Re: How to determine a single pixels alpha?
Re: How to determine a single pixels alpha?
- Subject: Re: How to determine a single pixels alpha?
- From: "John C. Randolph" <email@hidden>
- Date: Sun, 30 Sep 2001 16:11:00 -0700
On Sunday, September 30, 2001, at 03:30 PM, Brian Hook wrote:
At 05:36 PM 9/30/01 -0400, Jesse Grosjean wrote:
unsigned char* pixels = [aBitmapImageRep bitmapData];
int alpha;
// i'm trying to get to the pixel specified by aPoint. I'm
sure i should be able to index
// into this with an array somehow
"pixels[aPoint.x][aPoint.y]" but can't figure out how
// to do that. In any case I'm pretty sure this loop is where
i'm doing things wrong, the pick
// works correctly sometimes, but other returns the wrong
answer.
for(i= 0; i <= aPoint.x; i++) {
for (j = 0; j <= aPoint.y; j++) {
pixels++; // red
pixels++; // blue
pixels++; // green
alpha = *pixels++;
}
}
I don't know the ordering of pixels in BSBitmapImageRep off the top of
my head, but you seem to be stepping through it in x-major order, so
assuming that, you should be able to fetch the pixel you want directly
just doing:
unsigned char *pPixel = &pixels[ aPoint.x * 4 + aPoint.y * 4 *
imageWidth ];
Where "imageWidth" is the width of the bitmap data in pixels.
Then it's just a matter of:
if ( pPixel[ 3 ] == 0 )
return NO;
return YES;
There's one too many multiplies in the code above.
Here's an example I have of code that fills a 256x256 image with a
gradient:
static NSSize imageSize = {256.0, 256.0};
...
NSBitmapImageRep *bitmap = [[NSBitmapImageRep alloc]
initWithBitmapDataPlanes:nil
pixelsWide:imageSize.width
pixelsHigh:imageSize.height
bitsPerSample:8
samplesPerPixel:4
hasAlpha:YES
isPlanar:NO
colorSpaceName:NSCalibratedRGBColorSpace
bytesPerRow:0
bitsPerPixel:32];
imageBytes=[bitmap bitmapData];
y = imageSize.height;
while(y--)
{
x = imageSize.width;
while(x--)
{
int
pixIndex = 4 * (y*(int)imageSize.width+x);
imageBytes[pixIndex] = x; //red
imageBytes[pixIndex +1] = 255 - x; // green
imageBytes[pixIndex +2] = 0; // blue
imageBytes[pixIndex +3] = 255; //Alpha
}
}
-jcr
"I fear all we have done is to awaken a sleeping giant and fill him with
a terrible resolve." -Admiral Isoroku Yamamoto, Dec 7, 1941.