Question

I’m trying to apply contrast and brightness to a bitmap in memory and I’m completely lost. Currently I’m trying to use Magick++ to do it, but if one of the other APIs would work better I’m all ears. I managed to find Magick::Image::sigmoidalContrast() for applying the contrast, but I can’t figure out how to get it to work. I’m creating an image, passing it the buffer pointer, then calling that function, but it doesn’t seem like it’s changing anything so my first though was that it’s making a copy and modifying that. Even so, I have no idea how to get the data out of the Magick::Image object.

Here’s what I got so far.

Magick::Image image(fBitmapData->mGetTextureWidth(), fBitmapData->mGetTextureHeight(), "RGBA", MagickCore::CharPixel, pixels);
image.sigmoidalContrast(1, 20.0);

The documentation is useless and after searching I could only find hints that the first parameter is actually a boolean, even though it takes a size_t, that specifies whether to add or subtract the contrast, and the second value is something I have no idea what to pass so I’m just using 20.0 to test.

So does anyone know if this will work for contrast, and if not, then how do you apply contrast? And likewise I still have no idea how to apply brightness either and can’t find any functions that look like they would work.

Answer 1

Figured it out; The function for contrast I was using was correct, and for brightness I ended up using image.modulate(brightness, 100.0, 100.0);. To get the data out of the image object you can grab the pixels of the entire image by doing

const MagickCore::PixelPacket * magickPixels = image.getConstPixels(0, 0, image.columns(), image.rows());

And then copy the magickPixels data back into the original pixels that were passed into the image constructor. An important thing to note is that the member MagickCore::PixelPacket::opacity is not what you would think it would be. If the pixel is completely transparent you’d think the value would be 0, right? Well for some reason ImageMagick is doing it opposite. So for full transparency the value would be 255. This means you need to do 255 - opacity to get the correct value.

Also be careful of the MAGICKCORE_QUANTUM_DEPTH that ImageMagick was compiled with, as this will change the values drastically. For my code MAGICKCORE_QUANTUM_DEPTH just happened to be defined as 16 so all of the values were a range of 0 to 65535, which I just fixed by doing realValue = magickValue >> 8 when copying the data back over since the texture data is unsigned char values.

Just for clarification on how to use these functions, since the documentation is horrible and completely wrong, the first parameter to signmoidalContrast() is actually a boolean, even though the type is a size_t, that specifies whether to increase the contrast (true) or reduce it (false), and the second is a range from 0.00001 to 20.0. I say 0.00001 because 0.0 is an invalid value so it just needs to be some decimal that is close to but not exactly 0.0.

For modulate() the documentation says that each value should be specified as 1.0 for no change, which is completely wrong. The values are actually a percentage so for no change you would specify 100.0.

I hope that helps someone because it took me all damn day to figure this stuff out.