I'm also learning, but I think in this case the viewport is not sized correctly to draw to the texture. I think you need something like this:
; Draw a blue triangle ***this is the part that isn't working*** (glDrawBuffer GL_COLOR_ATTACHMENT0) ; draw to color attachment 0 (glDisable GL_TEXTURE_2D) ; do not use the texture when drawing the triangle (glViewport 0 0 tw th) ; set the viewport to the size of the texture (get tw and th from the size of the png file bitmap) (clear 0.0 0.0 1.0 1.0) (set-color 0.0 1.0 0.0) (glDisable GL_BLEND) (draw-triangle -1.0 0.0 1.0 0.0 1.0 1.0) ; switch to screen framebuffer (glBindFramebuffer GL_FRAMEBUFFER 0) (glViewport 0 0 W H) ; set the viewport to the size of the framebuffer (set W and H from resize handler) Does that help? Thanks, Dave On Fri, Apr 6, 2018 at 2:05 PM, Jens Axel Søgaard <jensa...@soegaard.net> wrote: > Hi All, > > I am looking into opengl and have so far learned to draw directly to the > screen. > However I'd like to do some offscreen rendering and have therefore tried > to use a framebuffer. > The program below does the following: > > 1. Makes a framebuffer > 2. Adds a texture (a plt logo) > 3. Draws a blue triangle on top of the logo > 4. Switches to the screen > 5. Draw a green triangle > 6. Copies the logo with the blue triangle to the screen. > > My problem is that the blue triangle is missing in the result. > > The result: https://imgur.com/NBJf2DG > See program below. > > Ideas welcome. > > /Jens Axel > > > > #lang racket/gui > (require opengl opengl/util ffi/vector) > > (define logo-texture #f) > (define (load-logo-texture) > (unless logo-texture > (set! logo-texture (load-texture "plt-logo-red-gradient.png" #:repeat > 'both))) > logo-texture) > > (define (clear r g b a) > (glClearColor r g b a) > (glClear (bitwise-ior GL_COLOR_BUFFER_BIT GL_DEPTH_BUFFER_BIT))) > > (define (set-color r g b) > (glColor3f r g b)) > > (define (reset-color) > (set-color 1. 1. 1.)) > > (define (draw-triangle x1 y1 x2 y2 x3 y3) > (glBegin GL_TRIANGLES) > (glVertex3d x1 y1 0.) > (glVertex3d x2 y2 0.) > (glVertex3d x3 y3 0.) > (glEnd)) > > (define (example) > ; make framebuffer > (define fbo-ids (glGenFramebuffers 1)) ; this returns a vector of > names > (define fbo (u32vector-ref fbo-ids 0)) ; get the first framebuffer > ; swith to the new framebuffer > (glBindFramebuffer GL_FRAMEBUFFER fbo) ; initialize framebuffer object > ; load logo texture if needed > (load-logo-texture) (unless logo-texture (error)) > (glFramebufferTexture2D GL_FRAMEBUFFER GL_COLOR_ATTACHMENT0 > GL_TEXTURE_2D logo-texture 0) > (define status (glCheckFramebufferStatus GL_FRAMEBUFFER_EXT)) > (define status-complete? (= status GL_FRAMEBUFFER_COMPLETE_EXT)) > > ; Draw a blue triangle ***this is the part that isn't working*** > (glDrawBuffer GL_COLOR_ATTACHMENT0) ; draw to color attachment 0 > (glDisable GL_TEXTURE_2D) ; do not use the texture when > drawing the triangle > (clear 0.0 0.0 1.0 1.0) > (set-color 0.0 1.0 0.0) > (glDisable GL_BLEND) > (draw-triangle -1.0 0.0 1.0 0.0 1.0 1.0) > > ; switch to screen framebuffer > (glBindFramebuffer GL_FRAMEBUFFER 0) > ; red background > (clear 1.0 0.0 0.0 0.0) > ; green triangle > (set-color 0.0 1.0 0.0) (draw-triangle 0.0 0.0 1.0 0.0 0.0 1.0) > ; Copy logo with blue triangle to screen > (reset-color) > (glEnable GL_TEXTURE_2D) > (glEnable GL_BLEND) > (glBlendFunc GL_ONE GL_ONE_MINUS_SRC_ALPHA) ; settings for > pre-multiplied alpha. > (define vertex-array (f64vector -0.5 -0.5 0.5 -0.5 0.5 0.5 -0.5 > 0.5)) > (define texcoord-array (s16vector 0 2 2 2 2 0 0 > 0)) > (let-values (((type cptr) (gl-vector->type/cpointer vertex-array))) > (glVertexPointer 2 type 0 cptr)) > (let-values (((type cptr) (gl-vector->type/cpointer texcoord-array))) > (glTexCoordPointer 2 type 0 cptr)) > (glEnableClientState GL_VERTEX_ARRAY) > (glEnableClientState GL_TEXTURE_COORD_ARRAY) > > (glDrawArrays GL_QUADS 0 4) > (glDisableClientState GL_TEXTURE_COORD_ARRAY) > (glDisableClientState GL_VERTEX_ARRAY) > (glDisable GL_TEXTURE_2D) > (void)) > > ;;; > ;;; MODEL > ;;; > > (define frames-per-second 60.) > (define milliseconds-per-frame (/ 1000. frames-per-second)) > (define time 0.) > > (define (on-tick) > (set! time (+ time 1.)) > (send gl on-paint)) > > ;;; > ;;; GUI > ;;; > > (define (resize w h) > (glViewport 0 0 w h)) > > (define (example2) > (define v (* 2. pi (/ time 60))) > (define c (cos v)) > (define s (sin v)) > > (glClearColor 0.0 0.0 0.0 0.0) > (glClear GL_COLOR_BUFFER_BIT) > > (glBegin GL_TRIANGLES) > (glColor3f c 0. 0.) > (glVertex3d c 0 0) > (glColor3f 0. s 0.) > (glVertex3d 0. s 0.) > (glColor3f 0. 0. 1.) > (glVertex3d -1. -1. 0.) > (glEnd)) > > (define my-canvas% > (class* canvas% () > (inherit with-gl-context swap-gl-buffers) > (define/override (on-paint) (with-gl-context (λ() > (example) (swap-gl-buffers)))) > (define/override (on-size width height) (with-gl-context (λ() (resize > width height) (on-paint)))) > (super-instantiate () (style '(gl))))) > > (define win (new frame% [label "Racket Rosetta Code OpenGL example"] > [min-width 200] [min-height 200])) > (define gl (new my-canvas% [parent win])) > > (new timer% > [interval (exact-floor milliseconds-per-frame)] ; in milliseconds > [notify-callback on-tick]) > > (send win show #t) > > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. 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