Trying to extract pixel values from a given PNG image

Question

Trying to understand PNG format.

Consider this PNG Image:

Answer 1

Although it might be interesting to learn about what PNG Images actually are, and how the image is actually represented in the file, you don't need to know this to generate a PNG file.

Note that PNG uses lossless compression, which means you won't get two bytes per pixel.

You can generate your image in a program and output it in PNG format using many of the libraries that there are out there. For example, you can make your image in OpenCV and then output it with imWrite. One of the parameters can make it output to a PNG.

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If you have the gray-scale 16-bit pixel values, then you can put them into a Mat.

Then convert that to an IplImage: Converting cv::Mat to IplImage*

Then you can output it to a file.

Answer 2

Just for completeness (eboix's answer is right on the spot)

11- What is the following data?

C6 25 AA 3E 00 00 00 C2

Each chunk ends with a CRC (4 bytes), and starts with 4 bytes that tell its length. So, C6 25 AA 3E is the CRC of the previous chunk (IHDR) and 00 00 00 C2 (194) is the length of the following (IDAT) chunk.

In the same way, the last 4 bytes is the CRC of the IEND chunk.

Answer 3

I did not look too carefully but from looking at the structure...

Q11. C6 25 AA 3E = CRC32 00 00 00 C2 = Size of next chunk

Q13. check the png spec's you referred to earlier that looks like the IDAT chunk you allready know the compression applied to it!

Q15. AE 42 60 82 = CRC32