Technical Q&A

QTMCC 13 - Setting Export Quality (06-December-1999)

Q: We currently have a file which we would like to export in a different format. We're using GetGraphicsImporterForFile() and GraphicsImportExportImageFile() to do this. However, we didn't see how to change the compression factor if we're saving as a JPEG file. Is this possible?

A: This is possible with just a little more work than using GraphicsImportExportImageFile(). The idea is to use the Graphics Exporter Component APIs with a specific call to GraphicsExportSetCompressionQuality() or GraphicsExportSetTargetDataSize() to set the compression quality for the export.

Here's an overview of the steps to follow:

Select your Export Component. In the code below, we get the JPEG Export Component.
Specify the source for the export. This can be a Graphics Importer instance, a QuickDraw Picture, GWorld, PixMap, or a segment of compressed data described by an image description record.
Tell the Export Component where the input is coming from. The code below uses a Graphics Importer Instance.
Tell the Export Component the destination of the exported data. The code below uses a file.
Specify your settings. A variety of Get and Set functions are available to manipulate various graphics exporter settings. There are two examples in the code below.
1. Explicitly tell the exporter we want low quality.
2. Ask the exporter to fit the file to a given size constraint and ramp the quality accordingly.
Do the export. The code below will write out the data and create a JPEG file.

Graphics Exporter Components Documentation can be found in the QuickTime 4 API Documentation.

Example:

OSErr DoEasyExportToJpeg()
{
    SFTypeList      myTypeList = {kQTFileTypeMovie, 0, 0, 0};
    StandardFileReply     myReply;
    GraphicsImportComponent theImportComponent = NULL;
    GraphicsExportComponent theExportComponent = NULL;
    OSErr       rc = noErr;
 
    StandardGetFilePreview( NULL, 1, myTypeList, &myReply );
    if (!myReply.sfGood) goto bail;
    
    // 1. Open the JPEG Exporter Component.
    // We'll connect an Import Component to this Export Component
    // and very easily export a JPEG file.
    rc = OpenADefaultComponent( GraphicsExporterComponentType, kQTFileTypeJPEG,
          &theExportComponent );
    if (rc) goto bail;
    
    // 2. Find the Graphics Import Component for our file.
    rc = GetGraphicsImporterForFile( &myReply.sfFile, &theImportComponent );
    if (rc) goto bail;
 
    // 3. Connect the Importer to the Exporter.
    rc = GraphicsExportSetInputGraphicsImporter( theExportComponent, theImportComponent );
    if (rc) goto bail;
 
    StandardPutFile( "\pExport File As...", "\pMyJpeg.jpg", &myReply );
    if (!myReply.sfGood) goto bail;
    
    // 4. We're going to export this to a file.
    rc = GraphicsExportSetOutputFile( theExportComponent, &myReply.sfFile );
    if (rc) goto bail;
    
    // 5a. Set the image compression quality.
    rc = GraphicsExportSetCompressionQuality( theExportComponent, codecLowQuality );
    if (rc) goto bail;
    
    // 5b. Alternatively you could use GraphicsExportSetTargetDataSize().
    // This API let's you specify the desired data size in bytes; the JPEG exporter will
    // decrease the quality until it reaches the specified size or bottoms out on the
    // quality setting.
    //rc = GraphicsExportSetTargetDataSize( theExportComponent, 25000 ); // Max size 25k
    //if (rc) goto bail;
    
    // 6. Do the export, write the file.
    rc = GraphicsExportDoExport( theExportComponent, NULL );

bail:
 if ( theImportComponent != NULL )
     CloseComponent( theImportComponent );
 if ( theExportComponent != NULL )
         CloseComponent( theExportComponent );

    return rc;
}

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