ABAP Ray Tracer – Part 3 – The Skeleton
I am writing a ray tracer with ABAP by studying the book “Ray Tracing from the Ground Up“.
A ray tracer is able to create computer generated imagery.
If you like to know more about my motivation behind this endeavour, check out my first blog.
In my second blog I converted the first C++ classes to ABAP, which are needed to patch together my first ray tracer. On the way of conversion I managed to tackle some language feature shortcomings of ABAP, for example overloading.
In this third blog I am going to get the most rudimentary ray tracer running. The book calls that the skeleton ray tracer.
I’m gonna explain the basic concept behind a ray tracer and give you code examples from my implementation to highlight this concept.
Also I let you take part how I struggled with getting my rendered results displayed.
The Skeleton Ray Tracer
The following figure gives you an idea of how ray casting works.
At this stage, my ray tracer will not be able to handle lights, shadows and cameras, these are features for the coming chapters.
After this blog I’ll be able to render a single object from the position of a window, here titled as “Image”.
So how does a ray tracer work? The book boils it down to:
1. Define some objects 2. Specify a material for each object 3. Define some light sources 4. Define a window whose surface is covered with pixels 5. For each pixel 6. Shoot a ray towards the objects from the center of the pixel 7. Compute the nearest hit point of the ray with the objects (if any) 8. If the ray hits an object 9. Use the object's material and lights to compute the pixel color Else 10. Set the pixel color to black
(Kevin Suffern, 2007, p. 46)
I am not implementing 2., 3., so I skip these and radically simplify 9.
1. Define some objects
Get me a world and build things upon it!
REPORT zart_main. ... FORM render. DATA(world) = NEW zcl_art_world( ). world->build( ). ... ENDFORM.
My world consists of a single sphere. Spectacular!
CLASS zcl_art_world IMPLEMENTATION. ... METHOD build. build_single_sphere( ). ENDMETHOD. METHOD build_single_sphere. ... me->sphere->set_center_by_value( '0.0' ). me->sphere->set_radius( '85.0' ). ENDMETHOD.
4. Define a window whose surface is covered with pixels
I’m gonna look into the world by defining a window of 200 by 200 pixels.
Each pixel has a height and width of 1 unit.
Note: A pixel is not a dot, it’s a square in space. Its area is defined by
pixel_size * pixel_size.
CLASS zcl_art_world IMPLEMENTATION. METHOD build_single_sphere. me->viewplane->set_hres( 200 ). me->viewplane->set_vres( 200 ). me->viewplane->set_pixel_size( '1.0' ). me->viewplane->set_gamma( '1.0' ). ... ENDMETHOD.
5. For each pixel
The following code triggers the whole rendering process.
REPORT zart_main. ... FORM render. ... world->render_scene( ). ... ENDFORM.
Now trigger the calculation row by row.
CLASS zcl_art_world DEFINITION. METHOD render_scene. ... DATA row TYPE int4. DATA column TYPE int4. WHILE row < vres. column = 0. WHILE column < hres. ... ADD 1 TO column. ENDWHILE. ADD 1 TO row. ENDWHILE. ENDMETHOD.
6. Shoot a ray towards the objects from the center of the pixel
Build me a ray, which is facing towards the sphere (
i_z = -1) and originate the ray 100 units away from the world origin, from the center of each pixel.
CLASS zcl_art_world DEFINITION. METHOD render_scene. ... "Loop DATA(ray) = zcl_art_ray=>new_default( ). ray->direction = zcl_art_vector3d=>new_individual( i_x = 0 i_y = 0 i_z = -1 ). ray->origin = zcl_art_point3d=>new_individual( i_x = pixel_size * ( column - hres / '2.0' + '0.5' ) i_y = pixel_size * ( row - vres / '2.0' + '0.5' ) i_z = '100.0' ). DATA(pixel_color) = tracer->trace_ray( ray ). "Loop End ... ENDMETHOD.
7. Compute the nearest hit point of the ray with the objects (if any)
shade_rec aren’t used in this version of the ray tracer, but it will be in future versions.
Right now all I care about is, whether the ray hits the sphere (
CLASS zcl_art_single_sphere IMPLEMENTATION. METHOD trace_ray. DATA t TYPE decfloat16. DATA(shade_rec) = zcl_art_shade_rec=>new_from_world( _world ). _world->sphere->hit( EXPORTING i_ray = i_ray IMPORTING e_tmin = t e_hit = DATA(hit) CHANGING c_shade_rec = shade_rec ). ... ENDMETHOD.
8. If the ray hits …
...an object Use the object's material and the lights to compute the pixel color Else Set the pixel color to black
If the ray hits the sphere, I’m gonna taint the pixel red, or otherwise black.
CLASS zcl_art_single_sphere IMPLEMENTATION. METHOD trace_ray. ... IF hit = abap_true. r_result = zcl_art_rgb_color=>new_copy( zcl_art_rgb_color=>red ). ELSE. r_result = zcl_art_rgb_color=>new_copy( zcl_art_rgb_color=>black ). ENDIF. ENDMETHOD.
First Attempt: #-Character
At this point, I am getting a pixel color, but how the heck am I going to display that?
A quick google search revealed that this might be more difficult than I expected.
For verifying if my code is worth something, I turned to the good old character approach.
Whenever I am hitting the sphere, I’m gonna write a #-character at the row/column position.
CLASS zcl_art_world IMPLEMENTATION. METHOD display_pixel. ... "Write the row counter in front of each line IF x = 0. WRITE /1(*) y NO-GAP. ENDIF. "Write a #-character at the column position, "if the pixel isn't black IF r > 0 OR g > 0 OR b > 0. WRITE AT x(1) '#'. ENDIF. ENDMETHOD.
And then it revealed itself …
… as an egg. 🙂
Second Attempt: Pixel
Alright, my code seems to work. But I really need to be able to display pixels with a certain RGB color value.
I didn’t find anything which let me place at
y coordinates a pixel with a certain color.
After reading an old blog, my gut feeling told me that this is going to need extra effort on my end.
Though I still was looking for an easy way out, which made me tackle that problem from two sides.
I asked the community, if someone still has Thomas Jungs full fledged Bitmap image processing class laying around, which he posted in response to helping that fellow from above. I was hoping to just copy & paste the bitmap part I need and be done with it.
And while the community was digging in their SAP graveyards, searching for a 10 years old class, I was reading up on how to:
- create a bitmap
- display a bitmap
As Thomas mentioned, Wikipedia has an excellent article regarding the Bitmap file format, dissecting all the parts, like how a bitmap header is structured. That allowed me to come up with my own abstraction.
CLASS zcl_art_bitmap DEFINITION PUBLIC FINAL CREATE PUBLIC. PUBLIC SECTION. TYPES: BEGIN OF pixel, x TYPE int4, y TYPE int4, r TYPE int4, g TYPE int4, b TYPE int4, END OF pixel. METHODS: constructor IMPORTING i_image_height_in_pixel TYPE int4 i_image_width_in_pixel TYPE int4, add_pixel IMPORTING i_pixel TYPE pixel, build RETURNING VALUE(r_bitmap) TYPE xstring. ... CLASS zcl_art_bitmap IMPLEMENTATION. METHOD add_pixel. DATA: r TYPE x LENGTH 1, g TYPE x LENGTH 1, b TYPE x LENGTH 1. _converter->convert( EXPORTING data = i_pixel-r IMPORTING buffer = r ). _converter->convert( EXPORTING data = i_pixel-g IMPORTING buffer = g ). _converter->convert( EXPORTING data = i_pixel-b IMPORTING buffer = b ). CONCATENATE _data b g r INTO _data IN BYTE MODE. IF i_pixel-x + 1 = _image_width_in_pixel. CONCATENATE _data _remaining_bytes INTO _data IN BYTE MODE. ENDIF. ENDMETHOD. METHOD build_header. DATA magic_number TYPE x LENGTH 2. _converter->convert( EXPORTING data = _co_magic_number_in_ascii IMPORTING buffer = magic_number ). DATA file_size TYPE x LENGTH 4. DATA(bmp_file_size_in_byte) = get_bmp_file_size( ). _converter->convert( EXPORTING data = bmp_file_size_in_byte IMPORTING buffer = file_size ). ... CONCATENATE magic_number file_size _co_application_specific _co_application_specific offset dib_header_size image_width image_height num_color_palates bits_per_pixel _co_bi_rgb_compression raw_bitmap_size print_resolution print_resolution _co_num_colors_in_palettes _co_important_colors INTO _header IN BYTE MODE. ENDMETHOD. ...
REPORT zart_main. ... FORM display USING i_bitmap type xstring. "now we go from XSTRING back to binary characters TYPES binary_row TYPE x LENGTH 256. DATA binary_rows TYPE TABLE OF binary_row WITH DEFAULT KEY. CALL FUNCTION 'SCMS_XSTRING_TO_BINARY' EXPORTING buffer = i_bitmap TABLES binary_tab = binary_rows. "and now we prepare everything for display DATA url TYPE cndp_url. CALL FUNCTION 'DP_CREATE_URL' EXPORTING type = 'IMAGE' subtype = 'BMP' TABLES data = binary_rows CHANGING url = url. DATA(container) = NEW cl_gui_custom_container( container_name = 'PICTURE' ). DATA(picture) = NEW cl_gui_picture( parent = container ). picture->load_picture_from_url( url ). picture->set_display_mode( picture->display_mode_normal ). ENDFORM.
Et voilá… My first rendered image.
That was quite a moment for me … at 2:45 AM … ?
I wrote the glue code to orchestrate the necessary classes, which are involved in ray tracing an image.
Also, I managed to display pixel color values on a classical Dynpro screen, with the help of my own bitmap file format abstraction class (
The fundamentals of my ray tracer are working.
I am now able to start adding features to my ray tracer.
One sphere is boring. I want more objects, and more objects I’m gonna give you.
The next thing on the menu is to render multiple objects, aka more spheres and more planes at the same time.
Also I’m gonna explain a bit about converting method parameter signatures between C++ and ABAP.