Many of the Doom engine graphics, including wall patches and sprites, are stored in the WAD files in a special picture format. Notably excepted are the textures for floors and ceilings, which are known as flats.
Details of the picture format in Doom are given in the Unofficial Doom Specs.
A picture header gives its width and height, and offset values. Following the header are pointers to data for each column of pixels; the number of these pointers is equal to the picture width.
The data for each column is divided into posts, which are lines of colored pixels going downward on the screen. Each post is described by its starting height (relative to the top of the picture) and number of pixels, followed by a value for each of the pixels. Picture descriptions can (and do) skip over some pixel positions; these pixels are transparent. (Since transparent pixels are not changed when drawing a particular picture, whatever was drawn into the frame buffer previously will show through.)
Each pixel is given as an unsigned byte (and thus is valued from 0 to 255). The pixel value is first used as an index into the current COLORMAP, which gives a new pixel value (from 0 to 255) adjusted for the desired light level. (At full brightness, the pixel value is unchanged.) Then this new pixel value is written into the frame buffer. The actual red, green, and blue values corresponding to the palette index in the current palette are stored in the VGA graphics card's 8-bit hardware palette.
Note that gamma correction, a user-adjustable setting that can lighten the colors for dark-looking monitors, is handled when setting the game's palette and not when actually drawing the graphics themselves. This avoids an additional indirection.
Programming algorithms
Converting to a doom picture
This algorithm will convert some pixel data (eg, from a windows bitmap file) to a doom picture.
Notes: ------ Byte = 0 - 255 Word = 0 - 65535 DWord = 0 - 4294967295 dummy_value = Byte, those unused bytes in the file (excerpt from UDS: "..left overs from NeXT machines?..") picture_* = Word, the maximum width for an image in doom picture format is 256 pixels pixel_count = Byte, the number of pixels in a post Pixel = Byte, the pixel colour column_array = array of DWord, this holds all the post start offsets for each column Algorithm: ---------- begin write picture_width to file write picture_height to file write picture_top to file write picture_left to file while loop, exit on x = picture width increase column_array by 1 write memory buffer position to end of column_array y = 0 operator = true do while loop, until y = picture height get Pixel value if the pixel = transperant_colour and operator = false then dummy_value = 0 write dummy_value to memory buffer operator = true otherwise, if pixel != transperant_colour and operator = true then row_start = y pixel_count = 0 dummy value = 0 write above post data to memory buffer offset = current post position in memory buffer operator = false otherwise, if pixel != transperant_colour and operator = false then increment current post pixel_count if offset > 0 and pixel count > 0 then previous_offset = current post position seek back in memory buffer by offset - 2 write pixel_count to memory buffer seek back to previous_offset end block write pixel to memory buffer end block increment y by 1 end block if operator = true or y = height then Pixel = 0 write Pixel to memory buffer rowstart = 255 write rowstart to memory buffer end block increment x by 1 end block seek memory buffer position to 0 block_size = picture_width * size of dword allocate block_memory, filled with 0's, with block_size write block_memory to file, using block_size as size offset = current file_position free block_memory seek to position 8 in file from start for loop, count = 0, break on count = number of elements in column_array column_offset = column_array[count] + offset write column_offset to file end block write memory buffer to file
Converting from a doom picture
This algorithm will convert a doom picture to some pixel data.
Notes: ------ Byte = 0 - 255 Word = 0 - 65535 DWord = 0 - 4294967295 dummy_value = Byte, those unused bytes in the file (excerpt from UDS: "..left overs from NeXT machines?..") picture_* = Word, the maximum width for an image in doom picture format is 256 pixels pixel_count = Byte, the number of pixels in a post Pixel = Byte, the pixel colour column_array = array of DWord, this holds all the post start offsets for each column doom image = could be a file or memory stream Algorithm: ---------- create a image with a pixel format of 8bit and the doom palette, set the background colour to a transperant colour (cyan). read width from doom image (word) read height from doom image (word) read left from doom image (word) read top from doom image (word) create column_array with width number of elements for loop, i = 0, break on i = width - 1 column_array[i] = read from doom image, 4 bytes end block for loop, i = 0, break on i = width - 1 seek doom image to column_array[i] from beginning of doom image rowstart = 0 while loop, rowstart != 255 read rowstart from doom image, 1 byte if rowstart = 255, break from this loop read pixel_count from doom image, 1 byte read dummy_value from doom image, 1 byte for loop, j = 0, break on j = pixel_count - 1 read Pixel from doom image, 1 byte write Pixel to image, j + rowstart = row, i = column end block read dummy_value from doom image, 1 byte end block end block