![]() ![]() In the final step, if the pixel value was below 0.5 it was set to 1 otherwise it was set to 0. The ‘dealt with later’ refers to later in the Floyd – Steinberg process. The algorithm achieves dithering using error diffusion, meaning it pushes (adds) the residual quantization error of a pixel onto its neighboring pixels, to be dealt with later. Since the colors on paper were either 1 or 0, the Floyd – Steinberg dithering made a big difference to the outcome in making the image look like shades of gray. I used the luminosity method to get a gray value for each pixel, then the Floyd – Steinberg dithering algorithm. What you consider neutral is also heavily effected by the surroundings in which you view, your eyes are able to adjust their own gain functions to compensate for a certain amount of change, thus a piece of paper can look white to you even if the colour of the light shining on it is changed, although with highly coloured light you will still see it as being coloured.Ībout 20 years ago I had to read a 640 x 480 px image from a RGB frame buffer in an IBM-6000 workstation with AIX Unix and the Motif Window Manager on top of X-windows, and send it to a LaserJet printer that did not have PostScript and which only had dots, black or white for images. The mixing also assumes that the individual colour channels are linear, else your not really that close to how our eyes behave. Depending on the exact colours of R, G and B you will get different proportions. ![]() They are are saying what linear combination of R, G and B would you perceive as being without colour, i.e. Re: the magic numbers, well they are based upon a mixing assumption. In film the actual ‘colours’ vary mostly due to the stocks used, but there are some effects that vary with intensity of exposure so the camera settings can effect the results though this is minor compared to the stock and development. You then shine a light through the resultant film and bounce it off a screen, your eyes then view the result, In mathematical terms this is a highly non-linear transformation from scene-to-screen and is a function of many reductions from spectral to n-dimensional ‘records’ all of whose intensity functions are non-linear. Through various means this gets converted to either silver halide crystals or in colour film different dye’s are formed (eventually). In film or a digital sensor there is a spectral sensitivity function that determine which frequencies of light get absorbed/sensitise the emulsion based upon the chemical structure. ![]() in film or any system sensitive to the electromagnetic spectrum, can be considered as a reduction of an infinite number of frequencies (well infinite depending on your quantum view of the world I guess), into a smaller number, in a similar manner to a weighted sum histogram. The answer is none of the above methods are used for film. ![]() If you found this post useful, check out for daily tidbits of signal and image processing. And sometimes the three methods produce very similar results. However, some images look better using one of the other algorithms. The luminosity method works best overall and is the default method used if you ask GIMP to change an image from RGB to grayscale from the Image -> Mode menu. The lightness method tends to reduce contrast. The example sunflower images below come from the GIMP documentation. The formula for luminosity is 0.21 R + 0.72 G + 0.07 B. We’re more sensitive to green than other colors, so green is weighted most heavily. It also averages the values, but it forms a weighted average to account for human perception. The luminosity method is a more sophisticated version of the average method. The average method simply averages the values: (R + G + B) / 3. The lightness method averages the most prominent and least prominent colors: (max(R, G, B) + min(R, G, B)) / 2. How do you convert a color image to grayscale? If each color pixel is described by a triple (R, G, B) of intensities for red, green, and blue, how do you map that to a single number giving a grayscale value? The GIMP image software has three algorithms. ![]()
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