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colorsjpegrgbgrayscalelibjpeg-turbo

libjpeg not exact pixel values even with quality of 100

I am writing a program to read some text files and write it to a JPEG file using libjpeg. When I set the quality to 100 (withjpeg_set_quality), there is actually no quality degradation in grayscale. However, when I move to RGB, even with a quality of 100, there seems to be compression.

When I give this input to convert to a grayscale JPEG image it works nicely and gives me a clean JPEG image:

0  0  0 0 0
0  0  0 0 0
0  0  0 0 0
0 255 0 0 0
255 0 0 0 0

The (horizontally flipped) output is:

grayscale

Now when I assume that array was the Red color, and use the following two arrays for the Green and Blue colors respectively:

0 0  0  0 0
0 0  0  0 0
0 0 255 0 0
0 0  0  0 0
0 0  0  0 0


0 0 0  0 255
0 0 0 255 0
0 0 0  0  0
0 0 0  0  0
0 0 0  0  0

This is the color output I get:

RGB

While only 5 input pixels have any color value, the surrouding pixels have also gotten a value when converted to color. For both the grayscale image and RGB image the quality was set to 100.

I wanted to see what is causing this and how I can fix it so the colors are also only used for the pixels that actually have an input value?

Solution

  • You are getting errors from the RGB->YCbCr conversion. That is impossible to avoid in the large because there is not a 1:1 mapping between the two color spaces.