Why is bmp loading faster than png

Question

I have a sample wpf app here and wondering, why BMP is loading faster than PNG. Here is the exact setup:

- Windows 7
- Visual Studio 2013
- landscape.png, 1920x1080, 2.4mb
- landscape.bmp, 1920x1080, 5.6mb
- wpf-app, class MainWindow.xaml.cs constructor, no other code before

Code:

var sw = Stopwatch.StartNew();
var Bitmap1 = new Bitmap("landscape.bmp");
long t1 = sw.ElapsedMilliseconds;

sw.Restart();
var Bitmap2 = new Bitmap("landscape.png");
long t2 = sw.ElapsedMilliseconds;

So the BMP loads with around 6ms, the PNG needs 40ms.

Why is that so?

Answer 1

First, we need to understand how digital images are stored and shown, a digital image is represented as a matrix where each element of the matrix is the color of the pixel, if you have a grayscale image then each element is a uint8 (unsigned 8-bit integer) number between 0 and 255 and in some cases, it's an int8 (signed 8-bit integer) number between -128 and 127. if the element is 0 (or -128 in int8 version) the color is solid black and if the element is 255 (or 127 in int8 version) the color is solid white.

For RGB images each element of the said matrix takes 24 bit or 3 Byte to store (one Byte for each color), a very common resolution for digital cameras and smartphones is 3264 x 2448 for an 8-megapixel camera, now imagine we want to save a 3264 row matrix where each row has 2448 element and each element is 3 Byte, we need about 24 MegaByte to store that image which is not very efficient for posting on the internet or transferring or most of the other purposes. That is why we should compress the image, we can go for JPEG which is a lossy compression method and that means we do lose some quality or we can choose a lossless compression method like PNG which will give us less compression ratio but instead we are not gonna lose quality.

Whether we chose to compress the image or not, when we want to see the image, we can only show the uncompressed version of the image, if the image is not compressed at all, there is no problem, we show exactly what it is, but if it's compressed, we have to decode it first (uncompress it).

With all that being said, let's answer the question. BMP is a format for somewhat raw images, there is either no compression at all or much fewer compression techniques are used than PNG or JPEG but the file size is bigger. When you want to show a BMP image, because it's bigger, there is more data to read into memory, but when it is read, you can show it very faster because there is either no need for decoding or much less decoding is required, on the other hand when you want to show a PNG image, the image will be read into memory much faster but compared to BMP the decoding is going take more time.

If you have a very slow storage, BMP images will be shown slow. If you have a very slow CPU or your decoding software is not efficient PNG images will be shown slow.