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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 //———–

//———– // drawRect //———–

//———– // 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;

} @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!