UsingAlphaInImage

This little example should copy my image, which is a clear image with a rectangle, to my window. The background of the window is gray and the image consists of a red rectangle. The horizontal lines are copied like I would expect but the vertical lines are not drawn or a white bar is added. If I understand things correct, General/NSCompositeSourceOver should copy my image over my gray background (like it does for the horizontal lines). If you want to try things, simply create a cocoa application and put a custom view in the main window (make sure it is at least 200*200). The custom class is General/MyView. Simply add the code in a file that ends in .mm to avoid some errors.

What am I doing wrong?

#import <Cocoa/Cocoa.h>

@interface General/MyView : General/NSView { float frameHeight; float frameWidth; General/NSBitmapImageRep *imageCache; General/NSImage *mImage; unsigned char *imageDataPtr; int imageRowBytes; } @end

@implementation General/MyView

//———– // initWithFrame //———–

(id)initWithFrame:(General/NSRect)frameRect { if ((self = [super initWithFrame:frameRect]) != nil) { frameHeight=frameRect.size.height; frameWidth=frameRect.size.width;

      imageRowBytes = (((int)(frameWidth / 16) + 1) * 16) * 4;
      imageDataPtr = (unsigned char *)malloc(imageRowBytes * frameHeight);
      for (int i=0; i<imageRowBytes*frameHeight/4; i++) 
      {
          ((unsigned int *)imageDataPtr)[i] = 0xffffff00;
      }
      imageCache = [
          General/[NSBitmapImageRep alloc] 
  		initWithBitmapDataPlanes:&imageDataPtr
      pixelsWide:frameWidth
        pixelsHigh:frameHeight
      bitsPerSample:8 
      samplesPerPixel:4	
      hasAlpha:YES 		
  		isPlanar:NO
      colorSpaceName:General/NSDeviceRGBColorSpace
      bytesPerRow:imageRowBytes bitsPerPixel:(8 * 4)
  ];   mImage=General/[[NSImage alloc]init];   [mImage addRepresentation:imageCache];   // draw a rect   [self plotRect: 20 ypos:20 xpos2:150 ypos2:150 red:255 green:0 blue:0  alpha:0];

} return self; }

//———– // drawRect //———–

(void)drawRect:(General/NSRect)rect { General/[[NSGraphicsContext currentContext] setShouldAntialias:NO]; // gray background General/[[NSColor grayColor]set]; General/NSRectFill(rect); //image on top [mImage compositeToPoint:General/NSZeroPoint operation:General/NSCompositeSourceOver ]; }

//———– // plotRect //———– -(void) plotRect:(int) x1 ypos:(int) y1 xpos2:(int)x2 ypos2:(int) y2 red:(unsigned char)r green:(unsigned char)g blue:(unsigned char)b alpha:(unsigned char)a { int x,y; unsigned char ptr=imageDataPtr + (imageRowBytesy1 + x1 * 4); unsigned char ptr2=imageDataPtr + (imageRowBytesy2 + x1 * 4);

//plot horizontal line
for(x=x1; x<=x2; x++)
{
	*ptr++=r;	*ptr++=g;	*ptr++=b;*ptr++=255-a;
	*ptr2++=r;	*ptr2++=g;*ptr2++=b;*ptr2++=255-a;	
}

ptr=imageDataPtr + (imageRowBytes*y1 + x1 * 4);
ptr2=imageDataPtr + (imageRowBytes*y1 + x2 * 4);

//plot vertical line
for (y=y1; y<=y2; y++)
{
	*ptr=r; *(ptr+1)=g; *(ptr+2)=b; *(ptr+3)=255-a; 
	*ptr2=r; *(ptr2+1)=g; *(ptr2+2)=b; *(ptr+3)=255-a;
	ptr+=imageRowBytes; 
	ptr2+=imageRowBytes;
}

} @end

I doubt this is the root of your problem, but when you create a new General/NSImage you should never do a plain alloc/init. Instead, use initWithSize: and pass it the size you want.

When working with General/NSBitmapImageRep and alpha you need to pre-multiply the non-alpha components by the alpha component. Yes this is strange, but there you go. If you do not alpha does not work properly and you can get strange results. —- Yes, I was looking at that before but what I always forgot was to start by setting all pixels to 0x00000000 instead of 0xffffff00. Thanks!

CocoaDev