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PODelphi 6 : Alphablending works between sub-image and main Canvas, but not between sub-images
 

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  1. PODelphi 6 : Alphablending works between sub-image and main Canvas, but not between sub-images
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    <p>I have a Delphi 6 application where I have an array of TBitmaps each containing an image (sub-image) that was created by making a call to FillRect() with clWhite to flood the TBitmap's Canvas with white, followed by a call toTextOut() to print some text to the Bitmap's Canvas. </p> <p>I have a timer that updates the main Canvas, which belongs to a TImage component, with the contents of the array every Interval to reflect changes made to each sub-images X, Y coordinates since they are in motion. First I use FillRect() with clWhite to flood the main Canvas with white. To "print" the sub-images I simply iterate the array and call Alphablend() to "print" each sub-image on to the main Canvas. Each sub-image has a Z coordinate that is used to control the level of opacity. Sub-images with higher Z coordinates have a lower Alphablend() value to make them appear more transparent than those that are "up close".</p> <p>The alpha-blending works great with each individual sub-image and those that have higher coordinates do indeed appear to be more transparent than those with lower Z coordinates. However, I'm having a problem with sub-images that overlap in Z space. Alpha-blending between overlapping sub-images does not appear to work. Instead, the sub-image "behind" the overlapping sub-image is occluded by the white background belonging to the image that overlays it. What I want is for the two sub-images to overlap properly so it looks like two text strings are written on top of each other. Instead I see the front text string and everywhere the white background that comprises it's TBitmap's Canvas exists, wiping out the text string "behind" it.</p> <p>Can anyone tell me why alpha-blending is working between each individual sub-image and the main Canvas, but not between sub-images? I say between sub-images only in a temporal context and not as objects because they should all become part of the main Canvas as they print and I would expect alpha-blend to blend them properly, which is not happening.</p> <p>It's almost as if the "white" between sub-images is not being treated the same as the "white" between a sub-image and the "white" belonging to the main Canvas.</p> <p>FOLLOW-UP: In response to Andreas comment I set the PixelFormat of all sub-image TBitmap's to pf32bit. Now alpha-blending is taking place "between" sub-images. Unfortunately, the the sub-image's white areas are not being treated as a Transparent color. Instead the white background of the sub-image that is on top is being alpha-blended with the text belonging to the underlying sub-image creating a light gray frame around the sub-image on top as if it was smeared. To get the effect I want I need the white background of a sub-image to be completely ignored.</p>
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    1. USRobert Oschler
    1. USRobert Oschler
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    1. COI think you should give code and instructions to reproduce the problem, in as few steps as possible. Then you'll probably get a very accurate answer rather soon. Now we can only guess (e.g. "Have you tried to play with `PixelFormat`?").
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      1. PODelphi 6 : Alphablending works between sub-image and main Canvas, but not between sub-images
      1. USAndreas Rejbrand
    2. COOk Andreas. I'll go work something up. I've updated my description of the problem since it appears that since I set the PixelFormat of all the sub-images to pf32bit, alpha-blending is taking place between sub-images. Unfortunately, it's not treating the sub-image's white areas as Transparent so instead of the sub-image background completely occluding (wiping out) an underlying sub-image in Z space as before, it's being alpha-blended with the text of the overlayed sub-image, creating a smeared grey frame around the sub-image that is "on top".
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      1. PODelphi 6 : Alphablending works between sub-image and main Canvas, but not between sub-images
      1. USRobert Oschler
 

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