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6. Pixes

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Image Processing
Pixes (image processing)

Image processing

The pix objects are used to do image processing to pixel data. If you load in an image with [pix_image], then you can change what the image looks like before rendering it out

In general, processing images is extremely expensive, so you probably cannot have that many active pix objects. GEM only reprocesses images when the source image changes or one of the parameters for a pix object changes. This means that GEM will only process an image when something is different, instead of every frame. If you want to do a lot of processing at start up, but then not change anything once the patch is running, GEM will only do the computation once.
Modern CPUs use SIMD (Single Instruction - Multiple Data) (like MMX, SSE2, altivec) to make pixel-processing more effective (by processing data parallely). Until now, only the macOS version of Gem has support for SIMD for some pix-objects. MMX/SSE2 boosts will hopefully come in future Gem-releases.

The pix objects are divided into two general groups, those which take one input, and those which require two input images. For example, [pix_invert] will "invert" all of the pixels (if a pixel is white, it will change to black), while [pix_add] will add two images together.

Only some of the pix objects are described here. Look in the reference patches for explanations for the other pix objects.

[pix_invert] - invert the pixel data
[pix_add] - add two pixes together
[pix_mask] - create an alpha mask
[pix_convolve] - convolve a pix with a kernel


[pix_invert] inverts the pixels in an image. To use [pix_invert], simply make sure that you have already loaded an image into the chain. In the following patch, the fractal image will be inverted.

Here is the difference between the fractal image and the inverted version.


[pix_add] does what you would expect. It adds two images together.

This patch adds the fractal image with a car image. The processed image will often contain a lot of white pixels, because the data is just added together. This occurs in the resulting image, shown below.


[pix_mask] is used to create an alpha mask from another image. In the following example (gem_pix/gemMaskDancer.pd), the fractal image's alpha channel is replaced by the dancer image. If the [alpha] object was removed, then you would just see the solid fractal image (because the alpha channel wouldn't be used).

In other words, images are composed of a red, a green, a blue, and an alpha channel. The alpha channel is the transparency of the pixel. [pix_mask] only modifies the alpha channel and does not touch the red, green, or blue data.

The result is this image.


[pix_convolve] convolves pix data with a convolution kernel. Basically, you can get really nice effects if you choose the correct kernel...and garbage if you choose the wrong one.

Edge detection is done with a convolution kernel, as is smoothing. The biggest problem is that convolving an image is about the most expensive operation that you can do in GEM.

Look at gem_pix/gemPixConvolve.pd to get an idea of some of the kernels that you can send to [pix_convolve] and the effects that you can get.

If you want to learn the math behind convolution, then find any standard image processing (or audio processing book, this is just 2D convolution).


by IOhannes m zmoelnig last modified 2007-07-03 07:06 PM

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