The program code and XML files for all parts can be downloaded from github. Note that the full code is not found below - to run this, use github code.

### Going Somewhere

The cityscape in the previous part is floating in a void of black, empty space. To make it look more realistic, we need to show the ground and add some roads. I will cover the roads first. In the original demo (see introduction) the roads seem to stretch to infinity, and that is the effect I am after here, too. For infinity, "far enough" will do just fine; I have defined the coordinates so that the roads go to ±48,000 when the city in general sits within ±1,000.

There are two problems with using the road objects: (1) they very often have nodes (corner points) both behind and in front of the viewer, causing an issue with the perspective (scaling using the Z coordinate), and (2) sorting objects using their midpoint Z coordinate does not guarantee the correct drawing order, as the roads are so long. Hence, I am adding two new attributes to the Vectorobject class:

self.isFlat = 0 self.prio = 0 # priority order when drawn. Highest prio will be drawn first

The isFlat attribute is set to 1 when the object is created if it has only one surface. Then, before flattening from 3D to 2D and drawing it, the surface is cropped in 3D to the positive side of objMinZ (which is set to 100) in Z coordinates:

After cropping, applying perspective can be done the usual way by dividing X and Y with the Z coordinate.

The prio attribute is also set when creating the objects and can be defined in the XML file for each object. It is used is a simple way so that the higher prio objects will be drawn before anything with a lower prio, and sorting for the drawing order happens within each prio class separately. By giving the roads a higher priority than for the buildings, for instance, will make sure the roads are drawn first and the buildings after them. Otherwise the roads might appear to overlap the buildings.

If you look at the original demo, the roads should nicely vanish into the horizon, but for now they are just drawn in one constant color. This will be fixed in Part VIII.

if self.isFlat == 1: # for flat objects, build a list of transNodes for the surface by first cropping the necessary surface sides to minZ for surface in self.surfaces: surface.setVisible(1) # set all surfaces to "visible" flat_nodes = np.zeros((0, 3)) for node_num in range(len(surface.nodes)): node = self.rotatedNodes[surface.nodes[node_num], 0:3] # current node XYZ coordinates prev_node = self.rotatedNodes[surface.nodes[node_num - 1], 0:3] # previous node XYZ coordinates diff_node = node - prev_node # surface side vector # if both Z coordinates behind the viewer: do not draw at all, do not add a transNode if (node[2] < objMinZ and prev_node[2] >= objMinZ) or (node[2] >= objMinZ and prev_node[2] < objMinZ): # line crosses objMinZ, so add a "crop point". Start from previous node and add difference stopping to objMinZ flat_nodes = np.vstack((flat_nodes, prev_node + diff_node * ((objMinZ - prev_node[2]) / diff_node[2]))) if node[2] >= objMinZ: # add current node, if it is visible flat_nodes = np.vstack((flat_nodes, node)) # apply perspective using Z coordinates and add midScreen to center on screen to get to transNodes self.transNodes = (-flat_nodes[:, 0:2] * zScale) / (flat_nodes[:, 2:3]) + midScreen

After cropping, applying perspective can be done the usual way by dividing X and Y with the Z coordinate.

The prio attribute is also set when creating the objects and can be defined in the XML file for each object. It is used is a simple way so that the higher prio objects will be drawn before anything with a lower prio, and sorting for the drawing order happens within each prio class separately. By giving the roads a higher priority than for the buildings, for instance, will make sure the roads are drawn first and the buildings after them. Otherwise the roads might appear to overlap the buildings.

If you look at the original demo, the roads should nicely vanish into the horizon, but for now they are just drawn in one constant color. This will be fixed in Part VIII.

### Ground Control

The ground will be blue and nicely faded to black in the horizon. However, it will not be fixed; the fading will depend on the height of the viewer, and in the Python code I want to preserve the ability to rotate in true 3D, unlike the original purely vertical axis rotation. For this purpose I created a special ground object. There is not much special about the object itself - it is a flat square at ground level, just providing the information on rotation - but it is handled by custom code.

First, using the ground object rotated nodes, I want to find out a 3D square on that surface that has Z coordinates at either groundZ or -groundZ; this will make the fading work as intended with distance.

def groundData(self, midScreen, zScale, objMinZ, groundZ, groundShades, groundShadeNr): """ Calculate ground data for on a ground object. Assumes the ground object "covers the ground" reasonably and isFlat = 1, and the perimeter is concave. Ground settings are defined in VectorViewer. Returns an array of shape(groundShadeNr + 1, 4) where each row has X,Y of left edge and X.Y of right edge starting from most distant """ # find the most distant node maxZ = max(self.rotatedNodes[:, 2]) for nodenum in range(len(self.nodes)): if self.rotatedNodes[nodenum, 2] == maxZ: node = self.rotatedNodes[nodenum, :] break prev_node = self.rotatedNodes[nodenum - 1, :] if nodenum == len(self.nodes) - 1: next_node = self.rotatedNodes[0, :] else: next_node = self.rotatedNodes[nodenum + 1, :] # get a straight line where Z (ie, distance from viewer) is constant. Start with the mid of farthest of the two lines. # then find the point with matching Z coordinate on the other line. # special cases: next_node or prev_node as far as node. if node[2] == prev_node[2]: mid1_node = node mid2_node = prev_node else: if node[2] == next_node[2]: mid1_node = node mid2_node = next_node else: if next_node[2] > prev_node[2]: mid1_node = (next_node + node) / 2 mid2_node = node + (prev_node - node) * (mid1_node[2] - node[2]) / (prev_node[2] - node[2]) else: mid1_node = (prev_node + node) / 2 mid2_node = node + (next_node - node) * (mid1_node[2] - node[2]) / (next_node[2] - node[2]) if mid1_node[1] < mid2_node[1]: # make sure mid1_node X < mid2_node X mid1_node, mid2_node = mid2_node, mid1_node # adjust Z mid1_node = mid1_node * groundZ / mid1_node[2] mid2_node = mid2_node * groundZ / mid2_node[2] # finalize a square around object position mid2_node_back = self.position[0:3] + (self.position[0:3] - mid1_node) # from front left (mid1) to back right (mid2_back) mid1_node_back = self.position[0:3] + (self.position[0:3] - mid2_node) # from front right (mid2) to back left (mid1_back)

Then, having this 3D square, I will generate groundShadeNr (defined in VectorViewer class) slices of ground, the most distant being first. When drawing these in preset colors the fading effect will be accomplished. I am using 16 shades - on the Amiga 15 was the maximum - the more you use, the better the quality, and the slower the drawing. The nodes needed for the slices (groundShadeNr + 1 nodes) are also cropped to screen X coordinates, as the ground is very large and usually overlaps the screen by a lot.

# then generate arrays with necessary node data and transNode data left_nodes = np.zeros((groundShadeNr + 1, 3), dtype=float) right_nodes = np.zeros((groundShadeNr + 1, 3), dtype=float) # multipliers will span ground component span between groundZ/2 (furthest) and objMinZ mult = (mid1_node[2] / 2 - objMinZ) / ((mid1_node[2] - mid1_node_back[2]) / 2) # the most distant component (at groundZ). Most distant component will be very large (half of total) left_nodes[0,:] = mid1_node right_nodes[0,:] = mid2_node # other components from groundZ/2 to objMinZ for i in range(groundShadeNr): mult_i = mult * math.sqrt((i+1) / groundShadeNr) left_nodes[i+1,:] = (mid1_node * (1.0 - mult_i) + mid1_node_back * mult_i) / 2 right_nodes[i+1,:] = (mid2_node * (1.0 - mult_i) + mid2_node_back * mult_i) / 2 left_transNodes = (-left_nodes[:, 0:2] * zScale) / (left_nodes[:, 2:3]) + midScreen right_transNodes = (-right_nodes[:, 0:2] * zScale) / (right_nodes[:, 2:3]) + midScreen # crop these nodes to screen X edges diff_transNodes = right_transNodes - left_transNodes mult_nodes = right_transNodes[:, 0] / diff_transNodes[:, 0] left_transNodes = right_transNodes - np.multiply(np.transpose(np.vstack((mult_nodes, mult_nodes))), diff_transNodes) diff_transNodes = right_transNodes - left_transNodes mult_nodes = (midScreen[0] * 2) / diff_transNodes[:,0] right_transNodes = left_transNodes + np.multiply(np.transpose(np.vstack((mult_nodes, mult_nodes))), diff_transNodes) # the first component is "the top of the sky". if left_transNodes[0,1] < left_transNodes[1,1]: # "normal ground", add a node to the top of the screen if left_transNodes[0,1] < 0: # if ground already covers the whole screen, use the top node left_skynode = left_transNodes[0,:] else: left_skynode = np.array([0, 0]) else: # inverted ground ie. going upside down, add a node to the bottom of the screen if left_transNodes[0,1] > midScreen[1] * 2: # if ground already covers the whole screen, use the top node left_skynode = left_transNodes[0,:] else: left_skynode = np.array([0, midScreen[1] * 2]) if right_transNodes[0,1] < right_transNodes[1,1]: # "normal ground", add a node to the top of the screen if right_transNodes[0,1] < 0: # if ground already covers the whole screen, use the top node right_skynode = right_transNodes[0,:] else: right_skynode = np.array([midScreen[0] * 2, 0]) else: # inverted ground ie. going upside down, add a node to the bottom of the screen if right_transNodes[0,1] > midScreen[1] * 2: # if ground already covers the whole screen, use the top node right_skynode = right_transNodes[0,:] else: right_skynode = midScreen * 2 # add the first component and build an array of all the transnodes transNodes = np.vstack((np.hstack((left_skynode, right_skynode)), np.hstack((left_transNodes, right_transNodes))))

Since the ground is the first object to be drawn - it has the highest prio - and usually covers about half of the screen, I am also adding an extra component; see "the top of the sky" in the above code. By adding the part of the screen

*not*covered by the ground, I can skip clearing the screen altogether. In effect I am only clearing the part that will not be overwritten by the ground anyway, in an effort to save some time.

### Size Does Matter

With the new, big objects ground and roads, they will certainly be stretching outside of our screen. I already used cropping with the Z coordinate above, and now I am adding code to crop also X and Y transNodes i.e. 2D screen coordinates to fit the screen, so that we will not try to draw anything outside of the viewing area. The code below goes through a list of polygon nodes and makes the necessary deletions and modifications. It also converts the list of nodes to integer values.

def cropEdges(self, node_list, cropX = True, cropY = True): # crop to screen size. "Auto crop" does not seem to work if points very far outside. # takes list of nodes (X,Y) in drawing order as input. # returns list of nodes (X,Y) cropped to screen edges. # crop both X, Y, if cropX and cropY = True; X: i=0, Y: i=1 if len(node_list) > 2: for i in range(2): if (i == 0 and cropX == True) or (i == 1 and cropY == True): crop_nodes = [] # empty list prev_node = node_list[-1] for node in node_list: diff_node = node - prev_node # surface side vector # start cropping from prev_node direction, as order must stay the same if node[i] >= 0 and prev_node[i] < 0: # line crosses 0, so add a "crop point". Start from previous node and add difference stopping to 0 crop_nodes.append(prev_node + diff_node * ((0 - prev_node[i]) / diff_node[i])) if node[i] <= self.midScreen[i] * 2 and prev_node[i] > self.midScreen[i] * 2: # line crosses screen maximum, so add a "crop point". Start from previous node and add difference stopping to midScreen[i] * 2 crop_nodes.append(prev_node + diff_node * ((self.midScreen[i] * 2 - prev_node[i]) / diff_node[i])) # then crop current node if node[i] < 0 and prev_node[i] >= 0: # line crosses 0, so add a "crop point". Start from previous node and add difference stopping to 0 crop_nodes.append(prev_node + diff_node * ((0 - prev_node[i]) / diff_node[i])) if node[i] > self.midScreen[i] * 2 and prev_node[i] <= self.midScreen[i] * 2: # line crosses screen maximum, so add a "crop point". Start from previous node and add difference stopping to midScreen[i] * 2 crop_nodes.append(prev_node + diff_node * ((self.midScreen[i] * 2 - prev_node[i]) / diff_node[i])) # always add current node, if it is on screen if node[i] >= 0 and node[i] <= self.midScreen[i] * 2: crop_nodes.append(node) prev_node = node # for next i, copy results. Quit loop if no nodes to look at node_list = crop_nodes if len(node_list) < 3: break # convert to integers node_list = [(int(x[0] + 0.5), int(x[1] + 0.5)) for x in node_list] return node_list

Now the end result looks like this:

### To Blit or not to Blit?

On the Amiga, the Blitter co-processor is capable of drawing lines between the given coordinates. In addition, it has a special mode that only sets one bit (i.e. draws one pixel) per horizontal line. This comes in handy when combined with the Blitter fill operation, which will go through each horizontal line in an area bit by bit, and each time it encounters a bit that is set, it will turn area fill mode on if it was off, and off if it was on. So, by drawing the edges of a square using the special mode, it possible to fill that square with the fill operation.

Remember that the Amiga uses bitplane graphics so that one can choose the depth - the number of bitplanes - of the screen freely between one and five (special modes have six) and the number of colors available is two to the power of that, so with five bitplanes one gets 2^5 = 32 colors. If I needed a rotating cube, for example, like in Part III, I would only need three colors as that's the maximum number of surfaces drawn simultaneously. Obviously, I also need one color for the background. These four colors can be defined with two bitplanes. For any one pixel, the first bitplane defines the least significant bit of the color used, and the second bitplane the next bit. So, if both bitplanes are cleared, I would have color 00 (binary) = 0 (decimal) - this is the background. If I only draw a filled surface on the first bitplane, it will have color 01 = 1, and if only on the second bitplane, color 10 = 2. If both bitplanes are filled, the color is 11 = 3.

How to actually draw this on the Amiga then? To be efficient, the best way is not to draw the surfaces one by one, like the Python code and it's pygame.draw.polygon do. It's more efficient to first figure out for each edge of the cube if it needs to be drawn or not. For our cube, let's say there are three surfaces (1, 2 and 3) to be drawn, and we have defined that they will have colors 1, 2 and 3, respectively. The colors needed (in binary) will then be 01, 10, and 11. Furthermore, both surfaces 1 and 2 will have one edge in common with surface 3. In practice, then, I need to fill the area of surfaces 1 and 3 on bitplane 1, and the area of surfaces 2 and 3 on bitplane 2. So, instead of drawing and filling each surface with four operations (surface 1 on biplane 1, s. 2 on 2, s. 3 on 1, and s. 3 on 2) it will be much more efficient to draw the common boundaries of surfaces 1 and 3 on bitplane 1 and then fill it in one blitter fill operation, and then the common boundaries of surfaces 2 and 3 on bitplane 2 and then fill that in another blitter fill operation. In addition, the fill operation needs a "virgin" bitplane to work properly; trying to draw and fill a surface next to another already drawn and filled would mess up the second fill operation. Figuring out which edges need to be drawn in which colors is actually very simple by using an XOR logical operation on the edge colors with each surface drawn.

With multiple objects on top of each other, it is necessary to have separate bitplanes for drawing the next object, and then copy that to the main bitplanes on top of the other objects, again using the blitter.

Next: Part VI: Shadows