saving and loading images with common lisp opengl
I have created a graphic using Common Lisp, OpenGL, and glut. Specifically, I am using the library cl-opengl. I wish to save this graphic (which is made with connecting gl:vertex points) to an external file so that I can load it and manipulate it in future programs.
You can use glReadPixels function in your OpenGL frame rendering routine to get the RGB data, and then save it in a picture format with a fitting library. For example, there is the ZPNG in the quicklisp repository to write PNGs: http://www.xach.com/lisp/zpng/#sect-examples
Use an image library to read image data from the file. To follow my earlier example with the PNG format, you can quickload png-read and use it to extract the RGB data.
(png-read:read-png-file #p"/tmp/1.png")
(png-read:physical-dimensions p) ;returns width and height in meters
(png-read:image-data p) ;returns a three-dimensional array of bytes
If you are using OpenGL already, just enable orthogonal perspective mode, make a quad face with your image as texture and manipulate the mesh. If you want to paint on a 2D image buffer canvas, pick a library like cairo .
in my SBCL following works:
(ql:quickload '(:cl-opengl :cl-glu :cl-glut :zpng))
(defclass hello-window (glut:window) ()
(:default-initargs :pos-x 100 :pos-y 100 :width 250 :height 250
:mode '(:single :rgba) :title "hello"))
(defmethod glut:display-window :before ((w hello-window))
;; Select clearing color.
(gl:clear-color 0 0 0 0)
;; Initialize viewing values.
(gl:matrix-mode :projection)
(gl:load-identity)
(gl:ortho 0 1 0 1 -1 1))
(defmethod glut:display ((w hello-window))
(gl:clear :color-buffer)
(gl:color 0.4 1 0.6)
(gl:with-primitive :polygon
(gl:vertex 0.25 0.25 0)
(gl:vertex 0.75 0.25 0)
(gl:vertex 0.25 0.55 0))
(gl:flush))
(defmethod glut:keyboard ((w hello-window) key x y)
(declare (ignore x y))
(when (eql key #Esc)
(glut:destroy-current-window))
(when (eql key #r)
(let* ((mypng (make-instance 'zpng:png :width 250 :height 250))
(imagedata (zpng:data-array mypng))
(sample1 (gl:read-pixels 0 0 250 250 :bgra :unsigned-byte)))
(format t "read~%")
(dotimes (i (expt 250 2))
(multiple-value-bind (h w) (floor i 250)
(setf (aref imagedata (- 249 h) w 0) (aref sample1 (+ 2 (* i 4))))
(setf (aref imagedata (- 249 h) w 1) (aref sample1 (+ 1 (* i 4))))
(setf (aref imagedata (- 249 h) w 2) (aref sample1 (+ 0 (* i 4))))))
(zpng:write-png mypng #p"/tmp/readpixels.png"))
(format t "written~%")))
(defun rb-hello ()
(glut:display-window (make-instance 'hello-window)))
Pressing "r" saves the file in /tmp/readpixels.png
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