Image brightness for big images c#

Question

I use this link to add my program the capability to adjust the brightness of the image. This code is ok but it takes time to adjust the brightness(Image file size 1.8mb). When I try the lower quality image it instantly adjusts the image(Image file size 100KB). Is there any efficient way to adjust the brightness of the image.

Answer 1

The code seems to use GetPixel and SetPixel on regular Bitmaps. This is a bad idea because it is so slow.

To manipulate a single pixel of a Bitmap it must be locked (which Get/SetPixel do behind the scenes) and doing it on a pixel by pixel basis means that for a 1000x1000 sized image a million locking/unlocking operations must be performed. This creates an enormous overhead.

Method one

One way to avoid this is to lock the whole bitmap with the LockBits function. Now we can loop over the pixels and modify them.

Two notes about this method:

• What we now access are the raw bytes of each pixel, that is each channel separately: either BGR or BGRA, depending on the pixel format. This means that the channels are physically reversed from the usual RGB/ARGB format of the Color methods.

• To loop over the physical bitmap pixel rows we also need to add some stride to each row, which pads the rows to a multiple of 4 bytes. Also see here

For some examples you may want to browse over some of these posts. Note especially this one which uses a delegate to allow for flexible operations!

(Note that several of the posts use 2 or even 3 locked bitmaps because they aim at combining images..)

Method two

Another way to get around the overhead of locking pixels one by one are ready-made bitmap classes that help by locking themselves as a whole. Here and here are examples I didn't try myself.

Method three

Finally there is a very elegant method for image manipulation, which is both rather simple and really fast; also professionally crafted for best results: You can set up a ColorMatrix.

It will let you change brightness, gamma, hues and then some. Here is a very nice introduction.

The only drawback is, that is limited to some fixed operations, so you can't create custom filters for other fancy stuff, like photoshop-type layer modes or others, especially those that need to process neighbouring pixels e.g. for blurring..

But if all you want is changing brightness, this is what I would recommend!