doing wrong here?

]]> Tue, 17 Jan 2017 09:13:24 +0000 http://forum.codejock.com/forum_posts.asp?TID=23247&PID=75499&title=markup-rendering-with-alpha-channel#75499