> On 25 Jun 2017, at 09:29, Graham Cox <graham....@bigpond.com> wrote:
>
> You could get the colorSpaceName from the source image and use that. If
> there’s no conversion needed between the images it’ll probably be faster
> (though seems you are giving yourself the option of creating a grayscale
> version here).
The images are really b&w, but the source image claims to be
NSCalibratedRGBColorSpace.
So I prefer to control the colour here and not rely on the source image.
Which of these two lines is preferable:
NSString *colorSpaceName = useColour ? NSDeviceRGBColorSpace :
NSDeviceWhiteColorSpace;
NSString *colorSpaceName = useColour ? NSCalibratedRGBColorSpace :
NSCalibratedWhiteColorSpace;
?
>> NSInteger bitsPerPixel = samplesPerPixel * bitsPerSample;
>> NSInteger bytesPerRow = pixelsWide * bitsPerPixel / 8;
>
> In fact you can pass 0 for these last two and the function will calculate
> them for you. It’s better to do that because if there’s the slightest thing
> wrong with the parameters it will give up with an exception.
> The internal calculation can also round up bytesPerRow to something that will
> optimise copying performance.
I changed my code following your suggestion.
>> [ NSGraphicsContext setCurrentContext: context ];
>
> You may want to save and restore the current context around this, just to be
> sure your method isn’t going to have the unwanted side effect of changing the
> current context. It’s likely OK, but better safe than sorry.
I just did. Turns out that the oldContext seems to be nil. But as you rightly
said: “better safe than sorry”.
>> NSRect destRect = NSZeroRect; destRect.size = subRect.size;
>
> Just use NSZeroRect for destRect. This is interpreted to mean “the size of
> the destination image”, which is already <subRect>, rounded up. If you set
> the size here and it’s not a whole number for some reason, there is the
> potential for the very edge pixels not to get set, leaving a visible edge at
> the right and bottom of the result.
I tried this. But got a rather small image (just 88 bytes - usually 337 bytes)
and the resulting image looks rather empty. It has the right size, but not a
single pixel is set - all are transparent.
But heading your warning about non-whole numbers, I added at the very start:
- (NSData *)subImagePngDataWithRect: (NSRect)suxRect useColour: (BOOL)useColour
{
NSRect subRect = NSIntegralRect(suxRect);
…
}
> Otherwise, looks good. Does it do the job?
It did. And now it does even better.
Thanks a lot for your help. Very much appreciated!
Kind regards,
Gerriet.
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