Using ImageMagick++ to modify image contrast/brightness

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.

Answer 2

According to the Imagemagick website - for the command line but may be the same?

-sigmoidal-contrast contrastxmid-point

increase the contrast without saturating highlights or shadows.

Increase the contrast of the image using a sigmoidal transfer function without saturating highlights or shadows. Contrast indicates how much to increase the contrast. For example, near 0 is none, 3 is typical and 20 is a lot. Note that exactly zero is invalid, but 0.0001 is negligibly different from no change in contrast. mid-point indicates where midtones fall in the resultant image (0 is white; 50% is middle-gray; 100% is black). By default the image contrast is increased, use +sigmoidal-contrast to decrease the contrast.

To achieve the equivalent of a sigmoidal brightness change, use -sigmoidal-contrast brightnessx0% to increase brightness and class="arg">+sigmoidal-contrast brightnessx0% to decrease brightness.

On the command line there is a new brightness contrast setting that may be in later versions of magic++?

-brightness-contrast brightness{xcontrast}{%}}

Adjust the brightness and/or contrast of the image.

Brightness and Contrast values apply changes to the input image. They are not absolute settings. A brightness or contrast value of zero means no change. The range of values is -100 to +100 on each. Positive values increase the brightness or contrast and negative values decrease the brightness or contrast. To control only contrast, set the brightness=0. To control only brightness, set contrast=0 or just leave it off.

You may also use -channel to control which channels to apply the brightness and/or contrast change. The default is to apply the same transformation to all channels.

Brightness and Contrast arguments are converted to offset and slope of a linear transform and applied using -function polynomial "slope,offset".

The slope varies from 0 at contrast=-100 to almost vertical at contrast=+100. For brightness=0 and contrast=-100, the result are totally midgray. For brightness=0 and contrast=+100, the result will approach but not quite reach a threshold at midgray; that is the linear transformation is a very steep vertical line at mid gray.

Negative slopes, i.e. negating the image, are not possible with this function. All achievable slopes are zero or positive.

The offset varies from -0.5 at brightness=-100 to 0 at brightness=0 to +0.5 at brightness=+100. Thus, when contrast=0 and brightness=100, the result is totally white. Similarly, when contrast=0 and brightness=-100, the result is totally black.

As the range of values for the arguments are -100 to +100, adding the '%' symbol is no different than leaving it off.

If magick++ is like Imagick it may be lagging a long way behind the Imagemagick options