On Thursday 25 October 2007 10:18, norman wrote:

I would like to be able to produce a black and white negative image from an ordinary colour image. The effect I would like to produce would be to imitate what one would get if you took a colour, positive transparency and made a contact print on to either orthographic or lithographic film. Thus, blue would be black and red would be clear and transparent. Have you any suggestions how to approach this using the Gimp, please.

Colors->Components->Decompose (elseqwhee if you are on gimp 2.2, b ut still "decompose")

and pick only the "Hue" channel.

You cna tehn use colors->curves, or colors-->levels to equate your previous blue to black and your previous red to white.

js ->

Norman

It seems you would basically want an inverted copy of the blue channel with white regions being transparent. There are dozens of ways to accomplish this. I would propose the following:

Decompose you image to RGB components. In the new image, hide the red and green layers. Add a layermask to the blue layer, initialized to a "Grayscale copy of the layer".
Fill the blue layer with black.

OPTIONAL: Most orthochromatic film would respond to some extent to green, so repeat the last two steps on the green layer, make it visible, and set its opacity to a relatively low value (0-30%).

This method is a bit crude, but should provide something not entirely unsuitable. A more accurate approach would probably entail applying appropriate curves to the different color channels.

Quoting norman :

I would like to be able to produce a black and white negative image from an ordinary colour image. The effect I would like to produce would be to imitate what one would get if you took a colour, positive transparency and made a contact print on to either orthographic or lithographic film. Thus, blue would be black and red would be clear and transparent. Have you any suggestions how to approach this using the Gimp, please.

On Thursday 25 October 2007 23:18:04 norman wrote:

I would like to be able to produce a black and white negative image from an ordinary colour image.

I'm lazy, so desaturating on luminosity works well enough for me. If I wanted to alter the colours, I'd put the image through a colour-map first.

For example, I have a shot of an "evolutionary" sequence which runs from a little ape-like fellow through Magnon to a normal human standing tall, who then picks up a rake & starts to bend down again as a result, & so on through a jackhammer until he's sitting worshipfully at a computer keyboard.

I boosted the blue (Colours, Curves) a fair bit, then desaturated that for a slightly sharper-looking monochrome image.

Not sure why artists do this, but the modern-man figures were all Caucasian, & boosting the blue up fairly strongly, emphasised the contrast between these fellows & everyone else up to Mr Cro-Magnon, inclusive, who all had fairly mid-range colours..

Cheers; Leon

There should be no white in the resulting image. When you decompose to RGB ("Colors->Components->Decompose"), you end up with a grayscale image with three layers -- one each for the red, green, and blue components.

When you add the layermasks, you are basically making the the black parts of the layers transparent and the white parts opaque. When you fill the layer with black, you convert the opaque parts to black.

The end result is a layer consisting of black parts and transparent parts (your lithographic negative). There should be no white in your image.

Perhaps you could describe the steps you are taking which produced an overlay of white and black squares?

Quoting norman :

This is very interesting and to a learner like me not too complicated. Perhaps I did not do it quite right but I have what appears to be an overlay of small black and white squares. How do I get rid of that?

Oops, I just realized that what you describe may be attributed to the fact that GIMP will display a dark gray/light gray checkerboard to indicate transparent regions. Perhaps you have created your lithograph correctly but misinterpreted this representation of transparency.

You might try creating a new white layer and placing it at the bottom of the layerstack to show the regions where your image is transparent.