Re: Problem with setting color in NSBitmapImageRep

FROM : Jean-Daniel Dupas
DATE : Mon Mar 31 23:39:14 2008

Don't know why you got this problem, but if I had to fill a bitmap 
using a simple gradient, i would probably use either CGGradient 
(Leopard required) or CGShading (if you want a greater control of the 
shading function, etc…).

It can be done like this:

NSGraphicsContext *ctxt = [NSGraphicsContext 
graphicsContextWithBitmapImageRep:imageRep]; /* see doc for details 
about bitmap format supported */
CGContextRef cgctxt = [ctxt graphicPort];
CGShadingRef shading = CGShadingCreate(...); /* see Quartz 2D 
programming guide */
CGContextDrawShading(cgctxt, shading);
CGShadingRelease(shading);

This will be really faster than creating a NSColor object for each 
pixel.

In the same way, you can fill it with an uniform color using 
CGContextFillRect() function.

And is you want to use Cocoa drawing function instead, set your new 
context as the current context using -[NSGraphicsContext 
setCurrentContext:] and the you will be able to call NSBezierPath 
function for example.


Le 31 mars 08 à 21:30, Christian Kaiser a écrit :

> I'm struggling with setting color in a NSBitmapImageRep. I'm 
> computing the components of a NSColor according to a numeric value; 
> its basically a linear interpolation between all components of two 
> NSColors. I've got a function which is computing the new NSColor 
> based on a given value. If I call this function repeatedly using the 
> same value and setting the color using NSBitmapImageRep 
> setColor:atX:y:, I expect to have a uniform image if applying to all 
> pixels. But this is not the case...! What am I missing ?
>
> As I'm probably a little bit confusing, here the code producing my 
> strange image :
>
>
>
>  // Creating two colors which define a gradient. The color for our 
> image
>  // will be computed using this gradient.
>
>  NSColor *lightYellow =
>      [NSColor colorWithCalibratedRed:0.97 green:0.93 blue:0.65 
> alpha:1.0];
>    NSColor *darkRed =
>      [NSColor colorWithCalibratedRed:0.69 green:0.09 blue:0.11 
> alpha:1.0];
>
>
>  // Creating an empty image with the same color space.
>
>  NSBitmapImageRep *img = [[NSBitmapImageRep alloc]
>      initWithBitmapDataPlanes:NULL
>      pixelsWide:10
>      pixelsHigh:10
>      bitsPerSample:8
>      samplesPerPixel:3
>      hasAlpha:NO
>      isPlanar:YES
>      colorSpaceName:[lightYellow colorSpaceName]
>      bitmapFormat:0
>      bytesPerRow:0
>      bitsPerPixel:0];
>
>  // Assigning the same (???) color to all pixels.
>  int x, y;
>  for (y = 0; y < 10; y++)
>  {
>      for (x = 0; x < 10; x++)
>      {
>          NSColor *imgColor = [GradientColor gradientColorWithValue:
> 40.0
>              minColor:lightYellow maxColor:darkRed
>              minValue:30.0 maxValue:60.0];
>                    [img setColor:imgColor atX:x y:y];
>      }
>  }
>      [[img TIFFRepresentation]
>      writeToFile:@"/Users/chkaiser/Desktop/img.tif" atomically:YES];
>
>
> And the function computing the gradient color based on our values:
>
> +(NSColor*)gradientColorWithValue:(double)value minColor:(NSColor*)c1
>  maxColor:(NSColor*)c2 minValue:(double)v1 maxValue:(double)v2
> {
>    // Color components.
>  CGFloat comps1[5];
>  [c1 getComponents:comps1];
>  CGFloat comps2[5];
>  [c2 getComponents:comps2];
>    // Compute the multiplication factor for the value. This factor 
> has
>  // a value between 0 and 1.
>  double factor = (value - v1) / (v2 - v1);
>    // Compute the new components.
>  int i;
>  CGFloat newComps[5];
>  for (i = 0; i < [c1 numberOfComponents]; i++)
>  {
>      newComps[i] =
>          (factor * (comps2[i] - comps1[i])) +
>              comps1[i];
>  }
>    // Create the new color with the computed components.
>  NSColor *newColor = [NSColor colorWithColorSpace:[c1 colorSpace]
>      components:newComps count:[c1 numberOfComponents]];
>    return newColor;
>  }
>
>
> What is going wrong here ?
>
> I have not found any similar topic on this list until now, even if 
> it seems to me quite strange. Is there something obvious I am 
> missing ?
>
> --Christian Kaiser
>
>
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