Ernest Murphy ernie@surfree.com

June 30 2001

Sample code for this article is available.

Abstract:
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The COM API provides methods to load many useful picture formats, at least
.bmp. .ico, .wmf, .gif, and .,jpg. The use, care and feeding of this interface will 
be explained
 

Introduction:
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This tutorial is based on the work in Q218972 - How To Load and Display Graphics Files w/LOADPIC.EXE. Basically, all this tutorial consists of is a port of that C code to asm (no, not by a program, by the "see C, write ASM method).

There are quite a few steps to actually get a picture object, but these are always the same and may be contained in a procedure such as in the tutorial. 

What's a Picture Object Anyway?
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Picture objects are the COM/OLE standard way of loading picture files for ActiveX use. It's a nice, well behaved API you can use in any application.

A Picture object is any object that provides the IPicture interface. And IPicture does plenty of work for us. Let's look at it's properties and methods:
 

That's quite a few goodies to work with for one package. True, many seem to be wrappers for existing GDI methods, but here they are all contained in one nice package (hey, maybe that's why objects are so useful)
 

Creating a Picture Object
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A Picture object is created from a stream object, which is created from a.... OK, let's just jump to the code.
 
 

LoadPictureFile PROC pszFile:DWORD, ppPicture:DWORD     LOCAL hFile:DWORD, pvData:DWORD,     dwBytesRead:DWORD     LOCAL pstm:DWORD,  dwFileSize:DWORD, hGlobal:DWORD     LOCAL ghWnd:DWORD, bRead:DWORD ; This function loads a file into an IStream.     ; open file     invoke CreateFile, pszFile, GENERIC_READ, 0, NULL,            OPEN_EXISTING, 0, NULL     mov hFile, eax     .IF hFile == INVALID_HANDLE_VALUE          ; we had an error         mov eax, -1         ret     .ENDIF     ; get file size     invoke GetFileSize, hFile, NULL     mov dwFileSize, eax     .IF dwFileSize == -1          ; we had an error         invoke CloseHandle, hFile         mov eax, -1         ret     .ENDIF     mov pvData, NULL     ; alloc memory based on file size     invoke GlobalAlloc, GMEM_MOVEABLE, dwFileSize     mov hGlobal, eax     .IF !hGlobal        ; we had an error         invoke CloseHandle, hFile         mov eax, -1         ret     .ENDIF     invoke GlobalLock, hGlobal     mov pvData , eax     .IF !pvData        ; we had an error         ret     .ENDIF     mov dwBytesRead, NULL     ; read file and store in global memory     invoke ReadFile, hFile, pvData, dwFileSize,                      ADDR dwBytesRead, NULL     mov bRead, eax     .IF bRead == FALSE        ; we had an error         ret     .ENDIF     invoke GlobalUnlock, hGlobal     invoke CloseHandle, hFile     mov pstm, NULL     ; create IStream* from global memory     invoke CreateStreamOnHGlobal, hGlobal, TRUE, ADDR pstm     .IF_FAILED         ret     .ENDIF     .IF !pstm         ret     .ENDIF     ; Create IPicture from image file     .IF ppPicture         mov ecx, ppPicture         mov ecx, [ecx]         .IF ecx;gpPicture1             coinvoke ecx, IUnknown, Release         .ENDIF         invoke OleLoadPicture, pstm, dwFileSize, FALSE,                ADDR IID_IPicture, ppPicture         .IF_FAILED             coinvoke pstm, IUnknown, Release             ret         .ENDIF     .ENDIF     coinvoke pstm, IUnknown, Release     ret LoadPictureFile ENDP ;----------------------------------------------

The LoadPictureFile procedure takes a character string (psz) that contains the picture's filename, and assigns the picture object pointer to our pic object. It's smart enough to release an existing pic object if you pass one in. Actually, there is much more error handling in this routine then is usual, but I wanted to make this a reusable routine for you.

All we did here is use standard API methods to move a file into global memory. The first new COM method is the CreateStreamOnHGlobal method. A "stream" is OLE-speak for a storage medium where data may be streamed in or out. Of course, being a COM method it returns a stream object. Since all we need do is pass this on and release it, we'll pretend it isn't there.

The big action takes place with OleLoadPicture. This method will create the picture object for us from the stream object.
 
 

Using the Picture Object
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Once we have the picture object (which we call to be created inside the WM_CREATE message handler) we want to use it. IPicture does have a very nice Render method that is akin to a BitBlt instruction, in that it copies a selected portion of the Picture contained in the object to the device context you choose.

One real bit of true weirdness in this Render method is vertically it works backwards! I mean, if you have a 20x30 pixel bitmap and instruct it to render it starting at 0,0 and copy 20x30 piece, you will get an image that is upside down. I have no reason to explain this, just a work around. When Rendering this picture, instead tell it to place it at 0x30 (i.e., down by the Y dimension) and to copy 20x-30 pixels (i.e., work up and not down). 

The other bit of predictable weirdness is the Picture object works in high metric units, so we need to convert these.

So let's go to the code:
 
 

.ELSEIF uMsg==WM_PAINT     invoke BeginPaint, hWnd, ADDR ps     mov hdc, eax     .IF gpPicture1         ; get width and height of picture         coinvoke gpPicture1, IPicture, get_Width,                          ADDR hmWidth         coinvoke gpPicture1, IPicture, get_Height,                          ADDR hmHeight         ; convert himetric to pixels         invoke GetDeviceCaps,hdc, LOGPIXELSX         invoke MulDiv, hmWidth, eax, HIMETRIC_INCH         mov nWidth, eax         invoke GetDeviceCaps,hdc, LOGPIXELSY         invoke MulDiv, hmHeight, eax, HIMETRIC_INCH         mov nHeight, eax         xor eax, eax         sub eax, hmHeight         mov neghmHeight, eax         invoke GetClientRect, hWnd, ADDR rect         ; display picture using IPicture::Render         coinvoke gpPicture1, IPicture, Render, hdc, 0, 0,                   nWidth, nHeight, 0, hmHeight,                    hmWidth, neghmHeight, ADDR rect     .ENDIF     invoke EndPaint, hWnd, ADDR ps .ELSEIF

That's about it, except for one last bit of clean-up. When your app exits, and if you still have a picture object handy, you need to release it like so:
 
 

.IF gpPicture1       coinvoke gpPicture1, IUnknown, Release .ENDIF

 

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I hope you get some work out of this useful object.

Enjoy
 

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