How to draw 32-bit alpha channel bitmaps?

Answer 1

You need to do an alpha blend with your background color, then take out the alpha channel to paint it to the control.

The alpha channel should just be every 4th byte of your image. You can use that directly for your mask, or you can just copy every 4th byte to a new mask image.

Answer 2

The way I usually do this is via a DIBSection - a device independent bitmap that you can modify the pixels of directly. Unfortunately there isn't any MFC support for DIBSections: you have to use the Win32 function CreateDIBSection() to use it.

Start by loading the bitmap as 32-bit RGBA (that is, four bytes per pixel: one red, one green, one blue and one for the alpha channel). In the control, create a suitably sized DIBSection. Then, in the paint routine

• Copy the bitmap data into the DIBSection's bitmap data, using the alpha channel byte to blend the bitmap image with the background colour.
• Create a device context and select the DIBSection into it.
• Use BitBlt() to copy from the new device context to the paint device context.

You can create a mask given the raw bitmap data simply by looking at the alpha channel values - I'm not sure what you're asking here.

Answer 3

Painting it is very easy with the AlphaBlend function.

As for you mask, you'll need to get the bits of the bitmap and examine the alpha channel byte for each pixel you're interested in.

Answer 4

An optimised way to pre-multiply the RGB channels with the alpha channel is to set up a [256][256] array containing the calculated multiplication results. The first dimension is the alpha value, the second is the R/G/B value, the values in the array are the pre-multiplied values you need.

With this array set up correctly, you can calculate the value you need like this:

R = multiplicationLookup[alpha][R];
G = multiplicationLookup[alpha][G];
B = multiplicationLookup[alpha][B];

Answer 5

I'd suggest GDI+.

Answer 6

You are on the right track, but need to fix two things.

First use ::LoadImage( .. LR_CREATEDIBSECTION ..) instead of CBitmap::LoadBitmap. Two, you have to "pre-multiply" RGB values of every pixel in a bitmap to their respective A value. This is a requirement of AlphaBlend function, see AlphaFormat description on this MSDN page. T

The lpng has a working code that does the premultiplication of the DIB data.