Shading anomalies

Subject: Shading anomalies
From: Torsten Radtke <email@hidden>
Date: Fri, 3 Mar 2006 10:33:19 +0100
Delivered-to: email@hidden
Delivered-to: email@hidden

Hi there,

I discovered a problem with CGShadings on Tiger: If I draw a horizontal shading (i.e. the y-coordinates of start and end point are equal) and the red component of both the start and end points is equal to 1.0, the shading will be opaque (alpha values for both start and end point will be ignored).

If I change the red component for one of the points to a value less than 1.0, or change the y-coordinate of one of the points, the shading draws as expected. Here is the code:

- (void) drawRect: (NSRect) rect
{
  CGShadingRef shading;
  CGFunctionRef shadingFunction;
  CGColorSpaceRef colorSpace;
  CGFunctionCallbacks callbacks;

CGContextRef context = [[NSGraphicsContext currentContext] graphicsPort];

float inputValueRange[2] = {0.0f, 1.0f}; float outputValueRange[8] = {0.0f, 1.0f, 0.0f, 1.0f, 0.0f, 1.0f, 0.0f, 1.0f};

  callbacks.version = 0;
  callbacks.evaluate = evaluateGradient;
  callbacks.releaseInfo = NULL;

  colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);

shadingFunction = CGFunctionCreate(NULL, 1, inputValueRange, 4, outputValueRange, &callbacks); shading = CGShadingCreateAxial(colorSpace, CGPointMake (rect.origin.x, rect.size.height*0.5f), CGPointMake(rect.size.width, rect.size.height*0.5f), shadingFunction, false, false);

  CGContextDrawShading(context, shading);

  CGColorSpaceRelease(colorSpace);
  CGShadingRelease(shading);
  CGFunctionRelease(shadingFunction);
}

static void evaluateGradient(void *info, const float *in, float *out)
{
  float factor = in[0];

  // gradient from opaque white to transparent red

  out[0] = 1.0f*(1.0f - factor) + 1.0f*factor;
  out[1] = 1.0f*(1.0f - factor) + 0.0f*factor;
  out[2] = 1.0f*(1.0f - factor) + 0.0f*factor;
  out[3] = 1.0f*(1.0f - factor) + 0.0f*factor;
}

Am I doing something wrong or is this bug already known?

Thanks for any help,
Torsten


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